Symposium Organizers
Alexander O. Govorov Ohio University
Zhiming M. Wang University of Arkansas
Andrey L. Rogach Ludwig-Maximilians-Universtitaet Muenchen
Harry Ruda University of Toronto
Mark Brongersma Stanford University
GG1: Self-assembled Quantum Dots and Quantum Phenomena I
Session Chairs
Alexander Govorov
Gregory Salamo
Monday PM, November 26, 2007
Room 310 (Hynes)
9:30 AM - **GG1.1
Optical Properties of Quantum Dots and Quantum Posts.
Pierre Petroff 1 4 , H. Krenner 1 , J. He 1 , C. Pryor 2 , C. Morris 3 , M. Sherwin 3
1 Department of Materials, University of California-Santa Barbara, Santa Barbara, California, United States, 4 Department of Electrical & Computer Engineering, University of California-Santa Barbara, Santa Barbara, California, United States, 2 Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, United States, 3 Department of Physics, University of California-Santa Barbara, Santa Barbara, California, United States
Show AbstractWe discuss the growth and optical properties of InGaAs/GaAs self assembled quantum posts (QPs). The MBE grown QP is formed of a seed quantum dot (QD) connected to a short quantum wire and is capped by another QD. The QP length along the growth direction can be adjusted between 10 and 60nm. We briefly discuss the QP structural and chemical composition. Their optical properties measured by micro-photoluminescence (micro-PL) are compared to an 8-bands strain-dependent k.p model incorporating the detailed structure and alloy composition. The calculations for QPs shorter than 40nm, show full electron delocalization in the quantum wire part of the quantum post and the hole localization in the strain-induced regions at the ends of the QP. By embedding the QPs inside an n-i-p structure, measurements of the bias dependent micro-PL spectra show strongly tunable excitons transitions due to the Quantum Confined Stark effect. In addition, we find anti-crossings, which are consistent with delocalized electron and localized holes states. Thus, QP offers the possibility of controlling the strength of the electric dipole moment in the structure. We have measured dipole moments 40 times larger than those of isolated QDs. This opens up new possibilities for the studies of light matter interactions in the strong coupling regime.
10:00 AM - **GG1.2
Directed Self-Assembly of Coupled Nanostructures and their Behavior.
Gregory Salamo 1
1 Physics, University Arkansas, Fayetteville, Arkansas, United States
Show AbstractNovel and clever techniques for fabricating nanosize materials, one-atomic-layer-at-a-time, have simultaneously opened a door to a fantastic adventure. Nanosize materials simply do not behave as the bulk. Indeed, the rules that govern the growth and behavior of these tiny structures are only now being explored by many researchers with fascinating results throughout the world. In this talk we focus on semiconductor quantum dots that were produced by molecular beam epitaxy via the Stranski-Krastanov or droplet epitaxy growth techniques. In addition, we focus on developing an understanding of the underlying physics that gives rise to the quantum dot electronic structure and dynamical carrier processes both of which are important for device applications. For example, application of semiconductor quantum dots in high-performance lasers, quantum computations, or single electron transistors, implies carrier transfer from a continuum of states into the discrete atomic-like states of quantum dots. Efficiency of such transfer is determined to a great extent by the strength of the coupling between the quantum dot array and the carrier reservoir. The importance of this coupling to a carrier reservoir extends well beyond quantum dots to more complex structures, such as, quantum dot molecules, chains, three dimensional arrays, and even quantum rings. Excitingly, the ability to design and engineer such complex arrays of self-assembled of nanostructures is creating new opportunities to explore the underlying physics of carrier transfer and can potentially lead to novel devices. However, as is most often the case, it would be wise to investigate coupling between a carrier reservoir and quantum dots of a simple system before investigating more complex structures like quantum dot three dimensional arrays. Certainly a quantum well or wetting layer that collects carriers can act as a carrier reservoir from which lateral diffusion results in carrier capture by quantum dots. One might think that a simple example is a coupled quantum well-quantum dot array structure. However, even in this simple system the energy level structure suffers from a clear picture of carrier behavior. For example, quantum dots used in application are normally dense and generally not well separated spatially by long distances or infinite potential barriers. As a result, the electronic wavefunction of adjacent quantum dots can overlap, thus allowing carriers to travel from one quantum dot to another. This carrier transfer, even if slow, can affect carrier dynamics after optical excitation. In this talk we examine the underlying physics of coupling, both laterally and vertically, in semiconductor bilayer structures.
10:30 AM - **GG1.3
Electric Field Tuning of Exciton-biexciton Cascade in a Single Quantum Dot for Entangled Photon Pair Generation.
Marek Korkusinski 1 , Michal Zielinski 1 , Michael Reimer 1 , Robin Williams 1 , Pawel Hawrylak 1
1 Institute for Microstructural Sciences, National Research Council, Ottawa, Ontario, Canada
Show AbstractWe present theory and experiment describing the effect of the lateral electric field on excitonic and bi-excitonic resonances in a single self-assembled quantum dot. In model calculations the electron and hole single particle states of a single quantum dot in an electric field are described exactly by displaced harmonic oscillators. The multi-exciton states are expanded in terms of exact electron-hole configurations for any applied electric field strength. The Coulomb matrix elements for same particles are calculated exactly, and electron-hole Coulomb matrix elements are calculated numerically. The multi-exciton complexes are calculated using configuration interaction method with up to three shells for electrons and for holes. Electron-hole exchange is included and parametrized in terms of zero electric field anisotropic exchange splitting (AES). Results of calculations show that the lateral electric field modifies the attractive Coulomb interactions in exciton and biexciton while keeping the repulsive contribution unchanged. At a critical value of electric field the bi-exciton binding energy vanishes, leading to two indistinguishable recombination pathways for the biexciton. This produces entangled photon pair even in the presence of exciton states split by anisotropic exchange. The theoretical results are successfully compared with microscopic tight-binding calculations and with experiment on a single, site-selected InAs/InP quantum dots emitting in the wavelength range between 1300nm and 1550nm.[1] [1] Michael E. Reimer , Marek Korkusinski, Jacques Lefebvre, Jean Lapointe, Philip Poole, Geof Aers, Dan Dalacu, W. Ross McKinnon, Simon Frédérick, Pawel Hawrylak, Robin L. Williams, submitted to Phys.Rev.Letters.
11:30 AM - GG1.4
Controlling the Optical Properties of Self-Assembled Quantum Dots Using External Strain.
Garnett Bryant 1 , Michal Zielinski 2 3 , W. Jaskolski 3 , Javier Aizpurua 4
1 Atomic Physics Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 Instytut Fizyki, Uniwersytet Mikolaja Kopernika, Torun Poland, 3 Institute of Microstructural Sciences, National Research Council of Canada, Ottawa, Ontario, Canada, 4 , Donostia International Physics Center, San Sebastian Spain
Show AbstractPassive control of the optical properties of self-assembled semiconductor quantum dots is achieved by controlling dot size, shape and composition via growth. Active, dynamical control is needed. In self-assembled quantum dots, the local strain due to lattice mismatch between the quantum dot and barrier materials critically influences the dot electronic properties and optical response. Active control could be achieved via imposed external strain to induce or split level degeneracies, polarize optical transitions, or modify coupling in closely spaced dots, all critical capabilities for the use of dots in quantum information processing. Conversely, it was proposed recently that quantum dots could be used in optical cooling schemes to bring nanomechanical oscillators and cantilevers into the quantum limit for mechanical systems. To understand the coupling between externally imposed strains and the electronic states of the self-assembled quantum dots, we have developed an atomistic tight-binding theory of the confined states in quantum dots that incorporates the local strain due to atomistic lattice mismatch and the externally imposed strain due to applied stressors or to the bend in a vibrating nanomechanical oscillator. Both local strain and the externally imposed strain are important, so we include them on an equal footing via an atomistic valence force field approach, with the externally imposed strain modeled as a distortion of the lattice on the boundary of structure. A full tight-binding model including an sp3s*d5 orbital model and spin-orbit effects is used. To understand how applied stress can be used to actively control dot optical properties, we consider dots buried at different points in a nanobridge oscillator or cantilever. We study the dependence of the quantum dot electronic states and optical transitions on the coupling to bending modes of the oscillator. Both isolated and coupled dots are considered. Ten meV energy shifts of both the electron and hole state states are possible. Redshifts or blueshifts are possible, depending on how the stress is applied to the dot. Level crossings for hole states are found. State symmetries can be significantly distorted by the applied stress. Transitions can be strongly polarized parallel or perpendicular to the external strain or suppressed, depending on how the dots are stressed. The dependence on applied stress can be understood as a competition between the internal and external stress that can either enhance or suppress the local strain at the dot, depending on how the external stress is applied. These results are discussed to show how much active control is feasible.
11:45 AM - **GG1.5
Resonantly Controlled Light Emission Of Quantum Dots In Cavities.
Ken Shih 1
1 Department of Physics, University of Texas at Austin, Austin, Texas, United States
Show Abstract12:15 PM - **GG1.6
Time-resolved Measurements of Single Electron Spin Coherence in a Quantum Dot.
Maiken Mikkelsen 1 , Jesse Berezovsky 1 , Oliver Gywat 1 , Nick Stoltz 1 , Larry Coldren 1 , David Awschalom 1
1 Center for Spintronics and Quantum Computation, University of California, Santa Barbara, California, United States
Show AbstractThe non-destructive detection of a single electron spin in a quantum dot (QD) was recently demonstrated using a time-averaged magneto-optical Kerr rotation measurement [1]. This technique provides a means to directly probe the spin off-resonance, thus minimally disturbing the system. Furthermore, the ability to sequentially initialize, manipulate, and read out the state of a qubit, such as an electron spin in a quantum dot, is necessary for virtually any scheme for quantum information processing. Here, in contrast to previous time-averaged measurements, we have extended the single dot KR technique into the time domain with pulsed pump and probe lasers, allowing the observation of the coherent evolution of an electron spin state [2]. The dot is formed by interface fluctuations of a GaAs quantum well and embedded in a diode structure to allow controllable charging of the QD. To enhance the small single spin signal, the QD is positioned within a vertical optical cavity. Observations of coherent single spin precession in an applied magnetic field allow a direct measurement of the electron g-factor and a transverse spin lifetime of ~10 ns. These measurements reveal information about the relevant spin decoherence mechanisms, while also providing a sensitive probe of the local nuclear spin environment. The results represent progress toward the manipulation and coupling of single spins and photons for quantum information processing as well as quantum non-demolition measurements of a single spin.1. J. Berezovsky, M. H. Mikkelsen, O. Gywat, N. G. Stoltz, L. A. Coldren, and D. D. Awschalom, Science 314, 1916 (2006).2. M. H. Mikkelsen, J. Berezovsky, N. G. Stoltz, L. A. Coldren, and D. D. Awschalom, submitted for publication (2007).
12:45 PM - GG1.7
Optical Properties of Ordered Self-assembled Wires, Mono- and Bi- chains of InAs Quantum Dots.
Emanuele Uccelli 1 , Max Bichler 1 , Gerhard Abstreiter 1 , Anna Fontcuberta i Morral 1
1 Walter Schottky Institut, Technische Universität München, Garching Germany
Show AbstractSemiconductor quantum dots (QDs) have attracted in the past years significant interest worldwide as promising active media for advanced device applications and as systems enabling the investigation of new quantum physics phenomena. For all these studies and applications, controlling the assembly of QDs in a deterministic way is highly desirable. As previously shown, we were able to fabricate long range ordered chains of InAs QDs by combining selective growth with self-assembly. InAs nucleation was realized on a (110) facet consisting of AlAs nanostripes embedded in GaAs, that was obtained by in situ cleaving of a previously MBE grown AlAs/GaAs (001) heterostructure [1].Recently, we were able to “depict” a detailed phase diagram that shows under which growth conditions the formation of InAs QDs chain can be optimized. Moreover, we also obtained new and different nanostructures configurations on the cleaved surface, such as InAs wire array as well as mono- and bi- chain of InAs quantum dots. Indeed, combining high As4-vapor overpressure and very low InAs amount, the geometry of the AlAs stripes pattern acts as the selective factor for the nucleation of InAs wire or QDs chain structures [2,3].Here, we present a detailed investigation of the optical properties of the InAs nanostructures configurations at the cleaved facet. Intense sharp peaks around 1.3 eV at low temperature (10 K) have been registered for all the different growth scenarios (wire, mono- and bi- QDs chains), on samples capped by a thin GaAs layer. As expected, the trend in the excitonic emission for QDs located on AlAs stripe of different thickness confirms the geometrical evolution in the QDs dimensions. Differences in the QDs signals from similar and long studied InAs/GaAs (001) QDs system have been observed. To explain them, detailed Raman investigations have been started and pointed out an high In-Al intermixing in the dots and wire system, that also confirms our previous theoretical assumptions regarding the starting factor for the InAs nucleation only on the AlAs stripes.References:[1] Bauer J. et al., Long-range ordered self-assembled InAs quantum dots epitaxially grown on (110) GaAs, Applied Physics Letters 85, 4750 (2004)[2] Uccelli E. et al., Guided self assembly of InAs quantum dots on a cleaved facet, Proceeding of Materials Research Society 959 M 16.02 (2007)[3] Uccelli E. et al., Growth mechanisms of self assembled InAs quantum dots on (110) AlAs/GaAs cleaved facet, submitted to Superlattices and Microstructures (2007)
GG2: Self-assembled Quantum Dots and Quantum Phenomena II
Session Chairs
Monday PM, November 26, 2007
Room 310 (Hynes)
2:30 PM - **GG2.1
Mode Locking of Electron Spin Coherence in Singly Charged Quantum Dots.
Alexander Efros 1
1 Center for Computational Material Science, NRL, Washington, District of Columbia, United States
Show AbstractThe fast dephasing of electron spins in an ensemble of quantum dots is detrimental for applications in quantum-information processing. We show that dephasing can be overcome by using a periodic train of light pulses to synchronize the phases of the precessing spins, and demonstrate this effect in an ensemble of singly charged (In,Ga)As/GaAs quantum dots [1]. We first discuss the physical mechanism of this synchronization for pulses of different intensity [2, 3]. A periodic train of circularly polarized light pulses from a mode-locked laser synchronizes the precession of the spins to the laser repetition rate, transferring the mode-locking into the spin system. The mode-locking technique allows us to measure the single-spin coherence time to be 3 microseconds [1], which is four orders of magnitude longer than the ensemble dephasing time of 400 picoseconds. The interference also gives the possibility for all-optical coherent manipulation of spin ensembles, in which the electron spins can be clocked by two trains of pump pulses with a fixed temporal delay. After this pulse sequence, the quantum dot ensemble shows multiple echo-like Faraday rotation signals with a period equal to the pump pulse separation.[1] A. Greilich, D. R. Yakovlev, A. Shabaev, Al. L. Efros, I. A. Yugova, R. Oulton, V. Stavarache, D. Reuter, A. Wieck, and M. Bayer, Science 313, 341 (2006).[2] A. Greilich, R. Oulton, E. A. Zhukov, . A. Yugova, D. R. Yakovlev, M. Bayer, A. Shabaev, Al. L. Efros, I. A. Merkulov, V. Stavarache, D. Reuter, and A. Wieck, Phys.Rev. Lett., 96, 227401 (2006).[3] A. Shabaev, Al. L. Efros, D. Gammon, and I. A. Merkulov, Phys. Rev. B 68, 201305(R) (2003).
3:00 PM - GG2.2
Multi-color InAs and InGaAs Quantum Dot Photodetectors for Mid and Long-wavelength Infrared Detection.
Brandon Passmore 1 , J. Wu 1 , O. Manasreh 1 , Vas. Kunets 2 , G. Salamo 2
1 Department of Electrical Engineering and Microelectronics and Photonics, University of Arkansas, Fayetteville, Arkansas, United States, 2 Department of Physics, University of Arkansas, Fayetteville, Arkansas, United States
Show AbstractThe intersubband transitions in self-assembled InAs and In0.3Ga0.7As quantum dots grown by molecular beam epitaxy have been investigated for their use in mid- and long-wavelength infrared detection applications. The materials were characterized using x-ray diffractometry, photoluminescence, electrochemical capacitance-voltage and optical absorption. Devices were fabricated from the multiple quantum dot structures in order to measure the temperature dependence (4 – 300 K) of the photoresponse for normal incident operation. In addition, the dark current and photocurrent was measured in the temperature range of 4 – 300 K for the devices. For the two band infrared photodetector consisting of two stacks of n-type InAs and In0.3Ga0.7As multiple quantum dots, the photoresponse peaks were measured to be 5 – 7 µm and 10 – 14 µm, respectively. However, a broad-band photoresponse from InAs quantum dots embedded in an InGaAs graded well was measured in the spectral range of 3 – 12 µm. The transfer matrix method is used to estimate the peak position energies of the intersubband transitions in the two multi-color quantum dot infrared photodetector structures.
3:15 PM - GG2.3
Coupling Ga Nanoparticles with Semiconductors: The Impact of Charge-transfer Phenomena on the Ga Surface Plasmon Resonance.
Pae Wu 1 , Maria Losurdo 2 , Tong-Ho Kim 1 , Maria Giangregorio 2 , Giovanni Bruno 2 , April Brown 1
1 Electrical and Computer Engineering, Duke University, Durham, North Carolina, United States, 2 , Institute of Inorganic Methodologies and of Plasmas -- CNR, Bari Italy
Show AbstractSurface plasmon resonant metal nanoparticles (NPs) are widely exploited for a variety of applications including biosensors, waveguides, photon emission enhancement, and surface-enhanced Raman scattering. Traditionally Ag and Au, both noble metals, have been employed for plasmonic applications and their formation through chemistries in solution, with a high degree of shape and size control, is well-established. Gallium (Ga) plasmonic nanoparticles have recently gained traction in nonlinear optics and plasmonic applications such as optical switches and plasmonic waveguides. Ga NPs are also advantageous for device applications as they can be deposited in situ in the same apparatus where III-V semiconductor heterostructures are grown. This enables the pristine coupling of Ga NPs, which exhibit a wide plasmon resonance range unlike Au and Ag, with GaAs- and GaN-based photonic devices. We demonstrate the tailoring and exploitation of self-assembled plasmonic Ga NPs supported on both polar (GaN, N-polar, Ga-polar; SiC, Si-polar, C-polar; ZnO, Zn-polar, O-polar) and non-polar (e.g. Si) semiconductors. Novel factors presented and discussed in this contribution are:- splitting and tailoring of two plasmon modes, the longitudinal plasmon and the transverse plasmon that can be tuned from the far UV to the IR range; a correlation between the Ga NPs size and the wavelength of the two modes is established.- Thermal stability of Ga nanoparticles under very extreme temperature ranges (from -90 to 600°C), which enables Ga plasmon applications for harsh environments.- Demonstration of plasmonic ellipsometry employing a phase modulated spectroscopic ellipsometer (UVISEL – Jobin Yvon) that provides information both on the phase and amplitude of the reflected light. This powerful technique allows us to highlight and investigate, in real-time, plasmon tailoring during NP formation and plasmon perturbation phenomena due to interaction with surrounding surfaces and media.- Modification of the Ga plasmon resonance due to charge-transfer mechanisms. We examine the electronic phenomena due to charge transfer between the Ga NPs and the semiconductor surface affecting both the position and amplitude of the plasmon modes and demonstrate the sensitivity of the plasmonic behavior in response to semiconductor polarity and the accompanying semiconductor surface charge. We also investigate the modification of metallic nanoparticle optical behavior induced by charge-perturbation during interaction with electron donor and electron acceptor molecules. Finally, the plasmon-based detection of NO by hemin-functionalized Ga nanoparticles mediated by a charge-transfer mechanism is demonstrated.
3:30 PM - **GG2.4
Optical Spectroscopy of Quantum Dot Molecules.
Dan Gammon 1
1 , Naval Research Laboratory, Washington, District of Columbia, United States
Show AbstractIn this talk I will present our recent experiments on coherently coupled ‘diatomic’ quantum dot molecules. Individual pairs of vertically stacked InAs/GaAs QDs with tunnel barrier thicknesses ranging from 2 – 18 nm are being studied using optical spectroscopy through shadow masks. We find that the molecular spectra, though rich in structure, can be largely understood with a few key conceptual elements, involving coherent tunneling of either an electron or hole, Coulomb interactions, and spin. We have measured exciton and biexciton spectra of neutral and charged dots (both positively charged and negatively charged). Many of the properties that have been intensely studied over the last decade in single dots can now be systematically measured in molecules, including Stark shifts, Zeeman splittings, charged excitons, biexcitons, spin fine structure, and excited states. Each of these properties is greatly enriched by the coherent coupling between the two dots. I will present an overview of several of these results, emphasizing the similarities and differences between electron and hole tunneling.
GG3: Colloidal Nanocrystals: Plasmons, Excitons, and Thermal Effects I
Session Chairs
Monday PM, November 26, 2007
Room 310 (Hynes)
4:30 PM - **GG3.1
Optical Manipulation using Gold Nanoparticle Aggregates.
Jochen Feldmann 1
1 Photonics and Optoelectronics Group, Physics Department and CeNS, Ludwig-Maximilians-Universität (LMU), Munich Germany
Show AbstractAggregates of gold nanoparticles provide unique electromagnetic properties. In so-called "hot spots" intense Raman signals can be generated allowing for a detection of Raman spectra on a single molecule level.In this contribution it is shown that optical excitations of a protein-linked gold nanoparticle dimer not only leads to intense Raman signals but also provokes a change of the inter-nanoparticle distance on the order of 0.5 nm. As a second example gold nanoparticle aggregates linked by double stranded DNA are optically excited by nanosecond laser pulses. Herewith a controlled melting of the double stranded DNA can be achieved. This fast optothermal method allows for single basepair mismatch detection even in the presence of perfectly matching DNA.
5:15 PM - GG3.3
Size-Dependent Energy Transfer Processes in Mn(II)-Doped CdSe.
Remi Beaulac 1 , Paul Archer 1 , G. Salley 1 , Daniel Gamelin 1
1 Chemistry, University of Washington, Seattle, Washington, United States
Show AbstractColloidal Mn(II)-doped II-VI quantum dots are interesting materials for the study of magnetic and luminescent phenomena in quantum confined semiconductor nanostructures. In recent years, several reports have described luminescence, absorption and magnetism of Mn(II)-doped ZnS, CdS and ZnSe quantum dots. In general, the emission properties of these nano-scale materials behave much like their bulk counterparts, showing a size insensitive Mn(II) ligand-field emission with a lifetime of a few microseconds. In contrast, Mn(II)-doped CdSe nanoparticles are expected to behave differently from bulk because of the possibility of size-tuning the band-gap energy from below to above the Mn(II) emitting levels. For this reason, Mn(II)-doped CdSe offers an interesting opportunity for fundamental strudies of quantum confinement effects in doped semiconductors. Curiously, although photoluminescence spectra of self-assembled Mn(II) quantum dots prepared by vacuum deposition have been reported, the Mn(II) is either absent of only tentatively reported in the case of very high Mn(II) concentrations. Moreover, CdSe excitonic emission is observed despite the fact that the energy gap is greater than the Mn(II) excitation energy, in contrast with Mn(II)-doped CdS, ZnS and ZnSe. We recently presented a new method for preparing colloidal doped CdSe quantum dots.[1,2] Importantly, these particules show a giant Zeeman splitting of their excitonic transitions, as is expected for diluted magnetic semiconductor (DMS). The use of electronic absorption and magnetic circular dichroism (MCD) spectroscopies as probes of 3d transition metal dopant speciation in DMS quantum dots will be briefly described. Temperature-dependent photoluminescence of these particules has been measured and gives an insight on the kinetics of the energy transfer processes between the excited states of Mn(II)-CdSe. We propose a kinetic model that explains the paradoxical absence of Mn(II) emission reported before in Mn(II)-doped CdSe. [1] Archer, P. I.; Santangelo, S. A.; Gamelin, D. R., Nano. Lett., 7, 1037-1043 (2007).[2] Archer, P. I.; Santangelo, S. A.; Gamelin, D. R., J. Amer. Chem. Soc., accepted.
5:30 PM - GG3.4
CdSe:Te Nanocrystals: Band-Edge versus Te-Related Emission.
Andrey Rogach 1 , Thomas Franzl 1 , Josef Mueller 1 , Thomas Klar 1 , Jochen Feldmann 1 , Dmitri Talapin 2 3 , Horst Weller 2
1 Department of Physics and Center for Nanoscience (CeNS), Ludwig-Maximilians-Universitaet Muenchen, Munich Germany, 2 Institute of Physical Chemistry, University of Hamburg, Hamburg Germany, 3 The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractStrongly luminescent monodisperse CdSe nanocrystals in which a few Se atoms are substituted with Te atoms (CdSe:Te) provide a model system for studies of both band-edge and trap-related luminescence. Ensemble photoluminescence spectra of CdSe:Te nanocrystals are asymmetrically broadened and red-shifted in comparison to bare CdSe nanoparticles. Single particle luminescence measurements show that the bare CdSe and the CdSe:Te nanocrystals emit at distinctly different wavelengths and differ in line shape and linewidth. Individual CdSe:Te nanocrystals show two kinds of emission spectra, which have been ascribed by us to particles with one Te and with a few Te atoms per nanocrystal. Single particle measurements furthermore show that a single CdSe:Te nanocrystal can emit either from the band-edge states or from trap state(s) created by the Te atom(s), but not from both.
5:45 PM - GG3.5
Exploiting Bimodal Distribution to Enhance Photoluminescence Efficiency of Quantum Dot Arrays.
Krisztian Kohary 1 , Victor Burlakov 1 , David Pettifor 1
1 Materials, Oxford University, Oxford, Oxfordshire, United Kingdom
Show AbstractPhotoluminescence of semiconductor quantum dot arrays is significantly deteriorated by the presence of bad quantum dots quenching optical excitons via non-radiative decay channels. This process is highly facilitated by easy migration of excitons between aggregated quantum dots due to resonant Forster transfer processes. We propose to decrease the role of bad quantum dots by slowing down the exciton migration using a binary mixture of quantum dots, where one type of quantum dots serves as spacer quantum dots (SQDs) between the primary quantum dots (PQDs). We identified the maximum and minimum values of the photoluminescence efficiency for different spacer/primary quantum dot compositions. We found that for a given composition the photoluminescence efficiency is the highest for the random distribution of spacer and primary quantum dots. However, the photoluminescence efficiency dramatically decreases when clusters of SQDs and PQDs are formed. We studied and analyzed this cluster formation of quantum dots and the corresponding photoluminescence of the bimodal quantum dot array using the kinetic Monte Carlo simulation technique. By comparing our simulation results with those obtained experimentally we were able to determine the magnitude of the attractive pair-wise interaction between the quantum dots and explore possible strategies to achieve the highest photoluminescence efficiency of the PQDs in hybrid organic-inorganic materials for photo-electronics. (This research is funded by Hewlett-Packard Laboratories, Palo Alto, USA.)
Symposium Organizers
Alexander O. Govorov Ohio University
Zhiming M. Wang University of Arkansas
Andrey L. Rogach Ludwig-Maximilians-Universtitaet Muenchen
Harry Ruda University of Toronto
Mark Brongersma Stanford University
GG4: Colloidal Nanocrystals: Plasmons, Excitons, and Thermal Effects II
Session Chairs
Hugh Richardson
Andrey Rogach
Tuesday AM, November 27, 2007
Room 310 (Hynes)
10:00 AM - **GG4.1
Characterization of Heat Generation from Single Gold Nanoparticle Complexes.
Hugh Richardson 1
1 Chemistry and Biochemistry, Ohio University, Athens, Ohio, United States
Show AbstractMetal nanoparticles efficiently generate heat in the presence of electromagnetic radiation. This process becomes strongly enhanced under plasmon resonance and also depends on the shape and organization of the nanoparticles. In particular, the amount of heat generated and temperature increase depends on the number of nanoparticles in a complex. Metal nanoparticles can be used to induce a phase transformations when they are in thermal contact with a solid matrix, such as ice, and are optically driven. This effect can be used to quantitatively determine the amount of heat generated from single metal nanoparticle complexes using confocal Raman and photoluminescence microscopy. We use the phase transformation of ice to determine the temperature increase around a gold NP complex. With this, we can measure, not only the optical response of the NPs, but also thermal response. Because the heat generation is dependent upon the mesoscopic character of different clusters of NPs, the gold NPs are first immobilized on a glass substrate and then characterized with single particle spectroscopy. After characterization, the temperature profile around the NP complex during optical excitation is determined by measuring the relative amount of water and ice within the excitation volume. The thermal properties of the optically-excited gold clusters are then established by combining theoretical calculations with experimental results. This approach yields a quantitative measure of the amount of heat generation. Our results show for gold NP complexes in ice that at relatively low laser power (less than 10^5 W/cm2) liquid water encases the photo-excited gold particle and the temperature profile agrees with recent theoretical calculations. But at larger laser powers, a vapor cocoon surrounds the excited particle that shuts down thermal transport from the particle to the surrounding ice matrix, causing superheating of the particle. This phenomenon is also observed for gold NP complexes in liquid water where an insulating vapor cocoon is formed for laser powers in excess of 10^5 W/cm2.
10:30 AM - GG4.2
Plasmon-assisted Nanoscale Thermal Engineering: Principle and Applications.
Linyou Cao 1 2 , Mark Brongersma 1 2
1 Department of Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Geballe Laboratory of Advanced Materials , Stanford University, Stanford, California, United States
Show AbstractThe realization of controlled, nanoscale thermal environments has great fundamental and practical importance. Research in this area is largely driven by a desire to better control and monitor physicochemical or biochemical reactions and to develop thermally-controlled nanoscale devices. Here we present a novel approach by exploiting light-induced surface plasmon excitations in metallic nanostructures to generate localized, nanoscale controllable thermal environments. Theoretical calculations and experimental measurements collectively show highly spatial and temporal control over temperature field can be thereby achieved. This plasmon-assited nanoscale thermal engineering (PANTE) can be easily combined with other patterning techniques to generate pre-specified thermal environment. The potential capability of the PANTE in gaining superior control over chemical reaction or physical phase change are illustrated by its application in patterned local growth of individual semiconductor nanowires and carbon nanotubes and as pump in nanofluidic devices.
10:45 AM - GG4.3
Gold Nanoparticle Protein Conjugates: Study of Pulsed Laser Heating.
Joshua Alper 1 , Andy Wijaya 2 , Lauren DeFlores 3 , Andrei Tokmakoff 3 , Kimberly Hamad-Schifferli 1 4
1 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractIrradiation of Au nanorods with femtosecond laser pulses at the plasmon resonance can be used to heat the particles. We present here a study of how it can be used to selectively control activity of proteins that are conjugated to the nanorods. Au nanorod synthesis and linking to biomolecules is described, along with a biophysical characterization of the nanorod-biomolecule conjugates by electrophoresis, circular dichroism, and optical spectroscopy. Proteins include Ribonuclease S, Ribonuclease A and cytochrome c. The conjugates are irradiated with approximately 1 - 1000 femtosecond laser pulses close to the longitudinal plasmon frequency of the nanorods (λ = 790 nm). We study the effect of irradiation on the biomolecular structure and activity, as well as the resulting change in nanorod morphology. Effects of laser fluence, number of pulses and protein conjugation are explored.
11:30 AM - GG4.4
Tuning Exchange Interaction in Colloidal Nanocrystals.
Stefan Rohrmoser 1 , Andrei Susha 2 , Andrey Rogach 2 , Dmitri Talapin 3 , Horst Weller 4 , Richard Harley 1 , Pavlos Lagoudakis 1
1 School of Physics and Astronomy, University of Southampton, Southampton United Kingdom, 2 Photonics and Optoelectronics Group, Ludwig-Maximilians-Universität München, Munich Germany, 3 Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 4 Institute of Physical Chemistry, Universität Hamburg, Hamburg Germany
Show AbstractCompared to self-assembled quantum dots made by molecular beam epitaxy, colloidal nanocrystals can be produced with controlled size and shape, such as in quantum dots, rods or even tetrapods, while retaining a high optical and electronic stability. Shape control in the synthesis of colloidal nanocrystals offers unprecedented abilities to tune the interaction of solid state quantum structures with the environment, opening up the possibility of performing truly nanoscale manipulations of the optical and electronic properties. Entanglement schemes that utilise the broken degeneracy of the exciton fine structure in quantum dots have been proposed for applications in quantum information processing, where the degeneracy of exciton states is lifted by the structural shape asymmetry. The naturally occurring shape asymmetry of colloidal wurtzite nanocrystals allows for the growth of nanorods with precise control over the aspect ratio of the nanostructure, while advances in synthetic chemistry have made it possible for the growth of a novel class of core/shell nanocrystals that consist of two different materials grown with a strongly asymmetric shape. These nanorods facilitate electron penetration into the elongated CdS shell whereas the hole is confined inside the spherical CdSe core, which is preferentially situated at one end of the wurtzite structure. The ability to tune the aspect ratio of these core/shell nanorods allows us to control the strength of the exchange interaction, a powerful tool for investigating the electronic structure that dominate their optoelectronic properties. Application of strong external magnetic fields reveals a rich exciton fine structure and its dependence on the aspect ratio of the nanorods. Furthermore, we tune the exchange interaction between carriers in a single nanocrystal by modulating the electron-hole wavefunction overlap under an external electric field parallel to the nanorods and at the same time we probe the effect under an external magnetic field. The simultaneous application of external electric and magnetic fields on nanorods of fixed aspect ratio allows us to actively manipulate the exciton fine structure. This opens applications in spin-polarized magneto-electronics, spintronics and quantum computation.
11:45 AM - GG4.5
Self-Assembly and Conductivity of Nanocrystal Solids.
Dmitri Talapin 1 2 , Elena Shevchenko 2 , Maksym Kovalenko 2 3 , Jong-Soo Lee 2
1 Department of Chemistry, University of Chicago, Chicago, Illinois, United States, 2 The Molecular Foundry, LBNL, Berkeley, California, United States, 3 Institute of Semiconductor and Solid State Physics, University Linz, Linz Austria
Show Abstract12:00 PM - GG4.6
Dipolar Interactions Between PbSe Nanocrystals and Their Impact on Synthesis and Self-assembly.
Arjan Houtepen 1 3 , Rolf Koole 1 , Mark Klokkenburg 2 , Ben Erne 2 , Daniel Vanmaekelbergh 1
1 Condensed Matter and Interfaces, Utrecht University, Utrecht Netherlands, 3 Opto-Electronic Materials, Delft University of Technology, Delft Netherlands, 2 Van 't Hoff Laboratory for Physical and Colloid Chemistry, Utrecht University, Utrecht Netherlands
Show AbstractPbSe is one of the most studied nanocrystal materials because of its unique optical properties in the near infrared (NIR). In addition, PbSe quantum dots (QDs) hold great promise for the field of photovoltaics because of the recently observed multiple exciton generation (MEG) in these IV-VI nanocrystals. A controlled and reproducible synthesis of PbSe QDs is crucial to tailor the electronic or optical properties. We show that a small contamination of acetic acid in the synthesis mixture leads to star-shaped QDs, and that a perfect drying of the precursors is crucial for the synthesis of spherical nanocrystals with high monodispersity (< 5%)[1]. By tuning the amount of acetic acid one can tune the size of the star-shaped QDs from 10-150 nm. It is shown that monodisperse star-shaped nanocrystals form hexagonal close packed arrays with full alignment of their crystal planes. The formation of star-shaped nanocrystals can be explained by the presence of a permanent dipole moment in PbSe QDs, which causes an oriented attachment of clusters in the <100> direction. Using Wide Angle Electron Diffraction we have observed a high degree of atomic alignment between PbSe NCs in 2D and 3D self-assembled structures. This alignment can also be explained by a significant dipole moment on the nanocrystals. However, the origin and magnitude of the dipole moment in PbSe QDs is still under debate. To obtain quantitative information on the dipole moment, we have performed Cryo-TEM experiments on colloidal dispersions of PbSe NCs of various shapes. In Situ Cryo-TEM images reveal the spontaneous formation of linear chains of QDs in dispersion. We have performed a quantitative real-space analysis on a single-particle level and show that these chains correspond to one-dimensional dipolar equilibrium structures[2]. The dipolar pair-attraction is around 8 kBT at room temperature and is significantly larger than has been previously assumed.The results are relevant for all experiments on relatively concentrated NC dispersions, since the distribution of chain lengths can influence the response of the system to optical and electrical signals. For example, efficient energy-transfer may occur within clusters. Understanding the chain formation and the distribution of chain lengths is also important for the synthesis of anisotropic nanocrystals such as wires, stars and rings. Finally, the presence of significant amounts of surface charge has a strong effect on the electronic properties of the nanocrystals. The pair interactions we determined correspond to a potential drop of several tens of a Volt over the nanocrystals. This will distort the electronic wave functions and will likely lift the degeneracy of all electronic levels that are not spherically symmetric, leading to e.g. broadening of excitonic features in the optical absorption.1.A.J. Houtepen et al., J. Am. Chem. Soc. 128 (21), p. 6792-6793, 20062.A.J. Houtepen et al., Submitted
12:15 PM - GG4.7
New Materials for Nonlinear Optical Applications: The Nonlinear Refractive Index of PbSe Quantum Dot Suspensions and Thin Films.
Iwan Moreels 1 , Pascal Kockaert 3 , Dries Van Thourhout 2 , Zeger Hens 1
1 Inorganic and physical chemistry, Ghent University, Gent Belgium, 3 Service d'optique et acoustique, Université Libre de Bruxelles, Brussel Belgium, 2 Information Technology, Ghent University, Gent Belgium
Show AbstractColloidal PbSe nanocrystals (Q-PbSe) have a bandgap tunable around 1.3 and 1.55µm. This makes them excellent candidates for NIR optoelectronic applications, such as quantum dot LED’s or lasers (due to a high luminescence), and Mach-Zehnder interferometers (due to a high nonlinear refractive index). Here we present a detailed study on the nonlinear optical properties of Q-PbSe with the Z-scan technique. Different particle sizes between 3 and 6 nm were used. The nonlinear refractive index n2 and nonlinear absorption have been measured as a function of wavelength (1.2-1.75µm), optical intensity and nanocrystal concentration, both in suspension[1], and in a close-packed thin film. Our results demonstrate that n2 shows negative resonances near the optical transitions in Q-PbSe suspensions, with maximal values of -10^{-11}cm^{2}/W for a Q-PbSe concentration of 1µM. These resonances showed saturation behaviour. We attributed the high n2 in suspension to bi-exciton creation within the Q-PbSe. Measurements on close-packed films showed a negative n2 of the order of 10^{-8}cm^{2}/W. This value is 5 orders of magnitude larger than values for Si of AlGaAs at telecom wavelengths. At the first exciton transition, the films showed absorption saturation. These results, combined with the flexible processing of colloidal nanocrystals, suggests Q-PbSe thin films might be a promising material for all optical signal processing based on a high n2 or absorption saturation. [1] Moreels et al. Appl.Phys.Lett. 89, 193106 (2006)
12:30 PM - GG4.8
Carrier Dynamics Coupled Quantum Dot / Quantum Whell Systems.
Patanjali Kambhampati 1 , Manjunatha Dodderi 1 , Samuel Sewall 1 , Ryan Cooney 1 , Eva Dias 1
1 Chemistry, McGill University, Montreal, Quebec, Canada
Show AbstractThe CdSe/ZnS/CdSe core/barrier/shell layered nanostructure forms an electronically coupled quantum system that is a spherical analog to the quantum well superlattice. The CdSe core is spatially and spectrally separated from the CdSe shell by a wide gap ZnS shell. We recently employed steady state measurements to directly show that the two CdSe phases are electronically coupled [1]. Absorption from the shell phase results in emission from the core phase. In this manner, the core brightness is enhanced in a light harvesting manner. This material offers an opportunity to study charge transport in spherical nanoscale materials. Based upon knowledge of exciton relaxation dynamics in quantum dot cores [2], we extend our state-selective femtosecond methods to evaluate relaxation and transport dynamics in this quantum dot / quantum shell system. Here we will present new results on the ultrafast dynamics of radial charge transport in these materials. With a combination of excitonic state selectivity and femtosecond time resolution, we monitor the ultrafast relaxation dynamics of either the core or the shell, having optically excited either phase. The femtosecond pump/probe data on the core/barrier/shell system show dynamics on a distribution of timescales from <100 fs to >100 ps that are dramatically different from the quantum dot cores. These differences are suggestive of ultrafast charge transport between phases. The early time signals show evidence of spatially separated biexcitons. Finally, we present single dot data on the two-color blinking kinetics of these coupled quantum dot quantum shell systems. [1] “Light Harvesting and Carrier Transport in Core/Barrier/Shell Semiconductor Nanocrystals”, E.A. Dias, S.L. Sewall, and P. Kambhampati, J. Phys. Chem. C, 111, 708 (2007).[2] “Unified Picture of Electron and Hole Relaxation Pathways in Semiconductor Quantum Dots”, R.R. Cooney, S.L. Sewall, E.A. Dias, D.M. Sagar, K.E.H. Anderson, and P. Kambhampati, Phys. Rev. B., 78, 245311 (2007).
12:45 PM - GG4.9
Quantum Optics with Colloidal Nanocrystals in Solution.
David Bussian 1 , Anton Malko 1 , Yongfen Chen 1 , Jennifer Hollingsworth 1 , Han Htoon 1 , Victor Klimov 1
1 Center for Integrated Nanotechnologies and Chemistry Division, LANL, Los Alamos, New Mexico, United States
Show AbstractSemiconductor nanocrystal quantum dots (NQDs) have been increasingly utilized in developing technologies such as lasers, light-emitting diodes, and bioimaging. Traditional fluorophores often impose inconvenient limitations because of their narrow excitation spectra, broad emission bandwidth and rapid photobleaching. NQDs offer a competitive edge because of their size-dependent optical properties, high emission quantum yields, and robust surface functionality.Despite gaining popularity as labeling sources in bioimaging, no reliable method has been developed to characterize and identify single nanoparticles in solution, a requirement for efficient labeling at single-cell/single-NQD level. Previous attempts to quantify aggregation have utilized fluorescence correlation spectroscopy (FCS). This approach, while providing details about particle brightness and size, can be strongly affected by nanocrystal blinking and the propensity of NQDs to form small clusters. Here we present our recent results addressing the aggregation problem by combining FCS with photon pair correlation spectroscopy (PPCS). PPCS is a well-known method for determining the number of independent emitters per cluster and is insensitive to fluorescence intermittency. The combination of these two methods together with necessary theoretical treatment allows us to quantify, for the first time, the clustering degree of different nanocrystals in solution.We have simultaneously performed FCS and PPCS measurements and compared the average NQD number per cluster (N) extracted from each of the method. The extent of aggregation in a sample can be straightforwardly determined by the deviation of N between PPCS and FCS measurements. To validate the approach, we first measured dilute Rhodamine 590 solutions that have no clustering and found that the average number N can be determined with better than 5% accuracy. CdSe nanocrystals dispersed in organic solvents such as toluene or hexane have minimal clustering, usually less than 1.1 NQD/cluster and do not show considerable aggregation over time. To the contrary, commercially available water-soluble CdSe NQDs that are typically used for cell labeling show a tendency toward aggregation into small, 2-3 NQD clusters. In addition, the clustering degree of such dots increases significantly over time rendering their use in single-dot labeling somewhat problematic.
GG5: Exciton-plasmon Resonances in Hybrid Structures I
Session Chairs
Tuesday PM, November 27, 2007
Room 310 (Hynes)
2:30 PM - **GG5.1
Photoluminescence Properties of Silicon Nanocrystals near a Metal Thin Film.
Minoru Fujii 1
1 Department of Electrical & Electronic Engineering, Graduate School of Engineering, Kobe University, Kobe Japan
Show AbstractWe demonstrate the enhancement of photoluminescence (PL) from silicon nanocrystals (Si-ncs) by coupling of excitons to surface plasmon polaritons (SPPs) supported by a Au thin film. SPPs excited via excitons in Si-ncs were Bragg-scattered to photons by a 1- or 2-dimensional grating, and strong and directional PL was observed. From the angular dependence of PL spectra, dispersion relations of electromagnetic modes involved in the light emission process were obtained. The overall agreement between experimentally obtained and theoretically calculated dispersion relations confirmed that the strong and directional PL is mediated by SPPs. The PL decay rate of Si-ncs increased by placing a Au thin film on top and the wavelength dependence of the rate enhancement agreed well with that of the calculated SPP excitation rate. This suggests that the observed PL enhancement is due to efficient energy transfer from excitons to SPPs followed by efficient scattering of SPPs to photons.If the distance between Si-ncs and metal surface is increased, SPPs are not excited but the radiative recombination rate is modified via modification of photonic mode density by the existence of the metal surface. The continuous change of the distance resulted in the oscillation of the rate. By comparing the oscillation behavior with calculation, the radiative and non-radiative recombination rates, and also the internal quantum efficiency of excitons in Si-ncs were estimated independently. The relation between the radiative rate and the PL wavelength was on a single curve for all the samples studied. On the other hand, the non-radiative rate depended strongly on samples. Our results provide evidence that in Si-nc assemblies, majority of nanocrystals in samples do not contribute to photoluminescence and a small part of nanocrystals luminesce with high quantum efficiencies, and thus the total quantum efficiency is mainly determined by the number ratio of bright and dark Si-ncs in the assembly. We applied the same technique for silicon dioxide films containing Si-ncs and Er ions and observed similar but more pronounced oscillation of the decay rate of PL from Si-ncs. We show that the oscillation of the rate is reproduced quite well by calculation under the assumption that the energy transfer rate is proportional to the square of the photonic mode density. From the fitting by the model, the energy transfer rate and energy transfer efficiency could be estimated as a function of wavelength. The wavelength dependence of the energy transfer efficiency provides the evidence of resonant energy transfer.
3:00 PM - GG5.2
Exciton-Coupled Surface Plasmon Resonance Biosensor.
Mihail Bora 1 , Kemal Celebi 1 , Marc A. Baldo 2
1 Physics, MIT, Cambridge, Massachusetts, United States, 2 Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts, United States
Show AbstractWe present an exciton-coupled surface plasmon resonance (SPR) biosensor. It combines the sensitivity of conventional SPR sensors with a novel plasmon detection mechanism. Surface plasmons at the aqueous-gold interface are detected directly in the near field by coupling the electric field of the plasmon modes with exciton formation within the adjacent photovoltaic (PV) cell. The splitting of excitons into holes and electrons at the interface between donor and acceptor layers composing the PV cell generates a short circuit photocurrent [1]. Testing the sensor in the Kretschmann geometry demonstrates a strong dependence on the angle of incident light, in good agreement with numerical simulation data. The model predicts a 25% change in photocurrent upon binding of biomolecular species on the gold cathode due to the steep slope of the resonance. We discuss the potential for integration with an organic LED to create a compact near field SPR biosensor. [1] J.K. Mapel, K. Celebi, M. Singh, and M.A. Baldo, “Plasmonic excitation of organic double heterostructure solar cells,” Appl. Phys. Lett., vol. 90, no. 12, 121102:1-3, Mar. 2007.
3:15 PM - GG5.3
Enhanced Light Emission by Exciton-Surface Plasmon Coupling.
Koichi Okamoto 1 2 , Axel Scherer 3 , Yoichi Kawakami 2
1 PRESTO, Japan Science and Technology Agency, Kyoto, Kyoto, Japan, 2 Electronic Science and Engineering, Kyoto University, Kyoto, Kyoto, Japan, 3 Physics, California Institute of Technology, Pasadena, California, United States
Show AbstractSurface plasmons (SPs) offer the unique ability to localize and enhance electromagnetic fields. One interesting application is the construction of super bright light-emitting devices. Solid-state light-emitting devices are expected to eventually replace traditional illumination sources, however, their light-emission efficiencies are still substantially low. Recently, we reported a novel method to enhance this light emission efficiency by using energy coupling between excitons and SPs. We obtained a 16-fold increase in the photoluminescence intensity from InGaN/GaN quantum wells (QWs) when silver layers were deposited 10 nm above the QW.[1] Likewise, we succeeded in dramatic improvements of light emission from CdSe-based quantum dots and several organic light emitters.These enhancements should be attributed to exciton-SP coupling. Excitons can couple to SPs when a metal layer is grown within the near-field of the active layer, and when the bandgap energy is close to the electron vibration energy of SP. This exciton-SP coupling rate should be very fast because the SP has large density of states, and it increases the spontaneous emission rate and internal quantum efficiency (IQE) of emission. The SP energy can be extracted as light by providing roughness or nanostructure in the metal layer. We obtained several evidences to support the exciton-SP coupling mechanism. (1) Enhanced PL intensities decrease exponentially with increasing the distance between an active layer and a metal surface. (2) Obtained wavelength dependence of the PL enhanced ratios is clearly correlated to the dispersion diagrams of the SPs calculated with the dielectric functions. (3) We found that the internal quantum efficiencies were actually increased by measuring the temperature dependence of PL intensities. (4) We also found that the spontaneous emission rates were also dramatically increased by the time-resolved PL measurement.By using this technique, high-efficiency and high-speed light emission is predicted for optically as well as electrically pumped light emitters. This method is very simple and can be applied to various materials that suffer from low emission efficiencies, which include the indirect semiconductors. Other merit is wavelength tunability of enhanced light by controlling SP frequency based on the kinds of metal, surface roughness, or size/geometry of structures. Such tuning is very important to design and fabricate even more efficient devices for a wider spectral range. We fabricate several nano-grating structures on the metal layers and also simulated the localized SP modes by finite difference time domain (FDTD) calculation. The experimental results were well correlated to the calculated results, and we found that the both exciton-SP coupling and light extraction process can be controlled by the nanostructure geometries.[1] K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and A. Scherer: Nature Materials, 3 (2004) 601.
4:00 PM - **GG5.4
Exciton-plasmon Interactions in Metal-semiconductor Hybrid Structures.
Christoph Lienau 1
1 Institut für Physik, Universitaet Oldenburg, Oldenburg Germany
Show AbstractDuring the last years, there has been tremendous progress in applying highly spatially and temporally resolved optical techniques to the spectroscopy of semiconducting and plasmonic nanostructures. This progress is mainly based on combining ultrafast light pulses, providing temporal resolution down to 10 fs, with near-field optical techniques giving all-optical spatial resolution down to 10 nm. The use of ultrafast nano-optical techniques has been instrumental for the coherent control of optical excitations in single and electronically coupled semiconductor quantum dots as well as for imporving our understanding of coherent surface plasmon polariton dynamics in novel plasmonic nanostructures. This progress will briefly be reviewed. New and interesting physical properties arise when excitons and surface plasmon polariton excitations are coherently electronically coupled. Such coupling has been predicted to give rise to, e.g., surface plasmon polariton (SPP) amplification in optically pumped semiconductor gain media or even SPP lasing. So far, however, the optical properties of hybrid semicondcutor/metal nanostructures with coupled exciton/SPP excitations are understood only to a very limited extent. In the final part of this talk, we will report on first experiments exploring the interactions of excitons and surface plasmon polaritions in stacked layers of quantum wells and metallic nanostructures. It will be shown that such hybrid structure have the potential to explore and manipulate exciton-plasmon interactions - opening up interesting possibities for novel optoelectronic devices
4:30 PM - **GG5.5
Nanoscale Assemblies of Nanoparticles and Nanowires with Plasmon-Exciton Hybrid States.
Nicholas Kotov 1
1 Chemical Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractPlasmons and excitons are the two fundamental excitation states in nanotechnology. The resonance between them can be observed in specifically designing nanoscale assemblies from nanoparticles and nanowires [ Lee, et al Nano Lett. 2004, 4, 2323] connected by molecular springs from flexible PEG oligomers [J. Lee, et al Angew. Chemie. Intl. Ed., 2005, 44, 7439-7442] The polymeric linkers afford continuous and dynamic change of conformations in such structures leading to the variations of the distance between the nanoscale colloids reversibly changes depending on conditions or analyte concentration and can be evaluated by fluorescence measurements. Plasmon-exciton interactions result in tremendous enhancement of excitonic emission and for some systems in characteristic wavelength shift. Understanding plasmon-exciton interactions will contribute to the photonics and nanoscale optics and create a knowledge base for key technologies such as sensing [ Lee et al Nature Materials, 2007, 6(4), 291-295], lasing in nanomaterials, energy conversion, and nanoscale electronics.
5:00 PM - GG5.6
Simultaneous Surface Enhanced Raman and Fluorescence Spectroscopy of Single Conjugated Polymer Chains.
Manfred Walter 1 2 , John Lupton 1 2 , Klaus Becker 2 , Jochen Feldmann 2 , Gerald Gaefke 3 , Sigurd Hoeger 3
1 Department of Physics, University of Utah, Salt Lake City, Utah, United States, 2 Photonics and Optoelectronics Group, Physics Department and CeNS, Ludwig-Maximilians-Universität, Munich Germany, 3 Kekulé-Institut für Organische Chemie und Biochemie, Universität Bonn, Bonn Germany
Show Abstract5:15 PM - GG5.7
Single-Walled Carbon Nanotube Networks Decorated with Silver Nanoparticles; a Novel Graded SERS Substrate.
Yi-chieh Chen 1 , Robert Young 1 , Julie Macpherson 2 , Neil Wilson 3
1 Materials Science Centre, University of Manchester, Manchester United Kingdom, 2 Departments of Chemistry, University of Warwick, Coventry United Kingdom, 3 Departments of Physics, University of Warwick, Coventry United Kingdom
Show AbstractWe report on investigations of surface enhanced Raman scattering (SERS) from a nanostructure of single-walled carbon nanotubes (SWNTs) decorated with Ag nanoparticles. Ag is electrochemically deposited on a 2D network of SWNTs, and the nanometer diameter of the SWNTs helps to ‘pin’ the size of the deposited particles in the nanoparticle regime, forming nanoparticles of graded density and size away from the contact electrode. SERS effect of the Ag nanoparticle-SWNT structure by analyzing both the Raman intensity enhancement of SWNT spectra and the Rayleigh scattering spectra of the Ag nanoparticles was correlated with electron microscopy and AFM investigations on the morphology of deposited Ag nanoparticles. A strong correlation between localized surface plasmon resonance wavelength, particle size and density, laser excitation wavelength and the degree of SWNT Raman scattering intensity enhancement is found. This knowledge is used to increase the degree of enhancement by adjusting the nanoparticle size and density. These results demonstrate a tunable approach to control the formation of Ag nanoparticle for higher enhancement of Raman scattering from SWNTs, enabling high sensitivity SERS measurements. We will also present the applications of the nanostructure produced as a generic SERS substrate for the study of a variety of target molecules in air and under solution.
GG6: Poster Session: Colloidal Nanocrystals: Excitons and Plasmons
Session Chairs
Wednesday AM, November 28, 2007
Exhibition Hall D (Hynes)
9:00 PM - GG6.1
White-Light Emission from Core-Shell-Shell-Shell Semiconductor Nanocrystals.
Andrey Rogach 1 , Sameer Sapra 1 , Sergiy Mayilo 1 , Thomas Klar 1 , Jochen Feldmann 1
1 Department of Physics and Center for Nanoscience (CeNS), Ludwig-Maximilians-Universitaet Muenchen, Munich Germany
Show AbstractWe report the generation of white light from onionlike nanocrystals with a CdSe core and three shells comprised of ZnS/CdSe/ZnS. In these nanocystals, it is possible to decouple the light-emitting CdSe core and CdSe shell by introducing a high bandgap ZnS barrier between the two. White light emission with room temperature quantum efficiency as high as 30% is a result of the combination of orange and blue band-edge emission from the CdSe core and the CdSe shell, respectively. Varying synthetic conditions for these nanocrystals enables us to balance the relative strength of the orange and blue components in emission spectrum in order to tune shades of white light. Intra-nanocrystal excitation transfer has been demonstrated using time-resolved fluorescence measurements.
9:00 PM - GG6.10
A Detailed Quantification of the Composition, Surface Chemistry and Extinction Coefficient of Colloidal PbSe Quantum Dots.
Iwan Moreels 1 , David De Muynck 3 , Dirk Poelman 5 , Guy Allan 2 , José Martins 4 , Frank Vanhaecke 3 , Zeger Hens 1
1 Inorganic and physical chemistry, Ghent University, Gent Belgium, 3 Analytical chemistry, Ghent University, Gent Belgium, 5 Solid state physics, Ghent University, Gent Belgium, 2 ISEN, IEMN, Lille France, 4 Organic chemistry, Ghent University, Gent Belgium
Show AbstractColloidal semiconductor nanocrystals or quantum dots are a promising building block in bottom-up nanotechnology. For telecom applications, colloidal PbSe nanocrystals (Q-PbSe) are most suitable, as their optical properties can be tuned over the entire telecom wavelength range. Like other sterically stabilized colloidal particles, Q-PbSe consist of an inorganic, crystalline core surrounded by a monolayer of organic ligands. We have used Inductively Coupled Plasma Mass-Spectrometry (ICP-MS) and Nuclear Magnetic Resonance (NMR) spectroscopy to quantify this picture in the case of PbSe nanocrystals made from lead oleate (Pb-OA) and trioctylphosphine selenium (TOP-Se) precursors. ICP-MS results demonstrate that Q-PbSe are non-stoichiometric. They show a systematic excess of Pb that decreases with increasing particle size. Quantitatively, the Pb excess corresponds to a Pb termination of the nanocrystal surface. With NMR, we could establish that the Q-PbSe ligand shell is almost solely composed of OA, possibly with a few percent of TOP for larger particles. Quantitatively, the surface density of OA ligands (3.9±0.1 nm-2) matches the surface density of excess Pb, suggesting that Q-PbSe have a Pb-OA surface termination. Combining the ICP-MS analysis with UV-Vis-NIR absorbance measurements and particle size measurements by transmission electron microscopy, the molar extinction coefficient of PbSe colloids could be determined. At energies well above the bandgap transition, we find that it scales with the nanocrystal volume. Quantitatively, the resulting size independent absorption coefficient agrees with that of bulk PbSe. At the bandgap, this scaling law no longer holds. To circumvent errors due to differences in size dispersion, we integrate the extinction coefficient for the bandgap transition over energy. The resulting energy integrated extinction coefficient increases only linearly with particle size. From this energy integrated value, the oscillator strength of the bandgap transition can be calculated. We find an approximately linear increase with particle size with values ranging from 8 for 3 nm particles to 20 for 7 nm particles. These values are in close correspondence to the values predicted by tight binding calculations.
9:00 PM - GG6.12
Quantum Dot-Based Energy Transfer to Photodynamic Therapy Agents.
Smita Dayal 1 , Clemens Burda 1
1 chemistry, case western reserve university, Cleveland, Ohio, United States
Show Abstract9:00 PM - GG6.13
Site-Selective Grafting of PbSe Nanocrystals on SOI-based Microcavities.
Andras Pattantyus-Abraham 1 2 , Qiao Haijun 2 , Jeff Young 2 , Jingning Shan 3 , Frank van Veggel 3
1 Electrical and Computer Engineering, University of Toronto, Toronto, Ontario, Canada, 2 Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, Canada, 3 Department of Chemistry, University of Victoria, Victoria, British Columbia, Canada
Show AbstractWe demonstrate a novel method for patterning colloidal PbSe nanocrystals on Si-based photonic crystal microcavities. Oleate-capped PbSe nanocrystals were found to adhere to H-terminated Si surfaces preferentially over oxide and alkyl-terminated Si surfaces. Scanning probe lithography was used to oxidize locally a dodecyl monolayer on the Si surface, followed by aqueous HF treatment to remove the oxide and expose H-terminated Si areas, yielding patterned PbSe nanocrystals on the Si surface after exposure to a nanocrystal solution. Nanocrystals may thus be selectively localized any desired position on the Si microcavity surface.
9:00 PM - GG6.15
Optical and Thermal Properties of Bimetallic Nanoparticle Complexes.
Alyssa Thomas 1 , Hugh Richardson 1 , Alexander Govorov 2 , Michael Carlson 1
1 Chemistry and Biochemistry, Ohio University, Athens, Ohio, United States, 2 Physics and Astronomy, Ohio University, Athens, Ohio, United States
Show Abstract In recent years, much attention has been paid to the optical properties of metal and semiconductor nanoparticles (NPs). In the case of metal NPs, optical excitation of the NPs results in plasmons or collective oscillations of electrons at the NP surface. The plasmon frequency is dependent on the size, shape and composition of the NP and interaction of the plasmons with incident electromagnetic radiation results in strong absorption near the plasmon frequency. Gold (Au) and silver (Ag) NPs have plasmon frequencies in the visible region. By controlling their shape, size and spatial arrangement, metal NPs or bimetallic NP complexes can be spectrally tuned for a plethora of applications including incorporation into biological systems. Metal NPs can be used in molecular biology as drug or gene delivery agents, for the detection of biomolecules with zeptomole sensitivity and biological imaging. Another attractive property for using metal NPs in biomedical applications is heat generation under optical illumination, a process which involves absorption of incident photons and heat transfer to the surrounding matrix. The transfer of heat to the surrounding matrix can have interesting consequences and characterizing the NP surface temperature and the local temperature dynamics are currently critical parameters in developing applications such as thermal cancer therapies. The immense flexibility in surface modification and size/shape control enable the utilization of metal NPs and bimetallic NP complexes in specific targeting of biological molecules, such as cancer cell markers, for sensitive and cost effective diagnostics and therapies. This project will provide new details of heat flow and dynamic temperature fluctuations due to optical excitation of the bimetallic NP complexes and provide a greater understanding of the processes involved and allow for design of complexes with desired photothermal properties.
9:00 PM - GG6.16
The Eigenstate Spectrum of Biexcitons in Semiconductor Quantum Dots.
Samuel Sewall 1 , Ryan Cooney 1 , Manjunatha Dodderi 1 , Eva Dias 1 , Patanjali Kambhampati 1
1 Chemistry, McGill University, Montreal, Quebec, Canada
Show AbstractSemiconductor quantum dots can have a four body bound state called a biexciton. The binding energy of the biexciton is strongly dependent upon the size of the quantum dot by virtue of spatial confinement of the charge carriers. An understanding of biexcitonic processes bears relevance to fundamental many body processes in quantum dots, unraveling carrier relaxation dynamics, multiple exciton generation, optical gain, and utilizing quantum dots as single photon sources for quantum devices. The existence of the ground state of the biexciton has been established for some time. The ground state of the biexciton may be represented by a basis of ground state single excitons. Excited states of the biexciton were theoretically predicted to exist, and were subsequently experimentally detected. The difficulty in observing the ground state biexciton is primarily due to its weak binding energy relative to the inhomogeneous broadening; the lifetime is a relatively long 10 – 100 ps. The difficulty in observing the excited states of the biexciton is that the relevant experimental signature can be dynamically cancelled by state filling as the excited exciton cools. Furthermore, the excited state lifetimes of the excitons which are used to construct the biexciton are very short, ca. 100 – 500 fs [1, 2]. While excited states of the biexciton have been observed to exist, a clear and unambiguous measurement of the binding energies of the excited states of the biexciton has remained elusive. We have recently shown that a mixed time/frequency domain spectroscopic approach can yield state-to-state exciton dynamics [1]. Preliminary work has shown clear observation of the size dependence of the splittings between the first excited state and the ground state of the biexciton [3]. Here, we utilize this approach to yield the spectrum of a biexciton in a semiconductor quantum dot. To the extent that an exciton is akin to a hydrogenic atom, the biexciton can be considered akin to a hydrogenic molecule, with a full eigenstate spectrum. [1] “State-to-state exciton dynamics in semiconductor quantum dots”, S.L. Sewall, R.R. Cooney, K.E.H. Anderson, E.A. Dias and P. Kambhampati, Phys. Rev. B., 74, 235328 (2006).[2] “Unified Picture of Electron and Hole Relaxation Pathways in Semiconductor Quantum Dots”, R.R. Cooney, S.L. Sewall, E.A. Dias, D.M. Sagar, K.E.H. Anderson, and P. Kambhampati, Phys. Rev. B., 78, 245311 (2007).[3] “State-resolved processes in semiconductor quantum dots: biexciton interactions and surface trapping dynamics”, S.L. Sewall, R.R. Cooney, E.A. Dias, K.E.H. Anderson, and P. Kambhampati, Phys. Rev. B., Submitted (2007).
9:00 PM - GG6.17
Quantum Theory of Interacting Metal and Semiconductor Nanocrystals: Interference Effects, Fano Resonances, and Faraday Rotation.
Alexander Govorov 1
1 , Ohio University, Athens, Ohio, United States
Show AbstractThe optical spectra of structures composed of a quantum emitter and metal nanoparticles have peculiar lineshapes due to the exciton-plasmon interaction. At low temperatures, the exciton-plasmon interaction becomes partially coherent and the optical absorption spectra exhibit the Fano-interference effect [1]. At high temperatures and for strong decoherence, a new interference effect appears due to interplay between induced and external electric fields in the system. This interference effect can exists under certain conditions even at room temperature and manifests itself as an asymmetric line or deep minimum in the absorption spectrum. Circular dichroism of chiral molecules and Faraday rotation from semiconductor quantum dots in a magnetic filed can be sensitive tools to observe the room-temperature interference effect. The optical responses of hybrid structures can be understood in terms of local enhancement of electromagnetic fields, exciton-plasmon energy transfer [2], dissipative image currents in the metal component, and interference effects. A single metal nanoparticle can result in a few-fold enhancement. For a plasmon resonator made of a pair of metal nanoparticles, the plasmon enhancement effect may be much stronger. Absorption of light, Faraday rotation in a magnetic field, and CD are calculated for self-assembled and colloidal nanocrystals and for chiral molecules. [1] W. Zhang, A. O. Govorov, G. W. Bryant, Phys. Rev. Lett. 97, 146804 (2006).[2] J. Lee, A. O. Govorov, J. Dulka, and N. A. Kotov, Nano Letters, 4, 2323 (2004).
9:00 PM - GG6.18
Enhanced Spontaneous Emission in a Spherical Nanoparticle with a Silver Nanoshell by the Purcell Effect.
Wallace Choy 1 , Xue-Wen Chen 1 2 , Sailing He 2 , P. Chui 1
1 Department of Electrical & Electronic Engineering, the University of Hong Kong, Hong Kong China, 2 Centre for Optical and Electromagnetic Research and Joint Research Centre of Photonics of the Royal Institute of Technology (Sweden) and Zhejiang University, Zhejiang University, Hangzhou China
Show Abstract9:00 PM - GG6.19
Plasmon-Enhanced Organic Photovoltaic Cells with Metal-Core/Insulator-Shell Nanoparticles using an Improved Aerosol Deposition Method.
Shigeo Fujimori 1 2 , Jung-Yong Lee 1 , Rostam Dinyari 1 , Peter Peumans 1
1 Electrical Engineering, Stanford University, Stanford, California, United States, 2 Electronic & Imaging Materials Res. Labs., Toray Industries, Inc., Otsu, Shiga, Japan
Show AbstractMetal nanostructures can confine and guide electromagnetic energy on a nanometer scale over a wide spectral range that covers a large part of the solar spectrum. Metal nanoparticles can be embedded into the active region of organic photovoltaic cells to improve the power conversion efficiency and to tune the spectral response of the cell provided they are electrically insulated from the active region to prevent exciton quenching. We have recently shown that Au-nanoparticles (nanospheres and nanorods) with an ionic surfactant introduced in the active region of an organic photovoltaic cell using electric-field-assisted aerosol deposition enhanced the power conversion efficiency of these devices by 40%. We have developed an improved aerosol deposition process in which nanoparticle clustering is nearly completely suppressed, resulting in nearly ideal films of finely dispersed metal nanoparticles that can be deposited onto an organic device layer without damage to the organic layer. Essential to this process is the Coulomb explosion of the mist droplets into much finer droplets to prevent metal nanoparticle aggregation. The result is a deposition process in which isolated metal-core/insulator-shell nanoparticles can be deposited on a substrate from the vapor phase. The details and mechanisms of the aerosol deposition process will be described. Using thiol-modified Au-nanospheres that form stable dispersions, we have fabricated metal-organic composite photovoltaic cells in which the metal nanoparticles are located in the active region. Spectrally-resolved photocurrent and absorption efficiencies were carried out and confirm that the presence of the metal nanostructures directly leads to enhanced photocurrents. Our experimental results are in good agreement with optical modeling.
9:00 PM - GG6.2
Controlled Variation of Optical and Electronic Properties of Metal Nanoparticles by Tuning of Molecular Capping Density.
Jaydeep Basu 1 , Mundoor Haridas 1
1 Physics, Indian Institute of Science, Bangalore, Karnataka, India
Show AbstractOptical properties of metal, especially the noble metals, nanostructures has attracted enormous interest over the last decade or so due to their wide ranging current and future applications from biology to electronics. The most prominent features encountered in optical properties of noble metal nanoparticles is the presence of strong absorption in the visible region due to the surface plasmon resonance, generated due to the interaction between the electromagnetic field and the collective oscillations of conduction electrons. It has been well established that the plasmon resonance in these nanoparticles is very sensitive to the shape, size as well as the host matrix in which it is embedded. Recent observation of ferromagnetism and fluorescence in gold nanoparticles has further intensified research in this already fascinating area. It was recognized that a key parameter that controls the optical and electronic parameter of such nanostructures is the nature of their molecular capping and their interface morphology. However, systematic work has only just begun on relating the interface properties of these nanomaterials with their observed optical and electronic properties.In this regard we have made systematic study on the influence of polymeric capping agents, through their interaction with the nanoparticle surface and by variation of chain density on the surface, on the optical properties of gold nanoparticles. Both weakly interacting polymethyl methacrylate (PMMA) and thiol-terminated polystyrene (PS) were used to cap the gold nanoparticles and to control their shape and size. Sizes ranged from 2-10 nm and was estimated using TEM. We also estimated their surface capping density using EDX spectra in TEM as well as from thermogravimetric analysis (TGA). Using UV-Visible spectroscopy we were able to characterize the optical properties of the respective polymer capped nanoparticles in suitable solutions. Apart from the variation of the plasmon resonance both in terms of position and width we found that there is a strong influence of the capping density and interaction on the observed absorbance in the UV region. As is well known, for gold there is a strong influence of the interband absorption on the plasmon resonance. Using Mie theory to calculate the absorbance of these nanoparticles and using the size dependent dielectric function and an appropriate interband contribution, as applicable for bulk gold, we were unable to match the absorption data very well. However, using a recently proposed expression to model the interband dielectric function, analytically, we could vary this contribution and found strong variation (both blue and red shift with respect to bulk gold) in the interband part as a function of both size as well as capping concentration. Our work points to the crucial role of the capping and the interface morphology on optical and electronic properties of noble metal nanoparticles in general and their plasmonic behaviour in particular.
9:00 PM - GG6.20
Patterning and Bio-mediated Assembly of Nanostructures for Surface-Enhanced Raman Scattering and Surface-Plasmon-Enhanced Fluorescence.
Hong Ma 1 , Melvin Zin 1 , Kirsty Leong 1 , Ngo-Yin Wong 1 , Mehmet Sarikaya 1 , Alex Jen 1
1 Materials Science & Engineering, University of Washington, Seattle, Washington, United States
Show Abstract9:00 PM - GG6.21
Efficient Exciton Energy Transfer in Cross-Linked CdTe QDs in Dispersion.
Rolf Koole 1 , Bob Luigjes 1 , Rene Pool 1 , Masanori Tachiya 2 , Thijs Vlugt 1 , Celso de Mello Donega 1 , Andries Meijerink 1 , Daniel Vanmaekelbergh 1
1 Condensed Matter & Interfaces, Debye Institue, Utrecht University, Utrecht Netherlands, 2 , National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan
Show Abstract9:00 PM - GG6.22
Optical Transitions of PbSe QDs at Different Points in the Brillouin Zone.
Rolf Koole 1 , Arjan Houtepen 1 , Guy Allan 2 , Christophe Delerue 2 , Andries Meijerink 1 , Daniel Vanmaekelbergh 1
1 Condensed Matter & Interfaces, Debye Institute, Utrecht University, Utrecht Netherlands, 2 ISEN, Institut d’Electronique, de Microélectronique et de Nanotechnologie, Lille France
Show Abstract9:00 PM - GG6.3
The Influence of Anions on the Polyol Synthesis of Silver Nanostructures.
Sara Skrabalak 1 , Younan Xia 1
1 , University of Washington, Seattle, Washington, United States
Show AbstractIt is well-established that the shape and size of a metal nanoparticle can influence its chemical, physical, optical, and catalytic properties. Consequently, much effort has been extended to the development of high-yield, shape-controlled syntheses of various metal nanostructures. Here, we highlight both recent advances in the controlled synthesis of silver nanostructures (i.e., wires, cubes, etc.) prepared by the polyol reduction of AgNO3 and the role of various anions in controlling the shape of products. In a typical synthesis, separate solutions of poly (vinyl pyrrolidone) (PVP) and silver nitrate are injected into a vial containing heated ethylene glycol and various anions (e.g., S2-, HSO4-, SO42-). The PVP serves as a stabilizer against aggregation, a reductant, and a selective capping agent. Scanning electron microscopy was used to evaluate product morphology and the influence of the various anionic additives. By using a combinatorial synthetic approach, we have been able to quickly survey and evaluate a variety of anion additives on their potential to control the final product morphology.
9:00 PM - GG6.4
Nanodot Coupler Using Near-field Energy Transfer Among the Resonant Energy Levels of CdSe Quantum Dots.
Takashi Yatsui 1 , Wataru Nomura 2 , Tadashi Kawazoe 2 , Motoichi Ohtsu 2 1
1 SORST, JST, Bunkyo-ku Japan, 2 Department of Electronics Engineering, University of Tokyo, Bunkyo-ku Japan
Show AbstractAn optical far-/near-field converter is required to drive nanophotonic devices. We propose a nanodot coupler (NDC) using semiconductor quantum dots (QDs). This has the unique property of unidirectional energy transmission by controlling the resonant energy transfer, which should achieve greater energy transmission efficiency, without reflection in the device. Time-resolved spectroscopy of the NDC using CdSe revealed the strong energy transfer of CdSe QDs near larger QDs with resonant energy levels. The NDC consists of small (SQD) and large (LQD) QDs, in which the ground exciton level in SQD (ES1) and the first excited level in LQD (EL2) resonate. They are aligned closely, and LQD is used as the output terminal to couple with the nanophotonic device. An optical near field transfers among resonant energy levels along the SQD chain, and is transferred to EL2 in LQD and consequently relaxes to the ground energy level of LQD (EL1). Consequently, the reflection of the optical energy to SQD is negligible, and output energy can be obtained from LQD. We used CdSe/ZnS QDs with respective diameters, D, of SQD and LQD of 2.8 and 4.1 nm, so that the ground exciton level in SQD and the first excited level in LQD resonate. Solutions containing SQD and LQD were dropped separately on mica substrate, where the areas A, B, and C consisted of SQD, SQD and LQD, and LQD, respectively. We performed micro-photoluminescence (μ-PL) spectroscopy. Temperature-dependent μ-PL spectra were obtained with λ=306 nm and a 2-ps pulse width. From area A (C), we obtained a single PL peak at λ=540 nm (600 nm), taken from room temperature to 30K. While, from area B, on decreasing the temperature, the PL intensity at λ=540nm decreased, while the PL intensity at λ=600nm constant. Since the exciton decay time increases on decreasing the temperature, the strong near-field interaction couples the resonant energy level. Therefore, the extinction of the emission PL peak from the SQD originated from the energy transfer from SQD to LQD. Next, we performed time-resolved spectroscopy. We calculated the temporal evolution of the PL intensity based on multiple-exponential decay. The evaluated τ1 of AS (75 ps) is in good agreement with the literature. As the dot size increased, τ2 of BL was smaller than that of AS, while τ1 of BL (154 ps) was larger than that of AS. Since the decay time decreases on increasing D in proportion to 1/D3, the increased decay time of BL was estimated as 130 ps (=154–75×(2.8/4.1)3). Furthermore, although both AS and BS were obtained from SQD, the τ1 of BS was smaller than that of AS. These results originated from the energy transfer from SQD to LQD and the resultant dissipation in LQD. The estimated value of the increased decay time of 130 ps is considered the energy transfer time from SQD to LQD, as the estimated value is comparable to that of CuCl. Our results constitute a promising step toward designing an NDC to drive nanophotonic devices.
9:00 PM - GG6.9
A Revised Variable-range Hopping Model Explains the Peculiar T-dependence of Conductivity in Assemblies of Nanocrystals.
Arjan Houtepen 1 3 , Daan Kockmann 2 , Daniel Vanmaekelbergh 1
1 Chemistry, Utrecht University, Utrecht Netherlands, 3 Opto-electronic materials, Delft University of Technology, Delft Netherlands, 2 MESA+, University of Twente, Enschede Netherlands
Show Abstract
Symposium Organizers
Alexander O. Govorov Ohio University
Zhiming M. Wang University of Arkansas
Andrey L. Rogach Ludwig-Maximilians-Universtitaet Muenchen
Harry Ruda University of Toronto
Mark Brongersma Stanford University
GG7: Exciton-plasmon Resonances in Hybrid Structures II
Session Chairs
Wednesday AM, November 28, 2007
Room 310 (Hynes)
9:30 AM - **GG7.1
Plexcitonic Nanoparticle Complexes and Assemblies.
Naomi Halas 1 2 , Nche Fofang 1 2 , Joseph Slocik 3 , Felicia Tam 2 , Oara Neumann 2 , Rajesh Naik 3
1 ECE, Rice University, Houston, Texas, United States, 2 Laboratory for Nanophotonics, Rice University, Houston, Texas, United States, 3 Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright Patterson Air Force Base, Ohio, United States
Show AbstractCombining chemically fabricated plasmonic nanoparticles with excitonic materials of reduced dimensionalities, in a manner that facilitates strong coupling between certain plasmon resonances and excitonic states, is an important materials challenge. A composite nanomaterial consisting of a tunable plasmonic nanoparticle coated with a molecular layer of J-aggregates in a stable complex supports strong and controllable coupling between the plasmonic nanoparticle core and the excitons in the J-aggregate adlayer. A novel active coupling between plasmonic and excitonic nanoparticles (semiconductor quantum dots), mediated by coiled-coil peptide tether molecules, exhibits an optical modulation of the excitonic fluorescence when illuminated at the plasmon resonance of the metallic nanoparticle. These simple architectures offer new opportunities to design and alter the innate plasmonic and excitonic properties of materials, establishing new properties not achievable by either the plasmonic or excitonic constituents alone.
10:00 AM - **GG7.2
CdSe Quantum Dots in Single Plasmonic Nanocavities.
Ulrike Woggon 1 , Yuri Fedutik 1 , Vasily V. Temnov 1 , Oliver Schoeps 1 , Mikhail V. Artemyev 2
1 Physics, University Dortmund, Dortmund Germany, 2 , Insitute for Physico-Chemical Problems of Belarussian State University, Minsk Belarus
Show AbstractSurface plasmon-coupled excitations of light and electrons at a metal surface allow the localization of light to subwavelength volumes. Recent advances in plasmonics have demonstrated surface plasmon waveguiding and concepts of using plasmons in cavity quantum electrodynamics (cavity QED) are proposed. While silver wires or metallic nanoshells are discussed as examples for plasmonic nanocavities, optically active plasmonic cavities, i.e. nanocavities which are functionalized with nanoscale light emitters, are less studied. We report about a metal-isolator-semiconductor multishell system which unifies the local effect of enhanced spontaneous emission due to a modified density of states near metallic nanostructures with the excitation of surface plasmons (SP) and their propagation in a 1D-nanowire resonator. Such a prototype for an active plasmonic nanocavity allows us to operate and control exciton-plasmon-photon conversion as well as guiding of electromagnetic waves on a nanoscale. The composite nanosystem we present here as an example for plasmonic nanocavities consists of a wet-chemically grown Ag-wire core (100 to 200 nm in diameter, length between 4 and 40 μm) which is covered by a SiO2 shell of different, well-defined thickness d followed by an outer shell of homogeneously distributed, highly luminescent CdSe nanocrystals (NCs). The silver-nanowire cavity is functionalized with CdSe nanocrystals and optimized towards cavity quantum electrodynamics by varying the nanocrystal-nanowire distance d and cavity length L. Efficient exciton-plasmon-photon conversion and guiding is demonstrated along with a modification in the spontaneous emission rate of the coupled exciton-plasmon system. For the optimum QD-Ag wire distance of about 15 nm, the CdSe QD spontaneous emission rate is enhanced by a factor of ~2, both in absolute emission intensity and radiative decay time. From the modulation of the nanocrystal emission by the cavity modes a plasmon group velocity of 0.5c is derived. The plasmon propagation length LSP is measured for different nanocrystal emission wavelengths and yields 7 μm for 545 nm, 11 μm for 595 nm, 12 μm for 605 nm, 13 μm for 635nm and 30 μm for 809 nm. The obtained spectral dependence of LSP is in good qualitative agreement with the reported data about plasmon propagation on silver surfaces. Despite the very low and far from being optimized quality factor of that plasmonic nanocavity, the system presented here is an interesting candidate to explore quantum optical phenomena such as collective spontaneous emission into a single plasmon mode (superradiance) and photon statistics and temporal coherence properties of the emission in weakly coupled exciton-plasmon systems.
10:30 AM - GG7.3
Enhancement of Dye Fluorescence by Silver-copper Nanoparticles.
Sanchari Chowdhury 1 , Makoto Hirai 2 , Venkat Bhethanabotla 1 , Ashok Kumar 3 2
1 Department of Chemical Engineering, University of South Florida, Tampa, Florida, United States, 2 Nanomaterials and Nanomanufacturing Research Center, University of South Florida, Tampa, Florida, United States, 3 Department of Mechanical Engineering, University of South Florida, Tampa, Florida, United States
Show AbstractNumerous devices based on modified spontaneous emission have been demonstrated in recent years, because the enhancement of molecular fluorescence is of great interest due to the wide popularity of molecular-fluorescence-based measurements and devices. The increase in emission of fluorophores would greatly enhance the effectiveness of the applications. Metallic nanoparticles, which drastically alter the emission of vicinal fluorophores, have been utilized to achieve plasmonic enhancement of molecular fluorescence. Since the peak absorption attributed to surface plasmon resonance (SPR) of silver-copper (Ag-Cu) nanoparticles was tuned in the range of 437 to 763 nm by changing annealing temperature only, this investigation has been focused on the fluorescence enhancement of platinum octaethylporphyrin (PtOEP) conjugated to the nanoparticles. From the results of high-resolution transmission electron microscope (HRTEM), the Ag-Cu nanoparticles having the average diameter of approximately 15 nm were solid solution. The SPR wavelength tuning can be owing to the precipitation of Cu atoms induced by annealing. Moreover, PtOEP conjugated to the Ag nanoparticles possessed larger fluorescence emission intensity than the sample with no nanoparticles. The fluorescence enhancement resulted from the overlap with the emission wavelength of PtOEP and the absorption wavelength of the Ag nanoparticles. The change in fluorescence induced by the Ag-Cu nanoparticles will be also introduced in this talk.
10:45 AM - GG7.4
Near-field Optical Sspectroscopy of Resonant Surface Plasmon Coupled to Excitons in GaN/InGaN Quantum Wells.
Arup Neogi 1 , Tadashi Kawazoe 2 , Motoichi Ohtsu 2
1 , University of North Texas, Denton, Texas, United States, 2 , University of Tokyo, Tokyo Japan
Show AbstractExcitation of surface plasmons coupled to excitons in semiconductor quantum wells or quantum dots offer the potential to enhance the efficiency of light emitting devices, ultrafast all-optical modulators/switches, or realize novel sensing devices [1-3]. An appropriate combination of metal and dielectric such as Au on GaAs [4] or silver on GaN semiconductor system [1] can be designed to generate surface plasmons resonant to the exciton emission from quantum confined structure in the infrared or UV-visible wavelength regime respectively. In material system with surface plasmons resonantly coupled to excitons, the spontaneous emission from the quantum wells decay via the surface plasmon polariton modes at a very high rate (~ ps in semiconductor quantum wells). The enhancement in the radiative emission rate (or the Purcell Enhancement) in Ag/InGaN quantum well is observed to be over two orders in magnitude compared to uncoupled system resulting in enhanced light emission or faster modulation speed in optoelectronic devices. In the present work we present the direct excitation and observation of surface plasmon polariton emission from silver coated GaN/InGaN quantum well in the strong coupling regime. Theoretical estimation of surface-plasmon polariton dispersion reveals that bulk plasmon of silver (~ 3.76 eV) is reduced to 2.85 eV at the surface of GaN substrate due to resonant surface plasmons energy condition e'Ag(w) + e 'GaN(w) = 0, where e'Ag(w) and e'GaN(w) is the real part of dielectric constant of silver and GaN respectively [5]. However as the surface plasmon decay length in metal or in dielectric is rather short (4 - 40 nm in the visible wavelength range) and direct observation of the surface plasmon polariton mode in the far-field limit is not possible. Using near-field optical scanning spectroscope with a 50 nm probe and a spectral resolution of 0.05 nm, surface plasmons were excited at the interface of 8 nm Ag film on an InGaN/GaN quantum well emitting at 410 nm close to the surface plasmon energy. Near-field spectrum measured at various distance from the sample surface varying from 2.0 micron to 5 nm exhibit oscillatory luminescence feature. Near-field spectrum shows that luminescence from the quantum well reduces and the surface plasmon polariton emission from the silver layer is enhanced as tip approaches the metal coated surface. Surface plasmon polariton modes coupled to the excitons are observed at 3.14 eV. Time resolved photoluminescence in the near-field limit has been performed to investigate the energy transfer effect.[1] A. Neogi et al, Optics Lett. 30, 93 (2005)[2] A. Scherer, Nature Materials, 1, 73 (2005)[3] S. Bozevolnyi, et al, Nature, 440, 4594 (2006)[4] N. Sawaki et al, Appl,. Phys. Lett. (1999)[5] I. Gontijo, et al, Phys. Rev. B., 72, 10879 (1999)
GG8: Plasmonics: Antennas I
Session Chairs
Wednesday PM, November 28, 2007
Room 310 (Hynes)
11:30 AM - **GG8.1
Plasmonic Laser Antennas from the Near-ir to the Mid-ir: Physics and Applications.
Federico Capasso 1
1 School of Engineering and Applied Sciences , Harvard University, Cambridge, Massachusetts, United States
Show AbstractThis talk reviews our results on a class of new surface plasmon devices that consist of a resonant optical antenna integrated on to the facet of a semiconductor laser, termed plasmonic laser antennas. These devices allow intense and spatially confined optical fields to be generated in the near-field zone. They can be implemented in a wide variety of semiconductor lasers emitting in spectral regions ranging from the visible to the far-infrared. They are potentially useful in a broad range of applications including near-field optical microscopes, optical data storage, spatially resolved chemical imaging and spectroscopy.Optical antennas are single or coupled metallic nanoparticles in which optical excitation of surface plasmons can produce very high intensities in the optical near field due to the high curvature of the metal surfaces. The field enhancement is maximized when the wavelength is suitably matched to the size of the nanoparticle (resonant optical antenna). Physically this enhancement is due to the charges accumulating on both sides of the gap, analogous to a capacitor, thus generating an enhanced electric field in the near field zone. In this talk we will present the design, fabrication and measurements of both near-infrared ( 0.8 microns) antennas built on the facet of commercial diode lasers [1] and of mid-infrared ones (5.4 and 7.7 microns) fabricated on the facet of Quantum Cascade Lasers (QCLs). The optical properties (measurements and simulation of reflection spectra) of arrays of gold coupled resonant antennas on glass will also be described and sensor applications of such arrays fabricated on the facet of an optical fiber for surface enhanced Raman scattering will be discussed [2]. 1. E. Cubukcu, E. A. Kort, K. B. Crozier and F. Capasso, Appl. Phys. Lett. 89, 093120 (2006)2. E. J. Smythe, E.Cubukcu, and F. Capasso,OPTICS EXPRESS 15, 7447(2007) *Collaborators: Ert Cubukcu, Nanfang Yu, Elizabeth Smythe, Eric A. Kort, Kenneth B. Crozier
12:00 PM - GG8.2
Fabrication and Optical Properties of Cross-striped Ordered Arrays of Au Nanoparticles in Anodic Porous Alumina.
Toshiaki Kondo 1 2 , Kazuyuki Nishio 1 3 , Hideki Masuda 1 3
1 , Tokyo Metropolitan University, Tokyo Japan, 2 , Japan Society for the Promotion of Science, Tokyo Japan, 3 , Kanagawa Academy of Science and Technology, Kanagawa Japan
Show Abstract12:15 PM - GG8.3
Infrared Antennas for Near-field Microscopy.
Thomas Taubner 1 , Jon Schuller 1 , Mark Brongersma 1
1 Material Science, Stanford University, Stanford , California, United States
Show AbstractInfrared near-field optical microscopy promises to become a reliable method for material research: It is capable of acquiring spectroscopic information on the samples chemical (1), structural (2,3) and electronic (4) properties at nanoscale resolution. Because the method relies on light scattering from a sharp metallic tip, its resolution is not limited by the wavelength of the illuminating light (5), but rather by the tips properties like radius of curvature or material. While near-field probes have been fabricated to become resonant at visible freqencies -being called optical antennas- (6), no such effort has been done for the operation at mid-infrared frequencies yet. We show that metallic AFM tips can act as infrared antennas and resonantly convert electromagnetic radiation into highly localized near-fields, enabling high-resolution near-field microscopy. The light scattering of differently shaped metallic AFM probes is measured by means of far-field infrared spectroscopy, whereas their near-field enhancement is probed in a scattering-type infrared near-field optical microscope. Polarization behavior, different shapes and the possibility of tuning the resonances by Focussed-Ion Beam (FIB) modification are addressed.1.T. Taubner, R. Hillenbrand, F. Keilmann, Applied Physics Letters 85, 5064 (2004).2.R. Hillenbrand, T. Taubner, F. Keilmann, Nature 418, 159 (2002).3.A. Huber et al., Nano Letters 7, 774 (2006).4.B. Knoll, F. Keilmann, Applied Physics Letters 77, 3980 (2000).5.T. Taubner, R. Hillenbrand, F. Keilmann, Journal of Microscopy 210, 311 (2003).6.J. N. Farahani et al., Phys. Rev. Lett. 95, 017402 (2005).
12:30 PM - GG8.4
Plasmon Resonances of Aluminum Nanoparticles and Nanorods.
Yasin Ekinci 1 , Harun Solak 2 , Joerg Loeffler 1
1 Department of Materials, ETH Zurich, Zurich Switzerland, 2 Laboratory for Micro- and Nanotechnology, Paul Scherrer Institute, Villigen Switzerland
Show AbstractMetallic nanoparticles strongly scatter and absorb light at certain wavelengths due to the resonant excitation of charge density oscillations in nanoparticles known as localized particle plasmons [1]. Although plasmonic properties of metallic nanoparticles, in particular Au and Ag, have been investigated extensively, the literature on Al nanoparticles is surprisingly limited. Al is an excellent material for optical applications, showing low absorption down to a wavelength of 200 nm due to its free-electron-like character and high bulk-plasmon frequency [2]. However, localized particle plasmonic resonances of Al nanoparticles have not yet been demonstrated explicitly and experimentally. In this paper we characterize the plasmon modes of both Al nanoparticles of different sizes and nanorods with various aspect ratios. We demonstrate that Al nanoparticles exhibit sharp particle plasmon resonances, and discuss the experimental results using Mie’s theory, dipolar approximation and FDTD (finite-difference time-domain) calculations. We fabricated Al nanoparticle arrays on quartz substrates using EUV (Extreme Ultra Violet) interference lithography, physical vapor deposition, and lift-off. The smallest features were nanoparticles with diameters of 40 nm and periods of 90 nm. These show sharp and distinct plasmon resonances at wavelengths down to 260 nm. Our work demonstrates that plasmonic materials are available in the UV region. Extending plasmonics into the UV region will open up new research areas and may enable new applications such as SERS (surface enhanced Raman spectroscopy) in this spectral region. References:[1] C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles, Wiley, New York, 1983.[2] D. Y. Smith, “Optical properties of metallic aluminum,” in Handbook of Optical Constants of Solids, E. D. Palik, (ed.), Academic, Orlando, 1985.
12:45 PM - GG8.5
Interaction of Cylindrical and Spherical Nanoparticles with Focused and Collimated Light.
Ibrahim Kursat Sendur 1 , William Challener 2
1 , Sabanci University, Istanbul Turkey, 2 , Seagate Technology Research Center, Pittsburgh, Pennsylvania, United States
Show AbstractInteraction of focused light with spherical particles has been of interest for various applications, such as optical levitation, particle sizing, laser fusion, Raman scattering diagnostics, and laser beam/aerosol cloud penetration. There has been extensive literature on the calculation and measurement of optical trapping forces with micro-sized particles, since optical tweezers allow manipulation of small spherical particles without any mechanical contact. In this context, the interaction of plane waves and Gaussian beams with spheres has been thoroughly studied. Although the Gaussian beam approximation gives a good model for a focused beam of light, a more accurate representation for high NA beams can be developed based on Richards-Wolf vector field equations. There is a need for analytical and numerical calculations to investigate interactions of spherical particles with high NA beams defined by Richards-Wolf vector field equations.Although the interaction of collimated light with nanoparticles is well studied, a better understanding of the interaction of focused light with nanoparticles is necessary. This is particularly important for metallic nanoparticles due to their ability to support surface plasmons at optical frequencies. Our studies indicate that as the half-angle of the beam, a parameter that determines the size of the focused spot, is increased the electric field distribution deviates drastically from the plane-wave solution. Collimated or focused light can be used to excite surface plasmons on metallic nano-antennas. Collimated light has a single component in the k-spectrum. Focused light, on the other hand, has a wide k-spectrum distribution. For metallic nano-antennas, the interaction of surface plasmons and the optimum geometries to excite surface plasmons is different for collimated and focused light.In this study, we first compare analytical and finite element methods for solving scattering problems involving metallic spheres and cylinders illuminated with focused light defined by Richards-Wolf vector field equations. Both metallic and dielectric nanoparticles are investigated for linear and radial polarized incident electric fields. The effects of wavelength, incidence angle, dielectric index of the surrounding medium and the particle size and composition on the surface plasmon resonances of metallic particles are well known for incident collimated light. For focused light, the half beam angle is an additional factor that impacts the electric field distribution of the particles. The effects of various parameters are investigated using the finite element method for spherical and cylindrical metallic nanoparticles. Near-field radiation from metallic nano-antennas will be discussed for focused light. Effect of various parameters, including k-spectrum, will be discussed.
GG9: Plasmonics: Antennas II
Session Chairs
Manfred Bayer
Nicholas Kotov
Wednesday PM, November 28, 2007
Room 310 (Hynes)
2:30 PM - **GG9.1
Metal Nanoparticle Plasmonics: Towards Quantum Nanoantennas.
Garnett Bryant 1 , Emily Townsend 1 2 , Ryan Artuso 2 1
1 Atomic Physics Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 Department of Physics, University of Maryland, College Park, Maryland, United States
Show AbstractUnderstanding the nanooptics of metallic nanoparticles is critical for applications in nanometrology, nanosensors, nanoantennas, and nanooptical communication where large optical response is needed. This understanding becomes even more essential for hybrid structures, that combine semiconductor quantum dots with metallic nanoparticles, where quantum coherent nanooptics is desired. The response of metallic nanoparticles is provided by surface plasmons. We first describe the plasmonic excitations of single and coupled metallic nanoparticles based on classical calculations of their electromagnetic response. Comparison with experiment shows that classical theory provides an excellent model. For isolated particles, the response is dipole-like, similar to a classical radio antenna. The nanoparticle plasmon resonance scales with the particle length. However, in contrast to a classical antenna, the plasmon half wavelength is much larger than the nanoparticle size. In coupled systems, interaction across gaps distorts intraparticle dipolar response, localizes charge at the gaps, significantly redshifts the response and dramatically increases near fields. These effects become singular for nearly touching nanoparticles, drastically influencing the interparticle coupling. For nearly touching particles and for small nanoparticles, quantum effects, such as interparticle tunneling, surface scattering and state filling, play an increasingly important role in determining plasmonic response. Quantum effects will also play a key role in strongly coupled hybrid structures of metallic nanoparticles and quantum particles, such as semiconductor dots. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) are employed to study strongly coupled systems. DFT is used to determine how interparticle tunneling and intraparticle charging saturates the singular response of nearly touching particles. TDDFT is used to determine the quantum excitations of strongly coupled hybrid structures. Results from these DFT approaches will be discussed. Hybrid structures with multiple quantum dots coupled via metal nanoparticles have also been studied with the quantum dots treated quantum mechanically, via a density matrix approach, coupled by dipole interaction to classical metal nanoparticles. The exciton response in the quantum dot is broadened and shifted due to incoherent and coherent interactions with the metal nanoparticles. Incoherent energy transfer between semiconductor dots in hybrid structures is determined both with and without coupling to metallic nanoparticles. The quantum concurrence of entangled states between different dots is determined to understand how disentanglement is influenced by the exciton-plasmon interaction. These results will be discussed to assess prospects for quantum coherent communication and metrology.
3:00 PM - GG9.2
High-resolution Cathodoluminescence Imaging of Plasmonic Modes in Metal Nanowires.
Ernst Jan Vesseur 1 , Rene deWaele 1 , Martin Kuttge 1 , Albert Polman 1 , Henri Lezec 2 , Luke Sweatlock 2 , Harry Atwater 2 , Woo Lee 3 , Kornelius Nielsch 3 , Ulrich Goesele 3
1 , FOM-Institute AMOLF, Amsterdam Netherlands, 2 Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California, United States, 3 , Max-Planck-Institute of Microstructure Physics, Halle Germany
Show AbstractMetal nanowires are promising future components of integrated optical circuits, serving as sub-wavelength interconnects supporting surface plasmon polariton propagation. Optical resonances in metal nanowires with finite length have an organ pipe-like mode structure, caused by reflection of surface plasmons at the wire ends. We use cathodoluminescence imaging spectroscopy to study both the spatial and spectral characteristics of metal nanowire modes in more detail. Metal nanowires with lengths in the range 300-2000 nm, and widths in the range 30-150 nm were fabricated by using different methods, namely (1) electron-beam lithography followed by evaporation of polycrystalline Au and lift-off, (2) focused ion beam patterning of a single-crystal Au substrate, or (3) electrochemical growth inside a porous alumina membrane.Cathodoluminescence (CL) imaging spectroscopy involves excitation of resonances in a sample using a high energy electron beam. We use a 30 keV electron beam (waist < 10 nm) inside the chamber of a scanning electron microscope. Using a parabolic mirror, we collect light that is subsequently emitted. The electron beam is scanned across the sample, and for every electron beam position the CL emission is recorded and spectrally resolved by a spectrometer equipped with a CCD array. CL thus provides a measure for the excitation efficiency of modes in the sample as a function of electron beam position.We performed CL scans on different nanowire samples, showing that the light spectrum that is emitted by a nanowire is highly dependent on electron beam position. Analysis of the spectral scans shows that the signal from each wire can be decomposed into a number of resonant modes. At the resonance frequencies the mode intensity patterns are determined with sub-wavelength precision, from which spatial frequencies are derived.Combining data from wires with different lengths we are able to determine the dispersion relation that corresponds to surface plasmons propagating along a nanowire. The data also shows that for short wires, the organ-pipe model does not fully explain all resonances on the wire.
3:15 PM - GG9.3
Plasmonic Core-Shell Nanowire Waveguides and Resonators.
Carrie Hofmann 1 , Stanley Burgos 1 , Jennifer Dionne 1 , Luke Sweatlock 1 , Michael Filler 1 , Brendan Kayes 1 , Anna Hiszpanski 1 , Harry Atwater 1 , Ernst Jan Vesseur 2 , Rene de Waele 2 , Albert Polman 2 , Donald Sirbuly 3
1 Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, California, United States, 2 Center for Nanophotonics, FOM Insitute for Atomic and Molecular Physics, Amsterdam Netherlands, 3 Chemistry, Materials, and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractPlasmonic core-shell nanowires are potential candidates for the realization of the smallest dielectric core waveguide capable of sustaining surface plasmons and for ultra-small resonators with high quality factors for spectroscopy and active device applications. We analytically calculate the dispersion relations for infinitely long plasmonic core-shell nanowires with core diameter in the range 150-1000 nm. For 200 nm diameter SiO2 core and infinitely thick Ag cladding, both radiative waveguide and non-radiative surface plasmon modes are supported.
We begin by exploiting the low loss and high confinement in metal-cladded SiO2 nanowires in the investigation of waveguiding properties. High aspect ratio Si nanowires grown by Au-catalyzed chemical vapor deposition were oxidized in a tube furnace at 1100 °C to form the 200 nm diameter SiO2 core of the core-shell nanowires. The oxidized nanowires were subsequently coated uniformly with 100 nm Ag by thermal evaporation. Using the localized electron beam source in a scanning electron microscope and cathodoluminescence imaging spectroscopy, we find that SiO2 nanowires exhibit bright defect-band luminescence in the visible regime, specifically at λ=450 nm.
Plasmonic core-shell nanowire resonators were also investigated theoretically and experimentally. From finite difference time domain simulations, we determined the resonant modes and frequencies of a plasmonic core-shell resonator 1 µm long with 200 nm SiO2 core and 100 nm Ag cladding, and find confined modes with quality factors as high as 137. By analyzing the surface plasmon wavelength of each mode, we are able to map out the dispersion relations for resonators of finite length.
Plasmonic core-shell nanowire resonators were fabricated from Ag-coated SiO2 nanowires cut into various lengths ranging from 500 nm to 3 µm using focused ion beam nanofabrication. Spectrally-resolved cathodoluminescence imaging characterization of resonators of varying length will be presented, and the dispersion characteristics and cathodoluminescence of plasmonic nanowires with semiconductor cores (Si, ZnO, and GaN) will also be discussed.
3:30 PM - GG9.4
Enhancing Infrared Upconversion with Plasmonic Nanostructures.
Ewold Verhagen 1 , L. (Kobus) Kuipers 1 , Albert Polman 1
1 Center for Nanophotonics, FOM-Institute AMOLF, Amsterdam Netherlands
Show AbstractThe efficient concentration of light using metal nanostructures has great potential for applications in subwavelength guiding and addressing single molecules or quantum dots. Moreover, the large fields associated with highly confined plasmonic modes allow the enhancement of nonlinear effects, useful for sensing, data storage and next generation solar cells.We demonstrate the enhancement of nonlinear conversion of infrared to visible radiation by field enhancements in metal nanostructures using erbium upconversion. Erbium ions that are excited with 1.48 μm laser light can be promoted to states emitting at 980, 660 and 550 nm through upconversion processes that involve Förster transfer between excited ions. By placing the ions in the near field of suitably shaped plasmonic structures that are illuminated with infrared light, the upconversion is enhanced. At the same time, the erbium upconversion luminescence can be used as a local probe of the surface plasmon field intensity.We show that infrared-to-green upconversion can be enhanced by two orders of magnitude in the vicinity of nanoholes in a gold film. The efficiency of the process exhibits a marked dependence on aperture size and shape. The upconversion luminescence measurements are related to peaks in the transmission spectra of the nanohole arrays and compared to FDTD calculations. We attribute the enhancement to large field intensities and concentration close to the apertures due to the resonant excitation of plasmonic modes. The luminescence intensities of the various Er levels and the pump power dependencies thereof reveal differences between extended surface plasmon resonances on arrays of square apertures and localized resonances in annular aperture arrays. By collecting the upconversion luminescence in a scanning confocal microscope, two-dimensional maps of the surface plasmon propagation on the arrays are obtained. These provide insight in the coupling mechanism of light to surface plasmon modes, and the propagation and diffraction of these modes on the periodic array. Understanding these processes is crucial to optimize the amount of upconversion luminescence generated on the arrays.Furthermore, the arrays are used to launch surface plasmons into laterally tapered metal waveguides.1 Inside the waveguides, a pronounced interference pattern is observed that can be ascribed to the reflection of the surface plasmon beam at the edges of the waveguide. The upconversion luminescence intensity increases as the taper tip is approached, with a factor 4-5 compared to the start of the taper for the 550 nm emission. These results demonstrate the possibility of using nanostructured metals to control the enhancement of nonlinear effects.1 E. Verhagen, L. Kuipers, and A. Polman, Enhanced nonlinear optical effects with a tapered plasmonic waveguide, Nano Lett. 7, 334 (2007)
4:15 PM - **GG9.5
Active Plasmonic Components and Metamaterials.
Harry Atwater 1 , Henri Lezec 1 , Jennifer Dionne 1 , Carrie Ross 1 , Luke Sweatlock 1 , Domenico Pacifici 1 , Ken Diest 1 , Matthew Dicken 1 , Vivian Ferry 1
1 Applied Physics, California Institute of Technology, Pasadena, California, United States
Show AbstractRecent advances in plasmon dispersion control and actively modulated devices have enabled new plasmonic components including i) metal-insulator-metal plasmon waveguide ‘metamaterials’ that facilitate dispersion control to enable very high positive as well as negative refractive index in the visible and near infrared ii) all-optical modulation of plasmon propagation iii) electro-optic modulation of metal-insulator-metal resonator transmission iv) silicon and silica nanowire plasmonic resonators, and v) prospects for plasmonic solar cells. We expand upon recently reported work[1] on direct observation of negative refraction in the visible frequency range. Visible frequency negative refraction is achieved using coupled surface plasmon waves in thin metal-insulator-metal waveguides operating in a dispersion regime with anti-parallel group and phase velocities. We discuss the modes excited in negative refraction experiments across the visible frequency range, and also power flow in metal-insulator-metal waveguides excited in the normal dispersion regime, at the surface plasmon resonance and in the anomalous dispersion regime. By employing a lumped network circuit model we can derive the frequency-dependent permittivity and permeability across the negative refraction regime and show that both are negative.Metal-dielectric plasmon waveguides can serve as active switching elements when the dielectric refractive index can be actively modulated. We demonstrate all-plasmonic modulation in which the complex mode refractive index seen by a surface plasmon polariton at infrared free-space wavelength (1428 nm) is modulated via interband excitation of the dielectric medium at visible frequencies (514 nm). We also demonstrate electro-optic refractive index modulation in metal-dielectric-metal plasmon waveguides using low-voltage electro-optic modulation of both silicon and perovskites oxide dielectric layers.Metal-dielectric-metal nanowires represent a path to realization of ultrasmall resonators that can exhibit very high confinement and fiber- or waveguide-coupled pumping. We report here on synthesis of metal-dielectric-metal resonators formed by Ag coating semiconductor nanowires and tapered optical fibers and plasmonic mode characterization of these nanoresonators by spectrally-resolved cathodoluminescence emission imaging.The efficiency and cost effectiveness of photovoltaic cells can both be increased by reduction of the active semiconductor absorber layer thickness and ability to fabricate ultrathin absorber layers opens up new possibilities for solar cell device design. The strong mode localization of surface plasmon polaritons at metal-dielectric interfaces leads to strong absorption in semiconductors thin films, enabling a dramatic (10-100X) reduction in the semiconductor absorber physical thickness needed to achieve an optically thick film.[1] HJ. Lezec, J.A. Dionne, H.A. Atwater, Science 316 430 (2007)
4:45 PM - GG9.6
Enlarging the Bandwidth of Nano-scale Propagating Plasmonic Modes in Deep-subwavelength Cylindrical Holes.
Peter Catrysse 1 , Shanhui Fan 1
1 Electrical Engineering, Stanford University, Stanford, California, United States
Show AbstractSubwavelength cylindrical holes in optically thick metallic films always support a propagating HE11 mode near the surface plasmon frequency, regardless of how small the holes are. For holes filled with a uniform dielectric material, the bandwidth of the HE11 mode asymptotically approaches zero as the hole size is reduced to deep-subwavelength scales. We show that it is possible to create nano-scale propagating plasmonic modes with very large bandwidth in holes that are concentrically filled with two different dielectric materials, even when the hole radius goes to zero.
5:00 PM - GG9.7
Si Plasmonic Electro-Optic Modulator.
Kenneth Diest 1 2 , Jennifer Dionne 2 , Luke Sweatlock 2 , Harry Atwater 2
1 Materials Science, California Institute of Technology, Pasadena, California, United States, 2 Applied Physics, California Institute of Technology, Pasadena, California, United States
Show Abstract Electro-optic modulators are critical components for data routing and switching in future chip-based silicon photonic and plasmonic systems. Because of the opportunity for dense integration of modulators with low insertion loss into Si-on-insulator photonic waveguide networks, it would be highly desirable for the modulator active region to be comprised of Si itself. Here, we demonstrate near-infrared (l=1550nm) electro-optic modulation of light transmission in ultracompact metal-silicon-metal (MSiM) plasmonic waveguides. Modulators were fabricated by deposition of 400nm of silver onto the top and bottom surfaces of a suspended single-crystalline, n-doped Si membrane. Subsequently, Fabry-Perot resonators were defined by slits milled into the top and bottom Ag layers via focused ion beam fabrication. Si core dimensions were varied from 60nm-400nm, and Fabry-Perot slit spacings were varied from 1-10um in 50nm increments. Modulation in our MSiM structures arises from a voltage-induced change in the refractive index of the Si core. We find that these resonators exhibit modulation of the zero-field resonator transmission intensity by values from 20 to 185% with applied fields varied from 2-11 V/micron. Modulation is seen for both TE and TM modes in thicker samples (~400nm) which support both photonic and plasmonic modes, while only the TM plasmon mode is seen in thinner samples (~60nm). While plasmon propagation lenghts approach 10um, modulation can be observed for resonator dimensions as small as 1.32l x l/24, illustrating an ultracompact modulator design. Raman spectroscopy of the 520cm-1 crystalline Si vibration mode indicates a tensile strain of ~400MPa, which is sufficient to distort the centrosymmetric Si crystal structure and impart a change in the c(2) of the silicon. Carrier injection and thermo-optic effects have also been analyzed. Depending on the applied field, the temporal response of the MSiM modulator can be tuned from the nano-second to microsecond regime. The compatibility of these devices with CMOS technology, as well as applications to second-harmonic generation, will be explored.
5:15 PM - GG9.8
Plasmon Launching using Lithographically Defined and Synthesized Silver Nanowires.
Marleen vanderVeen 1 , Laetitia Bernard 1 , Eric Dufresne 4 , Mark Reed 1
1 Dept. of Electrical Engineering, Yale University, New Haven, Connecticut, United States, 4 Dept. of Mechanical Engineering, Yale University, New Haven, Connecticut, United States
Show AbstractMetallic nanowires have interesting optical applications and are promising platforms for sensing applications. The optical properties originate from the excitation of surface plasmon polaritons with light. The excitation of nanowires at one end and reemission of light at the other end due to edges and scattering centers could also become a means to selectively couple light into the region of molecules. We present direct observation of plasmon propagation along silver nanowires synthesized in solution and fabricated by e-beam lithography. E-beam lithography is a powerful tool for the fabrication of nanostructured architectures in allowing structures to be designed at will. The e-beam written films have the shape of nanowires but allow for defining the edges, sizes, and distance between plasmon scattering centers on the metal structure.By defining the nanostructures (e.g., size, geometry, and relative orientation) and by controlling the properties of the incoming light that creates the plasmon-polariton (e.g., frequency, polarization, spatial angle, and number of points) we aim at selectively couple light into the region of molecules with the objective to build a platform for sensing molecules.
5:30 PM - GG9.9
Efficient Grating Coupling into Slow Surface Plasmons.
Michael Preiner 1 , Ken Shimizu 1 , Nicholas Melosh 1
1 Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California, United States
Show AbstractWe demonstrate efficient (>25%) grating coupling into slow surface plasmon polaritons. Slow mode plasmons can exist in MIM (metal-insulator-metal) structures, and have attracted interest recently as both spectroscopic tools and as a mechanism for directing highly confined optical fields. We utilize subwavelength diffraction gratings to measure the dispersion of the slow mode, and demonstrate the ability to couple into high order (n>1) grating modes. The resulting slow plasmons are highly confined within the insulating layer (~12 nm thick) and create a local high field region within the dielectric where field intensities are 300-600x stronger than the incident field. Measurements of coupling efficiency allow us to calculate the fields within the device using finite difference modeling. The ability to create intense, well-defined fields within molecular-scale junction promises to be useful for a variety of spectroscopic techniques.
Symposium Organizers
Alexander O. Govorov Ohio University
Zhiming M. Wang University of Arkansas
Andrey L. Rogach Ludwig-Maximilians-Universtitaet Muenchen
Harry Ruda University of Toronto
Mark Brongersma Stanford University
GG10: Plasmonic Device Components, SERS Applications and Imaging
Session Chairs
Thursday AM, November 29, 2007
Room 310 (Hynes)
9:00 AM - **GG10.1
Enhancing the Efficiency of Solar Cells using Metal Nanostructures.
Peter Peumans 1
1 Electrical Engineering, Stanford University, Stanford, California, United States
Show AbstractMetals have unique optical properties that allows nanostructured metals to guide light on the nanometer scale. This ability can be exploited to increase the efficiency of the photovoltaic energy conversion process in several ways. I will discuss how nanostructured metals can be very conductive and transparent at the same time, making them useful as transparent electrodes in thin-film solar cells. I will present design rules for various nanostructure geometries. In addition, I will show both experimentally and theoretically how solution-processed metal nanowire random meshes can be used to approach the performance of ideal nanostructures. Metal nanostructures can also be embedded inside the active region of solar cells to increase the local optical absorption and to increase the energy transport in excitonic solar cells. I will discuss design rules that optimize the enhancement and will show experimental examples of solar cells whose efficiency was enhanced by the incorporation of metal-core/insulator-shell nanostructures of various geometries. Finally, I will show how aperiodic one-dimensional metallo-dielectric stacks can be used to both capture sunlight very efficiently to generate heat and to re-emit it as a spectrum tailored to the characteristics of a solar cell to realize thermophotovoltaic systems with efficiencies >30%.
9:30 AM - GG10.2
En Route to No-loss and Low-loss Nanoplasmonics.
Mikhail Noginov 1 , G. Zhu 1 , M. Mayy 1 , M. Bahoura 1 , B. Ritzo 1 , V. Gavrilenko 1 , V. Podolskiy 2 , V. Drachev 3 , V. Shalaev 3
1 Center for Materials Research, Norfolk State University, Norfolk, Virginia, United States, 2 Department of Physics, Oregon State University, Corvallis, Oregon, United States, 3 Birck Nanotechnology Center and School of Electrical & Computer Engineering, Purdue University, West Lafayette, Indiana, United States
Show Abstract Localized surface plasmons (SPs), resonant oscillations of free electrons in metallic nanostructures, and surface plasmon polaritons (SPPs), special type of electromagnetic waves coupled to electron density oscillations, comprise a very hot topic in modern photonic research. Surface plasmons have been named an enabling mechanism in a number of unique optical phenomena, including surface-enhanced Raman spectroscopy, extraordinary optical transmission and negative index of refraction. They are broadly used in photonic and optoelectronic devices, including waveguides, couplers, splitters, and quantum cascade lasers. Unfortunately, many applications of surface plasmons suffer from damping caused by absorption in metals. Over the years, several proposals have been made to compensate loss in localized and propagating surface plasmons by incorporating optical gain into dielectric medium. In this work, we have experimentally demonstrated conquering of surface plasmon loss by optical gain. In addition, we show the possibility of reducing the loss in metal without gain, via modification of its electronic states due to presence of adjacent dye-doped polymer. In particular: (1) We have achieved the compensation of loss in metal by gain in dielectric in the mixture of aggregated Ag nanoparticles and rhodamine 6G dye. The demonstrated six-fold enhancement of Rayleigh scattering is the evidence of enhancement of the localized surface plasmon (SP) resonance. (2) We have compensated loss in propagating SPPs at the interface between silver film and optically pumped polymer with dye. In contrast to the previous studies, where an elongation of the SPP propagation length was infinitesimally small, the value of gain achieved in our experiments, ≈ 420 cm-1, is sufficient to fully compensate SPP loss in high-quality silver films.Narrowing of emission band observed in the SPP mode above a critical threshold pumping energy suggests a laser-like emission involving surface plasmons.(3) In an attenuated total internal reflection setup, we have demonstrated that doping of polymer film adjacent to a silver film with high concentration of rhodamine 6G dye leads to narrowing of a ‘dip’ in the dependence of reflectivity R versus angle θ. This is indicative of elongation of the SPP propagation length. Fitting the experimental dependence R(θ) with the known analytical formula developed for a three-layer system, we have demonstrated a three-fold reduction of the imaginary part of the permittivity of silver in comparison to its value without dye-doped polymer film. Based on first principle modeling, we explain the observed effect by changing the work function in a thin layer of metal adjacent to dye-doped polymer. The corresponding shift of the absorption bands associated with bound electrons reduces the loss in silver in the visible spectral range. The reported results enable practical applications of a broad range of low-loss and no-loss photonic metamaterials.
9:45 AM - GG10.3
Optimizing Silica-Coated Metal Nanoparticles for SERS Detection.
Leif Brown 1 , Stephen Doorn 1
1 Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractSurface Enhanced Raman Scattering (SERS) is a spectroscopic technique of particular interest for a number of detection applications. SERS grants access to a sensitivity potentially comparable to fluorescence, but with the additional capability of obtaining full spectral fingerprints, thus leading to the ability to perform enhanced multiplexing. Silica-coated noble metal nanoparticles have been demonstrated to be a highly stable system that can provide the necessary SERS effect, while residing in a solution (as opposed to a macroscopic surface) based environment. Although these particles can be conveniently prepared, obtaining the best possible signal enhancements requires careful and methodical manipulation of several coating parameters. Some of the reactions involved compete with each other, and can contribute to significant signal loss in the end product. Here, we report work on optimizing preparation and coating conditions for obtaining bright SERS-active substrates based on both gold and silver nanoparticles, overcoated with silica. In addition to optimizing for intensity, adjustments in both materials and conditions produce materials suitable for laser excitation from green wavelengths (e.g., 532 nm) through red (e.g., 785 nm). Work on applying these materials to bioanalytical applications will be reported.
10:00 AM - GG10.4
Arrays of Well-defined Size-tunable Metallic Nano-cones for Plasmonic Applications.
M. Fleischer 1 , F. Stade 1 , A. Heeren 1 , D. Kern 1 , K. Braun 2 , C. Stanciu 2 , A. Meixner 2
1 Institute of Applied Physics, University of Tuebingen, 72076 Tuebingen Germany, 2 Institute of Physical and Theoretical Chemistry, University of Tuebingen, 72076 Tuebingen Germany
Show AbstractOne of the primary aims in the field of plasmonics consists in obtaining localized regions of high electrical field enhancement. These are especially pronounced in the direct vicinity of metallic tips, where plasmons are generated within the metal upon laser irradiation. The plasmon resonances depend strongly on the size and geometry of the metallic structures. To achieve pointed metallic nanostructures, focussed ion beam techniques (e.g. [1]) have been used as well as ebeam lithography and lift-off processes (e.g. [2]), but they usually only meet one of the two requirements of either sharp tips or reproducible structures at the nanometer scale. For this reason, we developed a fabrication process for arrays of well-defined metallic nano-cones [3] with tip radii of the order of ten nanometers.In this process, we use the high-resolution negative electron beam resist hydrogen silsesquioxane (HSQ) as an etch mask for an underlying metal stack on silicon. A gold layer, whose thickness will determine the maximum height of the cones, is buffered between a thin titanium adhesion layer and a sacrificial chromium layer. In an electron beam lithography step, multiple 30 µm x 30 µm arrays of dots with varying parameters are exposed in the HSQ resist. After development, the dots remain on the metallic layer as resist pillars, forming an etch mask for the subsequent ion milling step. Both the unprotected metallic layers and part of the HSQ mask are removed during ion milling. After removal of any residual etch mask, either truncated cones or cones with sharp tips are obtained, depending on the original size of the HSQ dots.Diameter and height of the cones are tunable via the etch mask diameter, gold layer thickness and ion milling time. The sidewall angle can be varied by tilting the rotating substrate with respect to the incident ion beam, and asymmetric cones can be achieved by placing the sample off-axis during rotation. The dimensions and tip shapes of arrays of cones for field enhancement purposes will be presented, whose crucial parameters are varied systematically during fabrication. The technology can easily be extended to larger field sizes as well as to a variety of different metals and geometries. First results of the optical and field-enhancement properties of the cones will be shown, which are investigated using a high-resolution near-field microscope. [1] E.J. Sanchez, L. Novotny, and X.S. Xie, Phys. Rev. Lett. 82, 4014 (1999). [2] P.J. Schuck, D.P. Fromm, A. Sundaramurthy, G.S. Kino, and W.E. Moerner, Phys. Rev. Lett. 94, 017402 (2005).[3] F. Stade, A. Heeren, M. Fleischer, and D.P. Kern, Microelectron. Eng. 84, 1589 (2007).
10:15 AM - GG10.5
Metallic Nanocrystals Near Ultrasmooth Metallic Film For SERS Application.
Jing Tang 1 , Zhiyong Li 1 , Duncan Steward 1 , Ekaterina Ponizovskaya 1 , Alexandre Bratkovsky 1 , R. Williams 1
1 Quantum Science Research Group, Hewlett-Packard Labs, Palo Alto, California, United States
Show AbstractMetallic nanocrystals, such as Ag or Au, have been extensively used to enhance Raman scattering in colloidal solutions and on rough surfaces. Few studies have examined such enhancement with nanocrystals on surfaces of well-controlled morphology and electronic properties. We report a systematic surface enhanced Raman scattering (SERS) study of metallic nanocrystals on ultra-smooth metallic films (RMS < 0.5 nm) obtained by template-stripping and also on smooth Si substrates. Significantly higher SERS signal is observed on the metallic substrate than on the Si substrate when uniform nanocrystal films form on both substrates. Theoretical modeling of the electromagnetic (EM) field is conducted by standard Finite Difference Time Domain (FDTD) methods. Our work indicates that the plasmon coupling between the metallic nanocrystals and the plasmon coupling of the metallic nanocrystals to the underlying metallic film are primarily responsible for the much higher Raman enhancement on the metallic film than on the Si substrate.
10:30 AM - GG10.6
Time-resolved Imaging of Surface Plasmon Polaritons in Nanostructures.
L. Chelaru 1 , S. Moellenbeck 1 , N. Buckanie 1 , F. Meyer zu Heringdorf 1
1 , University of Duisburg-Essen, Duisburg Germany
Show AbstractCoupling of light into Surface Plasmon Polaritons (SPPs) is regarded today as a modality of scaling down the lateral dimensions of optoelectronics devices below the diffraction limit.It is therefore of high technological and fundamental interest to understand the behavior of SPPs in metallic nanostructures and waveguides. We investigate the excitation and propagation of SPPs in single-crystalline Ag nanostructures of different shapes and sizes that are formed in-situ by self assembly during deposition on Si surfaces. Imaging of the SPPs is accomplished by time-resolved two-photon photoelectron emission microscopy (TR-2PPEEM). Here the photoelectrons are generated by femtosecond laser pulses through a nonlinear two-photon photoemission process (2PPE). Under these conditions the excited SPP wave is visualized as a result of the interference (beat pattern) between the laser pulses and the traveling SPP wave at the sample surface. We show that the propagation length of the SPP wave depends on the size of the Ag nanostructures, and that it can extend for several micrometers along the surface. We present time-resolved movies of the propagating SPP wave recorded with a pump-probe setup where we control the delay time between the exciting pump pulse and the photoemitting probe pulse. By comparing nanostructures of different geometry, we address the guiding of SPP waves, and the coupling of the light into and out of the nanostructures. Depending on the incidence angle of the propagating SPP wave on the nanostructure’s boundary, we observe either trapping of the SPP wave inside the metallic nanostructures, or a photoemission yield enhancement in the vicinity of the nanostructure caused by the electric field of the SPP wave.
10:45 AM - GG10.7
Towards Sub-diffraction Limited Microscopy with Visible-frequency Negative Index Lenses.
Jennifer Dionne 1 , Henri Lezec 1 , Kenneth Diest 1 , Jennifer Stockdill 1 , Harry Atwater 1
1 Applied Physics, California Institute of Technology, Pasadena, California, United States
Show AbstractConventional optical microscopy is limited by diffraction, which bounds the resolution to about half the wavelength of light used to image an object. Finer details of an object decay in the near field and are traditionally imaged using scanning probe or fluorescent techniques [1]. It was recently proposed that a lens composed of a negative index material (NIM) could be used to both focus and perfectly resolve objects [2]. A flat slab of a NIM would negatively refract light and amplify evanescent electromagnetic Fourier components, allowing for complete reconstruction of an object – including its subwavelength details – in the image plane. Two challenges exist to implement such a microscope: 1) achieving negative refraction in the visible and 2) transferring an image into the far-field through a NIM lens. Here, we expand on our recent report of visible frequency negative refraction [3] and present the first experimental demonstration focusing through a negative index material. Negative refraction is demonstrated at the interface of a Au/Si3N4/Ag prism (core thickness 50-150nm) sustaining a surface plasmon mode and a Ag/Si3N4/Ag waveguide (core thickness 500nm) sustaining a positive-index guided mode. Negative refraction is achieved by exploiting the unique dispersion properties of plasmonic modes, which are characterized by antiparallel group and phase velocities for frequencies between the bulk and surface plasmon resonant frequencies. By tuning the wavelength between 470nm and 530nm, negative indices are found to vary from -1.1 to -5.1. The imaging characteristics of such NIMs are explored by cascading Au/Si3N4/Ag flat lenses with Ag/Si3N4/Ag waveguides. ‘Objects’ are defined by subwavelength apertures of arbitrary size and shape inserted in the plane of the Ag/Si3N4/Ag waveguides. By thinning the bottom Ag cladding of the waveguide and overcoating with a self-developing resist (poly [disperse Red 1 Methacrylate]), the optical mode trajectory from the waveguide through the lens is traced out in the resist. Atomic force microscopy can then be used to observe topographic changes in the resist that indicate the focal and resolving properties of the NIM lenses. In one experiment, two circular apertures with a 100nm radius separated by 150nm were placed 0.5μm away from a 1μm-wide flat lens. At 514nm, focusing of these point sources could be observed both in the lens (at a distance of 1μm from the object) and outside the lens (at a distance 2μm from the object), verifying the subwavelength-resolving capabilities of NIM flat lenses. Applications to general two and three-dimensional far-field subwavelength image transfer will be discussed, and opportunities for magnification will be explored. [1] S. Hell, Science 316 (2007)[2] J. B. Pendry, Phys. Rev. Lett. 85 (2000)[3] H. J. Lezec, J. A. Dionne, H. A. Atwater, Science 316 (2007)
GG11: Nanowires
Session Chairs
Thursday PM, November 29, 2007
Room 310 (Hynes)
11:30 AM - GG11.1
Room-Temperature Polariton Guiding, Luminescence and Lasing in a ZnO Nanowire.
Sven Rühle 1 , Heng-Yu Li 1 , Bert van Vugt 1 , Daniel Vanmaekelbergh 1
1 Utrecht Faculty of Science, Condensed Matter and Interfaces, Utrecht Netherlands
Show AbstractSemiconductor nanowires hold promise for miniaturized optical and electrical devices, such as single-wire FETS for logics, bio-sensors, single photon-detectors and emitters, lasers and nanophotonic circuits for optical information transfer [1]. Nanowires from the semiconductor ZnO form a special case due to their striking optical properties: ZnO has a huge exciton oscillator strength in the near UV, a large exciton-binding energy of 60 meV, and a robust exciton emission. This has led to speculations on the occurence of strong light-matter interaction with room-temperature exciton-polariton formation in ZnO cavities and nanostructures [2]. We have performed spatially resolved luminescence spectroscopy of optically excited single ZnO nanowires at room temperature [3-5]. We observed a strong UV luminescence with a series of Fabry-Perot modes that reflect the exciton polariton eigenstates of the wire. Photons are predominantly emitted at the wire ends, and the emission from each mode was supra-linear in the polariton density which showed that polariton-polariton scattering governs the competition between guiding and emission. Polariton guiding and non-linear emission can be explained by the polariton dispersion curve. At higher excitation intensities, some of the polariton eigenmodes evolve into sharp laser peaks. We observe that also above the lasing threshold, strong-light matter interaction prevails; ZnO nanowires are hence the first example of room-temperature polariton lasers [5]. [1] Nanoribbon waveguides for subwavelength photonics integration. M. Law et al., Science 305, 1269 (2004). [2] Giant exciton-light coupling in ZnO quantum dots. B. Gil and A. V. Kakovin, Appl. Phys. Lett. 81, 748 (2002). [3] Exciton Polaritons Confined in a ZnO Nanowire Cavity.L. K. van Vugt et al., Phys. Rev. Lett. 97, 147401 (2006). [4] Phase-Correlated Nondirectional Laser Emission from the Endfacets of aZnO Nanowire. L. K. van Vugt et al., Nano Lett. 6, 2702 (2006)[5] Room temperature polariton lasing from a ZnO nanowire. S. Rühle et al, submitted.
11:45 AM - GG11.2
Solution-Processed Metal Nanowire Mesh Transparent Electrodes.
Jung-Yong Lee 1 , Steve Connor 2 , Yi Cui 3 , Peter Peumans 1
1 Electrical Engineering, Stanford University, Stanford, California, United States, 2 Chemistry, Stanford University, Stanford, California, United States, 3 Material Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractThin-film optoelectronic devices make use of transparent, conductive thin-film electrodes to provide electrical contact to the devices without impeding coupling of light into and out of the devices. Transparent, conductive metal oxides such as indium-tin-oxide (ITO) and Al-doped zinc-oxide (ZnO:Al) are commonly used for this purpose but are expensive and brittle. Moreover, there is an inherent trade-off between optical transparency and sheet resistance: thicker films (or higher doping concentrations) decrease the film sheet resistance, but also decrease the optical transparency. For a sheet resistance of Rsh=10 Ohm/sq, the solar photon flux-weighted optical transparency of ITO is TSolar=80.3%. We investigate whether nanostructured metal films can be both transparent and conductive with properties better than those of metal oxides. We first show that the optical transmission and sheet resistance of subwavelength periodic metal gratings is superior to that of ITO using electromagnetic modeling. For a sheet resistance of Rsh=10 Ohm/sq, the solar photon flux averaged transmittance is TSolar=88%. The design tradeoffs that govern the ideal grating geometry will be discussed. We then show that random meshes of Ag nanowires processed entirely from solution achieve a performance that approaches that of periodic metal gratings. The Ag nanowires are fused together by a low-temperature annealing step to achieve a low sheet resistance. A solar photon flux averaged transmittance of TSolar=80% for a sheet resistance of Rsh=8 Ohm/sq was achieved for such a solution-processed transparent thin-film electrode. Organic photovoltaic cells that use such transparent metal nanowire mesh electrodes exhibit improved photocurrents compared to identical cells fabricated on ITO-coated substrates. The ease of fabrication and high electrical and optical performance of the solution-processed transparent nanowire mesh electrodes will allow for their use in a broad class of nanostructures solar cells as a replacement for metal oxides.
12:00 PM - GG11.3
Silicon Nanowires with Gold Caps on Top used as Probes for Nano-Raman Spectroscopy.
Michael Becker 1 2 , Vladimir Sivakov 2 3 , Andreas Berger 3 , Peter Werner 3 , Samuel Hoffmann 4 , Johannes Michler 4 , Silke Christiansen 1 2 3
1 , University of Halle-Wittenberg, Halle Germany, 2 , Institute for Physical Hightechnology e.V., Jena, Thüringen, Germany, 3 , Max-Planck-Institut of Microstructure Physics, Halle, Sachsen-Anhalt, Germany, 4 , EMPA, Thun Switzerland
Show AbstractWe show that hemispherical gold droplets on top of silicon nanowires when grown by the vapor-liquid-solid (VLS) mechanism, can produce a significant enhancement of Raman scattered signals. Signal enhancement for a few or even just single gold droplets is demonstrated by analyzing the enhanced Raman signature of dye- and biomolecules. For this experiment, trenches (~ 800 nm wide) were etched in a silicon-on-insulator (SOI) wafer along <110> crystallographic directions that constitute sidewalls ({110} surfaces) suitable for the growth of silicon nanowires in <111> directions with the intention that the gold droplets on the silicon nanowires can meet somewhere in the trench when growth time is carefully selected. Another way to realize gold nanostructures in close vicinity is to attach a silicon nanowire with a gold droplet onto an atomic force microscopy (AFM) tip and to bring this tip toward another gold-coated AFM tip where dye molecules were deposited prior to the measurements. In both experiments, signal enhancement of characteristic Raman bands of those molecules was observed. This indicates that silicon nanowires with gold droplets atop can act as efficient probes for tip-enhanced Raman spectroscopy (TERS). In our contribution, we show that a nanowire TERS probe can be fabricated by welding nanowires with gold droplets to AFM tips in a scanning electron microscope (SEM). The SERS/TERS capabilities of silicon nanowires with gold caps on top which grow within the trench or have been attached to an AFM-tip can be calculated by means of finite element (FE)-simulations. With FE-simulations we calculated the electric field enhancement produced by silicon nanowires with different geometrical configurations. The field enhancement produced by a single hemispherical gold droplet on top of a silicon nanowire as well as the enhancement produced by a larger but facetted gold droplet (facetting occurs under certain growth conditions if the gold droplets become rather large ~ 500 nm in diameter) were simulated. The field enhancement generated by the different configurations of nanowires grown within the trench (touching, close proximity, larger distance) have also been simulated. The results of our calcuations are in very good agreement with the experimental results.
12:15 PM - GG11.4
Theoretical Study on the Spontaneous Emission in One-dimensional Semiconductor Nanostructures.
Xuewen Chen 1 2 , Wallace Choy 2 , Sailing He 1
1 Department of Optical Engineering, Centre for Optical and Electromagnetic Research, Zhejiang University, Joint Research Centre of Photonics of the Royal Institute of Technology (Sweden) and Zhejiang University, Hangzhou, Zhejiang, China, 2 Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong SAR China
Show AbstractOne-dimensional semiconductor nanostructures, such as nanowires (NWs), core/(multi-)shell nanocables (NCs) and nanotubes (NTs), have been under intensive research owing to their impressive functionalities in building up novel photonic elements such as light-emitting diodes (LEDs), lasers and biosensors [1-2]. These nanostructures can be approximately considered as cylindrically layered structures and their optical properties depend strongly on the structural parameters. For example, the quantum efficiency of a NW LED is different from that of planar multilayered thin film LED since radiative transition rates of the emitter in these structures are distinct [3-4]. In addition, the assignment of the total emission power to lateral-surface emission and waveguided emission or facet emission (propagating along the longitudinal axis of the nanostructure) also depends on the size of the nano-structure. These spontaneous emission (SE) properties are critical to semiconductor NWs or NCs for LED and fluorescence labeling applications but no systematic theoretical studies have been addressed yet. Here, we will present an efficient optical model to calculate the relative radiative transition rate and the lateral-surface emission for a cylindrically layered nanostructure with arbitrary number of layers and permittivity including lossy material like metal. As examples of practical interest, the SE properties of an isolated ZnO NW and the ZnO NW with a silver shell are comprehensively studied. We find that the efficiency of the emitter in the NW/NC shows large anisotropy, i.e. the emitter orienting along the longitudinal axis of the NW are much more efficient than the emitter with an orientation in the cross section of the NW since the transition rate of the latter is smaller that of the former by nearly an order of magnitude. In addition, there is a critical value of the radius of the NW, i.e. below the value almost no waveguided emission exists. For the silver coated NC, the radiative transition rate can be greatly enhanced due to the excitation of the surface plasmon. The effect of the plasmonic resonance to the SE properties and the dependence on the size of NW will be studied and discussed. [1] C.J. Barrelet, A.B. Greytak, C.M. Lieber, Nano Lett. 4, 1981 (2004)[2] O. Hayden, A.B. Greytak, D.C. Bell , Adv. Mater.17,701 (2005)[3] W. Lukosz, Phys. Rev. B 22, 3030 (1980)[4] X.W. Chen, W.C.H. Choy, S.L. He and P. C. Chui, J. Appl. Phys, 101, 113107 (2007)
12:30 PM - GG11.5
Optical properties of Ultrafine Gold Nanowires in the UV-NIR Regime.
A. Halder 1 , Ravishankar Narayanan 1
1 Materials Research Centre, Indian Institute of Science, Bangalore India
Show AbstractThe plasmonic properties of noble metal nanostructures critically depend on their size as well as shape. Especially metal nanowires are very useful in nanoelectronics and for device applications. We have synthesized ultrafine gold nanowires (~ 2 nm diameter) having the aspect ratio of about 1000 by a simple wet chemical method. Gold nanoparticles of 2 nm diameter are found to undergo oriented attachment to yield ultrafine single crystalline nanowires. These gold nanowires with high aspect ratios show strong longitudinal absorbance in the NIR region, indicating potential use for biomedical applications. In the current study, we present the tunable plasmonic properties of the gold nanowires and the growth kinetics of the nanowires in solution phase by UV-Vis-NIR spectroscopy and EELS in the transmission electron microscope.
12:45 PM - GG11.6
Chemical Control of Photoexcited States in Titanate Nanostructures.
Oliver Diwald 1 , Alexander Riss 1 , Thomas Berger 1 , Hinrich Grothe 1 , Johannes Bernardi 2 , Erich Knözinger 1
1 Institute of Materials Chemistry, Vienna University of Technology, Vienna Austria, 2 Institute of Solid State Physics, Vienna University of Technology, Vienna, Vienna, Austria
Show AbstractLayered TiO2-based nanostructures have attracted much attention due to a broad spectrum of potential photochemical and electrochemical applications including photocatalysis, solar cell technology and gas sensing devices. The photochemical activity of these materials depends on the branching ratio between essentially three pathways photoexcited states can undergo: they can either recombine under photoluminescence emission or generation of heat, or become persistently trapped [1] or undergo interfacial charge transfer at the particle surface. Some of these competitive processes can be tracked by means of electron paramagnetic resonance and photoluminescence spectroscopy.We synthesized titanate nanowires and –tubes by TiO2 anatase powder treatment in aqueous alkaline solutions. The resulting structures are composed of layered sheets which consist of edge-sharing [TiO6] octahedra [2]. A specific photoluminescence process which originates from the deactivation of exciton states trapped in [TiO6] units was observed. It was found that exchange of intercalated ions provides means to adjust the branching ratio between radiative exciton deactivation, on one hand, and charge separation, on the other. Thus, chemical control over the photoelectronic properties of layered oxide structures has become feasible and the role of various ions in between the layered sheets will be discussed [3].[1] Berger, T.; Sterrer, M.; Diwald, O.; Knözinger, E.; Panayotov, D.; Thompson, T. L.; Yates, Jr, J.T. J. Phys. Chem. B 2005, 109, 6061[2] Orzali, T.; Casarin, M.; Granozzi, G.; Sambi, M.; Vittadini, A. Phys.Rev.Lett. 2006, 97, 156101[3] Riss, A.; Berger, T.; Grothe, H.; Bernardi, J.; Diwald, O.;Knözinger, E. Nano Lett. 2007, 7, 433
GG12: Nanocrystals: Optical Properties
Session Chairs
Thursday PM, November 29, 2007
Room 310 (Hynes)
2:30 PM - GG12.1
Controlled Modification of Erbium Photoluminescence Lifetime in Silicon Dioxide Film with Metallic Overlayers.
Nanfang Yu 1 , Jiming Bao 1 , Federico Capasso 1 , Thomas Mates 2 , Mariano Troccoli 3 , Alexey Belyanin 4
1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States, 2 Materials Department, University of California, Santa Barbara, Santa Barbara, California, United States, 3 , Argos Tech, LLC, Santa Clara, California, United States, 4 Department of Physics, Texas A&M University, College Station, Texas, United States
Show AbstractErbium is a rare-earth element of paramount technological importance for photonics. In bulk SiO2, the lifetime of Er is determined by spontaneous emission as well as non-radiative transitions such as through defects or phonon excitation. In this work, we show a strong enhancement of the photoluminescence (PL) decay rate of the 1.54μm transition in Er-implanted SiO2 films coated with different metals when the Er-metal separation is much smaller than the transition wavelength. The physical mechanisms behind the lifetime modification are the surface-plasmon-mediated energy transfer between the quantum dipole of the Er atom and the metal as well as free-carrier absorption in the metal. Samples were prepared by implanting Er ions at various depths D in thermally grown SiO2 films. An optically thick Cr or Ti film was thermally deposited on the implanted side of the sample. 980nm wavelength optical pulses were used to excite Er PL through the substrate and the intensity of the PL signal vs time was recorded. A fit of the time trace of the PL allowed to find the value of the lifetime. As can be seen from Table 1, the total PL lifetime for deep implantation is close to the typical value found for Er in bulk SiO2 (15ms) and strongly decreases as D shrinks. The PL lifetime for each sample was calculated and the values found are in excellent agreement with the experimental results (see Table 1). The lifetime was calculated by deriving the back reaction electromagnetic field at the Er ions and the total energy flux leaving the ions. The distribution of the implanted Er ions was considered. The quantum efficiency q, which accounts for the non-radiative transitions through adjacent defects or phonon excitation, is the only fitting parameter. For Cr and Ti coated samples the averaged q are 0.76 and 0.73, respectively, both with a small standard deviation of about 9%. For the same implantation energy, the Cr coated sample has slightly shorter Er lifetime than the Ti coated sample. This is because the skin depth is larger in the case of Cr and therefore more energy is dissipated into the metal. In conclusion, we report systematic measurements of the lifetime of the 1.54μm transition of Er implanted in SiO2 films coated with different metals. We demonstrate a strong reduction of the lifetime up to a factor of 20, which can be efficiently controlled by varying the distance between Er ions and the metal overlayer. Our measurements and calculations of the lifetimes are in excellent agreement and allow a determination of a 75% radiative efficiency of Er in SiO2, which is a crucial parameter for optical devices.
2:45 PM - GG12.2
Ultrafast Spin Flip Dynamics in Isolated and Coupled Lead Chalcogenide Nanocrystals.
Justin Johnson 1 , Kathrine Gerth 1 , Qing Song 1 , Gregory Scholes 2 , Arthur Nozik 1
1 , National Renewable Energy Lab, Golden, Colorado, United States, 2 Chemistry, University of Toronto, Toronto, Ontario, Canada
Show AbstractTransitions within the lowest manifold of exciton states in lead chalcogenide nanocrystals (NC) are thought to be mediated by exchange and spin-orbit interactions. Utilizing transient grating spectroscopy on a sub-50 fs timescale, we resolve the spin flip relaxation dynamics of excitons in various sizes of PbS, PbSe, and PbTe NCs. The trends observed suggest that both indirect (via spin-orbit coupling) and direct (via exchange interactions) spin flip mechanisms play a role in the dynamics of small NCs of PbS and PbSe, with the indirect mechanism dominating in larger NCs, especially in PbTe. Preliminary investigations of spin flip relaxation dynamics are also reported for semi-ordered NC arrays coupled to varying degrees by controlled treatment of cast films. Connections can also be drawn between these fundamental studies and device considerations (including multiple exciton generation) involving these potentially functional low band gap NCs.
3:00 PM - GG12.3
Carrier Relaxation and Recombination Kinetics in Si Nanocrystals.
Wieteke de Boer 1 , Salvatore Minissale 1 , Tom Gregorkiewicz 1
1 Van der Waals-Zeeman Institute, University of Amsterdam, Amsterdam Netherlands
Show AbstractSi nanocrystals (NCs) embedded in a SiO2 matrix are intensively studied as a possible basis for a light-emitting device. In spite of that, carrier relaxation and recombination processes in Si-NCs are still not fully understood. In particular, the role and influence of oxygen- and interface-related trapping states remains unclear.In this contribution, we present optical investigations of electron/hole relaxation in Si-NCs. Two experimental methods have been used. In time-resolved photoluminescence experiments we look at the exciton-related emission using time-correlated photon counting detection mode. With this experimental method the kinetics of exciton generation and recombination are displayed. These show that in addition to the long microsecond radiative decay, another faster recombination takes place on the nanosecond time scale. The relative amplitude of this fast component shows a dependence on the excitation power, thus indicating its non-radiative nature (Auger). These fast non-radiative processes are investigated further with a time-resolved absorption spectroscopy, which is used to monitor relaxation of carriers in the Si-NC. To accomplish this we used femtosecond time-resolved pump-probe absorption set-up; free carriers are generating with a pulsed tunable Ti:Sa-pumped OPA operating in the UV and visible, and subsequently probed by a white continuum probe beam. In the experiment the effect of the primary pulse on the absorption of the secondary pulse is followed. The results reveal ultrafast carrier relaxation processes with kinetics in subpico- to nanosecond scale. In the contribution, we combine the results obtained by these two methods and discuss the carrier recombination processes taking place in Si-NCs. We compare these to the case of materials additionally doped with Er3+ ions [1]. In that context, we address the possibility of carrier trapping at oxygen-related electron levels and multiple exciton generation in Si-NCs.[1] I. Izeddin et al., Phys. Rev. Lett. 97, 2007401:1-4 (2006).
3:15 PM - GG12.4
Controlled Integration of Nanocrystals on the Surface of Group III-Nitride Light-Emitting Epitaxial Heterostructures.
Sergio Pereira 1 , Tito Trindade 1 , Manuel Martins 1 , Ian Watson 3 , Diane Zhu 2 , Colin Humphreys 2
1 CICECO, University of Aveiro, Aveiro, 3810-193, Portugal, 3 Institute of Photonics, University of Strathclyde, Glasgow, G4 0NW, United Kingdom, 2 Dept of Materials Science & Metallurgy, University of Cambridge, Cambridge, CB2 3QZ, United Kingdom
Show AbstractCurrently an extensive range of colloidal nanocrystals (NCs) can be readily produced by diverse chemical methods. These nanomaterials present novel physical properties and can be regarded as building blocks to the nanofabrication of smaller, energy efficient and faster devices. However to form a functional device with reasonable complexity, these nano-building blocks have to be conveniently arranged and connected. This requires incorporation into an interconnectable, electrically and/or optically active framework. Such endeavour is a key challenge in nanoscience and nanotechnology. Here we address this via a novel and inexpensive approach that exploits the spontaneous formation of nanoscale inverted hexagonal pits (IHPs) at the surface of In-containing III-nitride epitaxial heterostructrures. We demonstrate that these morphological features, which can be designed with a suitable size, depth and density, can effectively organize and control the incorporation of NCs at the surface of efficient light emitting devices. Nanoengineering of the relative sizes of NCs and IHPs allows the number of NCs incorporated into each hexagonal pit to be controlled precisely. Although the selective incorporation of virtually any kind of NCs into the IHPs is possible due to interfacial capillarity forces acting during the deposition method, as a proof of concept, this work will focus on the nanomanipulation of Au nanocrystals with sizes ranging from 30 to ~5 nm. It will be shown that each nano-pit can accommodate either individual NCs, or well-defined “clusters” of NCs. Controlled self-assembly, with areal densities up to 109/cm2, was achieved with precisions below the limits of conventional lithographic techniques, over macroscopic length scales.The successful integration with nanometer-scale control of colloidal NCs onto the surface of III-nitride semiconductor heterostructures will bring together the benefits of bottom-up and top-down approaches towards the creation and control of nanostructures. As complementary realms of activity, the combination of both methodologies will certainly contribute to the achievement of novel functional nanodevices, and also give new scientific insights of a more fundamental kind. In this context, new properties may arise through the exploitation of cross-coupling effects, namely by the unrivalled versatility of wet chemistry to produce a wide range of nanomaterials, to be combined with the ease of charge injection and the high efficiency of light emission of group III-N bipolar devices. Such synergetic incorporation also opens the way to investigate and tailor the physical properties of elementary NC clusters, down to isolated individual NCs, as well as the coupling of elementary excitations (e.g. electronic, photonic and plasmonic) between the nitride framework and the nanoscale building blocks.
3:30 PM - GG12.5
Raman Characterization of Silicon Nanoclusters Embedded in Fused Silica.
David Barba 1 , Francois Martin 1 , Guy Ross 1
1 EMT, INRS, Varennes, Quebec, Canada
Show AbstractSilicon nanocrystals (Si-nc) and amorphous silicon (a-Si) aggregates either produced by silicon implantation in fused silica have been studied by micro-Raman spectroscopy. More accurate than the x-ray diffraction technique, which is usually based on the Scherrer’s law to determine the size of nanocrystallites, the Raman technique is also non-destructive and straightforward, compared to the characterization by transmission electron microscopy (TEM). Under certain experimental conditions, relevant information regarding both the dimension and the size distribution of Si-nc can be extracted from the energy shifts and the spectral distortion of the TO-phonon excitation, observed near 520 cm-1. Such a spectral deconvolution is performed using a phenomenological phonon confinement model, as well as an ab-initio description of the Si-nc sublayer. Results are presented for different implantation energies and ion fluences. These data compare well with the direct observation by TEM of non-uniformly depth distributed silicon nanocrystals. Moreover, measurements recorded for different Raman probing depths gives evidence of chemical composition changes within the fused silica matrix. The depth evolution of the phonon peak associated with the Raman signature of a-Si near 470 cm-1 shows that the formation of amorphous clusters is larger in the region containing a high concentration of silicon in-excess. As for some Si-nc systems prepared by SiOx layer deposition, this latter feature indicates that the presence of amorphous silicon cannot be neglected in heavy-implanted Si-nc systems.
GG13: Optical Properties of Two Dimensional Nanostructures
Session Chairs
Thursday PM, November 29, 2007
Room 310 (Hynes)
4:15 PM - GG13.1
Exciton-Polaritons in GaAs/AlGaAs Multiple Quantum Well System Near Bragg Resonance.
Vladimir Chaldyshev 1 , Alexei Shkolnik 1 , Vadim Evtikhiev 1
1 , Ioffe Institute, Saint Petersburg Russian Federation
Show AbstractA periodic GaAs/GaAlAs QW structure has been designed and grown, which was tunable to obtain the electromagnetic Bragg resonance and the exciton-polariton resonance at the same photon energy. The optical reflection spectra from such structure are formed by interplay of three different contributions, namely (i) the reflection from the air/semiconductor interface, (ii) the Bragg reflection due to periodic modulation of the background indices of refraction being different for the wells and barriers and (iii) the resonant reflection from the periodic system of exciton-polaritons in quantum wells. The exciton-polariton features under near-Bragg conditions were studied by contactless electroreflectance technique in the spectral range covering ground states of the heavy-hole and light-hole excitons associated with two lowest electron and hole subbands. A quantitative analysis of the experimental CER line shape has been done along with quantum-mechanical calculations, which revealed the characteristic energies and broadening parameters for different excitonic transitions. In particular, the systems of four and thirty two QWs exhibit spectral features with the characteristic broadening of 1.8 meV and 2.2 meV at 17 K, respectively. Comparison with theoretical calculations shows that both the radiative and non-radiative contributions are important to the total broadening of the optical reflection respond from the system of 32 QWs. For the system of 4 DQWs the radiative broadening is small and non-radiative broadening mechanisms dominate. Its major origin at 17 K is likely to be the roughness of the well/barrier interfaces and internal interfaces within short-period GaAs/AlAs superlattice of the barrier. Broadening due to interaction with phonons, defects and impurities makes minor contribution at 17 K, however, this mechanism becomes more important at higher temperatures.
4:30 PM - GG13.2
Near Infrared Intersubband Detectors Fabricated from Cubic and Hexagonal GaN/AlN Superlattices.
Eric DeCuir 1 , Emil Fred 1 , Omar Manasreh 1 , Joerg Schoermann 2 , Donat As 2 , Klaus Lischka 2 , Morgan Ware 3 , Greg Salamo 3
1 Department of Electrical Engineering, University of Arkansas, Fayetteville, Arkansas, United States, 2 Department of Physics, University of Paderborn, Paderborn, North Rhine-Westphalia, Germany, 3 Department of Physics, University of Arkansas, Fayetteville, Arkansas, United States
Show AbstractNear infrared intersubband absorption and photoconductivity was observed in molecular beam epitaxial grown cubic and hexagonal GaN/AlN superlattices. Characterization of intersubband transitions in these structures is used as a basis for fabrication of detectors operating in the near infrared regime. Electrochemical capacitance voltage technique is used to measure the three dimensional carrier concentrations in doped GaN layers. Control of the Fermi energy level in hexagonal structures is achieved through precise control of silicon impurities which enable sufficient electron densities necessary for intersubband absorption. However, in the absence of intentional doping in cubic materials, high background concentrations on the order of 1018cm-3 provided sufficient carriers for intersubband transitions to occur. Optical absorption and photoconductivity spectra are recorded using a Bruker 125HR Fourier-transform spectrometer in conjunction with a continuous-flow cryostat (4.2-300K) For device measurements, mesa structures were fabricated using reactive ion etching and Ti/Al/Ti/Au Ohmic contacts were formed using electron beam evaporation and rapid thermal annealing. Theoretical calculations of the bound state energy levels in these quantum well structures are made using a transfer matrix technique.
4:45 PM - GG13.3
Room Temperature Exciton-Polariton Devices Enabled by Layer-By-Layer Assembly of 5 nm Thick Films of J-aggregated Cyanine Dye.
Jonathan Tischler 1 , M. Bradley 1 , Yasuhiro Shirasaki 1 , Arto Nurmikko 2 , Vladimir Bulovic 1
1 Electrical Engineering and Computer Science, MIT, Cambridge , Massachusetts, United States, 2 Division of Engineering and Department of Physics, Brown University, Providence, Rhode Island, United States
Show AbstractHere we report results of the first room temperature exciton-polariton device consisting of layer-by-layer (LBL) assembled thin films of polyelectrolyte and J-aggregated cyanine dye inserted in a resonantly-tuned planar λ/2n optical microcavity with dielectric and metal mirrors. The device exhibits Rabi-splitting of ΩR = 125 ± 7 meV at room temperature with a polyelectrolyte/J-aggregate layer that is only 5.1 ± 0.5 nm thick [1, 2]. Furthermore, the linewidth of the lower energy polariton state, measured on resonance, is Γ = 12.1 meV. The device is therefore operating in a limit where the light-matter coupling strength (ΩR) significantly exceeds the dephasing processes (Γ) competing against the coherence of the interaction. The ratio ΩR/2Γ = 5.1 represents a 240% improvement over the previously best reported ratio for a room temperature exciton-polariton device [3]. These figures of merit are achieved by virtue of the nanostructured film’s large absorption coefficient of α ~ 1.0x106 cm-1 and by location of the 5.1 ± 0.5 nm thick layer at the anti-node of the microcavity. The signatures of strong light-matter coupling, namely Rabi-splitting and polaritonic dispersion, are observed in the reflectance, transmittance, and photoluminescence measurements of the device. Because strong coupling is achieved with such thin films, the majority of the microcavity modal volume is available for integrating other materials that could serve to stabilize the J-aggregate film chemically and structurally or leverage the coherent properties unique to strongly coupled states. LBL assembly is a versatile technique for integrating a variety of optically active materials such as colloidal quantum dots and fluorescent polymers and other J-aggregating dyes into the strongly coupled devices reported here. Moreover, the LBL process provides nanometer scale thickness control (~1.7 nm per polyelectrolyte/dye bi-layer), suggesting that this device can act as a platform for investigating fundamental physical phenomena such as non-radiative energy transfer and laser action in the strong coupling limit.[1] Bradley, et al., Adv. Mater. 17, 1881 (2005).[2] Tischler, et al., Phys. Rev. Lett. 95, 036401 (2005).[3] Connolly, et al., Appl. Phys. Lett., 83, 5377 (2003).
5:00 PM - GG13.4
Increasing the Rabi Splitting of Exciton-Polaritons in J-Aggregate Microcavity Systems.
M. Bradley 1 , Jonathan Tischler 1 , Yasuhiro Shirasaki 1 , Qiang Zhang 2 , Tolga Atay 2 , Arto Nurmikko 2 3 , Vladimir Bulovic 1
1 , MIT, Cambridge, Massachusetts, United States, 2 Division of Engineering, Brown University, Providence, Rhode Island, United States, 3 Department of Physics, Brown University, Providence, Rhode Island, United States
Show AbstractIn this study, we demonstrate the increase of Rabi splitting (ΩR) in J-aggregate exciton-polariton systems in order to diminish the overlap of the exciton-polariton energies with uncoupled J-aggregate states. Thin films of cyanine dye J-aggregates enable the observation of strong coupling between light and matter at room temperature due to their high absorption constant and narrow linewidth, potentially allowing for the development of an entirely new class of optoelectronic devices.[1,2] Recent developments in strong coupling in inorganic systems have demonstrated Bose-Einstein condensates (BECs) of exciton-polaritons at cryogenic temperatures, and BECs could potentially be formed even at room temperature, where an exciton-polariton laser was recently demonstrated.[3,4] Recent theoretical studies have suggested that overlap of exciton-polariton energies with uncoupled J-aggregate states could act as a significant source of decoherence in the system, and some experimental efforts have been made to characterize the exciton-polaritons in organic systems.[5-7] We show that by increasing the number of layer-by-layer (LBL) dip coating cycles that substrates undergo from 4.5 to 8.5 sequential immersions in cationic polyelectrolyte and anionic cyanine dye solutions, the Rabi splitting in the finished λ/2n microcavity device can be increased from ΩR~125 meV to ΩR~230 meV. Additionally we probe the dispersion and linewidths of the exciton-polariton states through the use of organic emitters deposited inside the cavity spacer material. We demonstrate that using organic emitters, such as aluminum tris-quinolate (Alq3), in the spacer allows for optical pumping of the exciton-polariton modes from inside the cavity. We show the lifetime of the luminescence from the organic spacer material via the exciton-polariton modes. The LBL method for depositing J-aggregate thin films provides for the deposition of high density J-aggregate thin films with nanoscale thickness precision, allowing one to tune the strong-coupling properties of exciton-polariton devices utilizing J aggregates.[1] Bradley et al., Adv. Mater. (2005).[2] Tischler et al., Phys. Rev. Lett. (2005).[3] Balili et al., Science (2007).[4] Christopoulos et al., Phys. Rev. Lett. (2007).[5] Agranovich et al., Phys. Rev. B (2003).[6] Song et al., Phys. Rev. B (2004).[7] Savvidis et al., Phys. Rev. B (2006).
GG14: Poster Session: Nanostructures: Plasmon Excitations and Other Optical Phenomena
Session Chairs
Friday AM, November 30, 2007
Exhibition Hall D (Hynes)
9:00 PM - GG14.1
Nano-particle Chains Coupled with Metal-dielectric Surfaces: Wave Guidance, Radiation, and Scattering.
Derek Van Orden 1 , Viltaliy Lomakin 1
1 Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, California, United States
Show Abstract9:00 PM - GG14.10
Metallic Nanowire Optical Antennas.
Edward Barnard 1 , Mark Brongersma 1
1 Materials Science & Engineering, Stanford University, Stanford, California, United States
Show AbstractA combined near-field and far-field study of the optical properties of metallic nanowire antennas is presented. These wires support resonant, surface plasmon excitations that enable concentration of light into deep sub-wavelength volumes. We have produced 50 nm diameter silver and gold nanowires of varying length using a template-assisted electrochemical process. We also have fabricated segmented wires with alternating Au and Ag regions using the same technique. After growth the template was dissolved and the nanowire suspension was then spun onto a glass substrate for optical characterization. The segmented wires were also used to generate antenna structures with nanoscale feed gaps by selectively etching wire segments. The size dependent optical properties of the wires were investigated using a dark-field optical microscope. Near-field scanning optical microscopy shows electromagnetic field concentration near the antenna surface and in the feed gap. The optical images were compared to full-field electromagnetic simulations. The results of this study could impact the design of templates for surface-enhanced Raman and non-linear spectroscopy as well as the work on nanoscale photodetectors and emitters.
9:00 PM - GG14.11
Near-field Characterization of Extraordinary Optical Transmission in Sub-Wavelength Aperture Arrays.
Aaron Lewis 1 2 , Michael Mrejen 2 1 , Hesham Taha 2 1 , Abraham Israel 2 1 , Mila Palchan 2
1 , Nanonics Imaging Ltd., Jerusalem Israel, 2 Applied Physics, Hebrew University of Jerusalem, Jerusalem Israel
Show AbstractOver 20 years ago Lewis and coworkers studied the transmission of light through sub-wavelength aperture arrays . A variety of aperture dimensions were studied down to 15 nm and light was detected through even the smallest apertures. Even fluorescence of pyrelene was reported through such aperture arrays. In 1998 Ebbesen and coworkers suggested that the extraordinary transmission (EOT) of aperture arrays was due to the excitation of surface plasmons in metals with surface states that could support such plasmonic propagation . In the intervening years since the report of Ebbesen et al there have been numerous studies on the far-field and spectral characteristics of the transmission of these arrays. However, there have been no reports on the nature of the distribution of light in the near-field on the surface of the array and on the alteration of this distribution as a function of distance from the surface. Such characterization is of course crucial for understanding the possible applications for such arrays.In this work, extra ordinary transmission through arrays of sub-wavelength apertures has been investigated using near-field scanning optical microscopy. For such studies arrays were fabricated to give maximum resonance enhancement of light transmission at the wavelength of illumination that was used (532 nm). To define this enhancement a design was employed that allowed in one field of view of a near-field image the investigation of single apertures of dimension that was similar to what was incorporated into the sub-wavelength hole array. Significant asymmetry in the transmission and the propagation of the light along the aperture array was detected. This non-uniformity could be explained by polarization of the incident light, edge effects and the geometry of the array. The results support a hypothesis of both enhanced transmission due to surface plasmons and a non-diffracting beaming as a function of distance effect in the propagation of the light from the array.
9:00 PM - GG14.12
Enhancement of Light Transmission Through Nanoslit in a Circular Metallic Film Flanked by Parallel Periodic Grooves.
Dinesh Kumar 1 , Kiyoshi Asakawa 1
1 Center for TARA, University of Tsukuba, Tsukuba Japan
Show AbstractThe directional beaming properties of subwavelength metallic aperture patterned with periodic corrugation make them interesting candidates for variety of optical devices. Due to excitation of surface plasmons, the light flux per unit area through the aperture is 5-10 times greater than the flux per unit area of the incident beam [1]. However the total transmittance of the reported structures is poor. It is important to enhance their transmittance for efficient utilization in light emitting devices such as optical nano-antenna (ONA) [2]. We propose a new type of ONA composed of a slit groove circular (SGC) structure for effective light coupling axially from a single mode (SM) fiber to an air bridge type two-dimensional photonic crystal waveguide (2DPC-WG) [3]. We report two different schemes for enhancing the total transmittance of such structures: (1) groove tapering and (2) varying the thickness of dielectric layer. The structure consists of a circular Ag film with a diametric nanoslit flanked by parallel periodic grooves of varying lengths. Different from the reported nano-antenna, we use the nanoslit instead of nano-circular hole because the cross-section of the 2DPC-WG is of thin rectangular shape. The important parameters of the structures are: Ag film thickness 300nm, radius 3 μm, groove depth 60nm, groove periodicity 400nm, duty cycle 0.5 and slit width 40nm. The SGC structure is sandwiched by two silicon (Si) thin films resulting in a stable membrane exhibiting plasmonic excitation and light emission in infrared region. We illuminate the SGC structure by a SM fiber in the FDTD simulation and found that its transmittance and reflectance in the Si medium are respectively 14 % and 33 % at λ = 1.7 μm. The output beam is narrow and highly directional in far field region. As a new result, tapering the depth of grooves uniformly towards the slit on input plane minimizes reflection and improves the total transmittance. For a taper index 0.82, defined as the ratio of depth of two consecutive grooves, transmittance improves to 24 % and reflectance reduces to 18 %. We observe that the variation of dielectric layer thickness on input plane of the structure has resonant effect on its peak transmission. The transmission varies periodically with layer thickness in a limited range. For a thickness 900nm of input side Si layer transmittance was found to be 21 %. With groove tapering index 0.82, the total transmittance increased to 32 % and reflectance reduced to 15 %.Tapering other parameters such as groove width etc., might further improve the transmission properties of slit groove structures in metal film. These investigations are underway and we expect enhanced directional transmission of SGC structure suitable for axial coupling of light from the SM fiber to the 2DPC-WG.[1] H. J. Lezec, et al, Science, 297 (2002) 820-822. [2] Tsutomu ISHI, et al, JJAP, 44 (2005), L 364–L 366. [3] K. Asakawa, et al, New J. Phys, 8 (2006) 208, 1-26.
9:00 PM - GG14.13
Single-Photon Emission from a Single Perylene Diimide Nanocrystal.
Sadahiro Masuo 1 2 , Akito Masuhara 3 , Mai Muranushi 1 , Shinjiro Machida 1 , Hitoshi Kasai 3 , Hidetoshi Oikawa 3 , Akira Itaya 3
1 Department of Macromolecular Science and Engineering, Kyoto Institute of Technology, Kyoto Japan, 2 , JST-PRESTO, Kawaguchi Japan, 3 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai Japan
Show AbstractThe advent of photon-based quantum cryptography and quantum information processing is increasing the need for such light sources that produce individual single photons. It is of particular importance that these single photons are produced in as controlled a manner as possible. The ultimate goal is to have completely characterized single photons, which are produced on demand. Single-photon emission has been demonstrated using a variety of quantum dots, defects in diamond, trapped atoms and ions in a cavity. Emission from a single molecule is also typical single-photon source because the molecules cannot emit two photons simultaneously. Generally, single-photon emission can be observed from so-called “single quantum systems”. However, it is considered that the multi-quantum systems, namely, multichromophoric systems can also be single-photon sources if one exciton remains as a result of exciton-exciton annihilation among the generated excitons. In this work, we demonstrate for the first time that the emission from a single nanocrystal consisting of organic dye molecules, namely, N,N’-bis(2,6-dimethylphenyl)- 3,4,9,10-perylenedicarboxyimide (DMPDI), shows photon antibunching. That is, even molecular assemblies can also be made to behave as single-photon sources by controlling the size at nanometer scale. DMPDI was synthesized according to a standard protocol, and its nanocrystals were prepared from a tetrahydrofuran solution by the reprecipitation method. The size of the prepared nanocrystals was estimated from a SEM image. For the photon antibunching measurement of single nanocrystals, a classical Hanbury-Brown and Twiss type photon correlation set-up in combination with pulsed laser excitation was applied. By combining the photon correlation measurement with the excitation laser-triggered TCSPC technique, the time traces of the emission intensity and lifetime, and the photon correlation were measured simultaneously. In the SEM image of the prepared nanocrystals, square- or rectangular-shaped nanocrystals are observed clearly, which suggests strongly that the nanocrystals are in crystalline state. This was confirmed by the X-ray diffraction measurement. The size of the long-axis ranges from 35 to 85 nm, and the average size is 50 nm. The single nanocrystals on a substrate were measured by above set-up. As a result, it was revealed that the single nanocrystals within a size range from 35 to 85 nm show photon antibunching even when more than one exciton in a crystal is generated by an intense single excitation pulse. On the other hand, single micrometer-sized crystals showed no antibunching. The present results indicate that molecular assemblies can also be considered as candidates for new single-photon sources.
9:00 PM - GG14.14
Composite Droplets: Evolution of InGa and AlGa Alloys On GaAs (100).
Kimberly Sablon 1 , Zhiming Wang 1 , Gregory Salamo 1
1 MicroEP, University of Arkansas, Fayetteville, Arkansas, United States
Show AbstractThere have been pioneering research efforts in the area of semiconductor nanostructures worldwide. As we continue to explore the playing ground of the formation of semiconductor nanostructures, one compelling investigation is that of elemental droplets for the formation of composite alloys using molecular beam epitaxy (MBE). Therefore, we present a comparative study for the evolution of utilizing Indium Gallium (InGa) and Aluminum Gallium (AlGa) alloys fabricated on GaAs (100) by means of simultaneous and sequential droplet formation. The composite alloys reported using the sequential approach lacks in its ability to precisely determine final alloy composition as well as consistency in the density of the droplets. Further, it results in a mix of In and Ga to form InGa and some In and Ga that is not uniform. Although this approach may be acceptable to materials that are deposited in equal proportions like in the case of AlGa, it is not suitable for materials such as InGa. This investigation reveals that the simultaneous approach for fabricating composite alloys is the optimum approach for producing InGa alloys with better control on composition so as to achieve better capabilities for tuning bandgaps or wavelengths for InGaAs lasers.
9:00 PM - GG14.16
Nitrogen δ-Doping in GaP (001) and Single-Photon Emission from Individual Nitrogen Pair.
Yoshiki Sakuma 1 , Michio Ikezawa 2 , Yasuaki Masumoto 2
1 , National Institute for Materials Science (NIMS), Tsukuba Japan, 2 Institute of Physics, University of Tsukuba, Tsukuba Japan
Show Abstract A stable solid-state single-photon source is highly required for several applications of quantum information technologies. Although clear single-photon emission has been realized in a variety of quantum dots (QDs), they suffer a serious problem of tuning wavelength of photons precisely due to the inherent broad inhomogeneous linewidth of QD ensemble. Lately, single-impurity centers in bulk semiconductors have attracted renewed attention for solving this problem because they provide accurately defined states in energy gap. Here, we report δ-doping techniques of nitrogen (N) atoms in gallium phosphide (GaP) and a good evidence of single-photon emission from a pair of N atoms for the first time. Nitrogen is known to be an optically active isoelectronic impurity in GaP. When N atoms substitute the phosphorus (P) sites, excitons bound to NNi pairs and isolated N atoms (i.e. NN∞) show a series of sharp emission lines at low temperatures, with varying energy positions dependent on the distance between two neighboring N atoms. To spatially distinguish the excitonic luminescence from each localized impurity state with a microscope objective, we focused on NNi pairs in a δ-doped layer with a surface doping density ([N]2D) around 1011 cm-2 or less. This leads to the surface density of any NNi pair, [NNi]2D, less than 1 μm-2. Since [NNi]2D is proportional to [N]2D2, the precise control of δ-doping is needed. The samples were grown at 650°C by a low-pressure MOCVD on GaP (001), using triethylgallium (TEGa) and phosphine (PH3). After growing buffer layers, dimethylhydrazine (DMHy) was supplied as N source and then GaP capping layers were grown. We found that one of the convenient ways of controlling the [N]2D is to vary the duration of PH3 pulse (tPH3) between DMHy supply and GaP capping, during which a portion of the adsorbed N atoms seem to be replaced by P. Secondary-ion mass spectroscopy measurement showed that [N]2D in the δ-doped layer can be controlled in the range from 5.0×1010 cm-2 to 5.0×1011 cm-2 by varying tPH3. Micro-photoluminescence spectra at 30K, selected from several different locations on the sample with a δ-doped layer of [N]2D ~1.3×1011 cm-2, showed appearance and disappearance behavior of excitonic lines from NNi (i=3-6), indicating the presence of single NNi pairs within the observation area of objective. Finally, a photon correlation measurement using Hanbury Brown and Twiss setup revealed a distinct photon antibunching for a NN4 peak, proving the single-photon emission. Thus, since the inhomogeneous linewidth from N isoelectronic impurities is very narrow, individual NNi pair in GaP is a potential candidate for an energetically defined, reproducible single-photon source.
9:00 PM - GG14.2
Surface Plasmon Resonance in Nanoscopic Hole Arrays.
Su Yeon Lee 1 2 , Se Gyu Jang 1 2 , Chul-Joon Heo 1 2 , Seung-Man Yang 1 2
1 Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon, Chungnam, Korea (the Republic of), 2 National Creative Research Initiative Center for Integrated Optofluidic Systems, KAIST, Daejeon, Chungnam, Korea (the Republic of)
Show Abstract Recently, surface plasmons or plasmonics have received great attention because of their potential applications such as surface plasmon resonance biosensors and surface enhanced Raman spectroscopy (SERS). Up to now, a number of methods using focused ion beam (FIB), nanoimprint lithography, and self-assembled colloidal template followed by electrochemical deposition of noble metals have been proposed to fabricate nanostructures with hole or void arrays, in which both surface and localized plasmons take place. Plasmon mode of nanostuctured metal films containing close packed hexagonal arrays of uniform voids could be controlled by film thickness and void diameter. Furthermore, these structures exhibited very promising SERS enhancements due to the strongly localized electromagnetic field. Although aforementioned methods are effective for fabricating the nanostructured metal films, the integration of these structures into simple, low-cost devices for chemical or biological sensing is still challenging. We report here a versatile method that can generate hierarchical patterns of metallic nanocavities. First, monolayer of monodisperse colloidal particles was spun-cast on polymer films, and the particles were partially embedded in polymer layer. After the removal of colloidal particles, nanostructures with hole arrays were created by metal sputter deposition. This method which is simple, cost-effective is able to integrate nanostructures with hole arrays into biosensors based on surface plasmon resonance.
9:00 PM - GG14.3
Enhanced OLED Performance via Surface Plasmon Resonances Induced by Various Metal Nanostrucutres.
Dong-Yu Kim 1 , Hong-Ju Park 1 , Seok-Soon Kim 1 , Doojin Vak 1
1 Dept. of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju Korea (the Republic of)
Show AbstractSurface plasmons (SPs) are charge density oscillations confined to metallic nanoparticles and metallic nanostructures. These oscillations can occur at the interface between a metal and a dielectric medium. The collective response of electrons in surface of conductor to optical fields, so-called surface plasmon resonance, strongly affects the spectroscopy of nearby molecules. The frequency and intensity of the SP absorption bands are characteristic of the type of material, and are highly sensitive to the size, size distribution, and shape of the nanostructures, as well as to the environments which surround them. Of great significance is the opportunity of tuning the wavelength of SPs, which is highly desirable for various applications of metallic nanoparticles such as single molecule detection using surface-enhanced Raman scattering and near-field microscopy. However, due to the quenching of emission by SPs in the light emitting device, only few attempts have so far been made for SPs to be applied for organic light emitting devices. Recently, it was reported that the presence metal thin film in the light emitting device can enhance the light emission. But several problems exist for metal nanoparticles or metal thin film to apply for organic light emitting devices (OLED). Examples of these problems would be phase separation and aggregation of metal nanoparticles. In our presentation, we overcome these problems and demonstrate enhanced photoluminescence intensity of organic fluorophores, as well as, enhanced OLED performance by using the coupling of the SPs, induced by the silver and gold nanostructures, with organic fluorophores. Metal nanostructures fabricated by thermal evaporation method, co-sputtering method and pulse-current electrodeposition method were located between the indium tin oxide substrate and hole tranport layer as a spacer, to prevent the quenching of emission. The thickness of this spacer layer affects the enhancement of emission ratios because SPs are evanescent waves that decay exponentially with distance from the metal surface. An OLEDs using various metal nanostructures fabricated by thermal deposition method and spin coating method. As a result, the maximum intensity of the electroluminescence of light emitting materials, included small molecules and conjugated polymers, enhanced compared to the reference sample. To find out the mechanism for the enhancement, we measured emission lifetime by Time-correlated single photon counting method. The enhanced strength of the emission is due to the increased radiative rates for emitting molecules near the metal nanostructures, when the wavelength of the modified surface plasmon resonances overlaps with the emission wavelength. Thus, efficient coupling of SPs to the emitters could take place. Our results suggest a simple way of improving efficiency of various OLEDs by introducing simple metallic nanostructures without disturbing common basic device architectures.
9:00 PM - GG14.4
Surface Plasmon Structures for Surface-Enhanced Raman Scattering.
Muhammad Khan 1 , He Huang 2 , Shanker Balasubramaniam 1 , Timothy Hogan 1
1 Electrical & Computer Engineering, Michigan State University, East Lansing, Michigan, United States, 2 Physics and Astronomy, Michigan State University, East Lansing, Michigan, United States
Show AbstractSurface-enhanced Raman scattering (SERS) can enhance the intensity of Raman radiation by many orders of magnitude for molecules bound to metallic nanostructures. SERS is generally thought to have been a result of excitation of surface plasmons in nanostructured metal, which greatly enhances local electric field experienced by the molecule. Common SERS substrates fabricated using noble metal colloidal, electrochemically roughened thin films, and nano-sphere lithography suffer from lack of homogeneity where only few hot spots yield high enhancement. We explore semiconducting nanowires and metallic nanorods as an economical, stable, and uniform SERS substrate for the detection of trace amount of chemicals and bio-species. We utilize bulk synthesized semiconducting nanowires as nano-scale structures that can be coated with noble metals or their colloidal forms to allow for excitation of surface plasmons over broad frequency range. The use of nanowires as SERS substrates has several advantages. The surface properties of these nanowires are highly reproducible and well defined as compared to other systems like colloidal aggregates, electrochemical roughening etc. The synthesized nanowires offer many unique features (sharp vertices, noncircular cross-sections, inter-wire coupling) that may lead to larger field enhancement factors. High density of nanowires means close interaction between adjacent nanowires, which enables SERS to manifest for a broad selection of excitation sources. We have synthesized germanium dioxide and zinc oxide nanowires using the vapor-liquid-solid growth mechanism utilizing a simple quartz tube furnace set up. The nanowires are grown either using thin gold film (5-15 nm) or colloidal gold (20 nm or 60 nm) as catalyst on substrates of silicon, quartz, and alumina. The resulting nanowires are dense (100-300 nm diameter, 10-40 μm long) and randomly distributed on the substrate. The nanowires are subsequently coated with thin films (10-15 nm) of noble metals (gold and silver) that provide surface plasmons active surface. We have also investigated silver nanorods on glass formed by grazing angle deposition using e-beam evaporation. These nanorods have a diameter of ~ 50-70 nm with lengths averaging 300-400 nm. These well aligned high aspect ratio and dense structures are ideal for excitation of surface plasmons. The synthesized structures have been characterized using SEM, TEM, and EDS. The SERS studies were conducted using EzRaman-L system from Enwave Optronics, Inc. Silver and gold-coated nanowires have been found to exhibit significant Raman enhancement for micro-molar concentrations of Rhodamine 6G and Nile blue. The silver nanorods yield stronger enhancement with good uniformity across the surface of the substrate and are promising candidates for SERS applications.
9:00 PM - GG14.5
Conjugated Polymer Emission and the Proximity of Long- and Short-range Surface Plasmon Modes.
Matthew Ng 1 , Graeme McPhee 1 , Paul Stavrinou 1 , Donal Bradley 1
1 EXSS, Physics Department, Imperial College London, London United Kingdom
Show AbstractConjugated polymer and metal interfaces are ubiquitous in many of today’s organic optoelectronic devices. Identifying the key physical attributes including optical effects on device performance from such interfaces is quite a challenging but potentially rewarding area. One difficulty is how to distinguish between conformational changes and electromagnetic effects since both have significant influence over the emission and absorption properties in a device [1, 2].In terms of conformational changes, conjugated polymer thin films exhibit interesting behaviour at precisely the thicknesses (<160nm) that are often employed in devices [1]. Of particular interest is the balance between radiative and non-radiative decay rates as the film thicknesses are varied. When a metallic surface is introduced, e.g., as an electrode, new non-radiative decay modes have to be considered. For thin metal films, emission from the polymer may efficiently couple to long- and short-range surface plasmon modes. Indeed, if the polymer film and additional layers comprising a device structure are thin in comparison to the wavelength this effect can be far more significant than interference effects. In the present work, the dependence of emission properties on metal film and polymer thicknesses are examined. Studies on bare polymer films are also used in conjunction with the metallic structures. For bare polymer films the excited state lifetime displays an almost linear dependence with film thickness. Similar overall trends are observed in the presence of the metallic surface. However when the polymer film is adjacent to a metallic surface, surface plasmon modes can present fast non-radiative decay channels. Consequently, the lifetime is further reduced. Surface plasmon modes are not the only fast mechanism available, but by varying the metal film thickness the precise influence of the surface plasmon modes can be identified. Further studies on the coupling between conjugated polymers and surface plasmon modes have examined the situation when a metal layer is sandwiched between two polymer layers. When the metal layer is thin, the surface plasmon modes associated with each polymer/metal interface may couple together. The coupled modes may then enable the transfer of energy from the donor layer to the acceptor layer [3]. Our results show some evidence for this energy transfer across metal thicknesses (around 5-15 nm) much greater than typical Förster radii (around 2-6 nm). As the metal thickness was increased (to >50 nm) the effects were no longer observed. [1] M. Campoy-Quiles et al., J Macromolecules, 39, pp. 7673-7680, 2006.[2] H. Becker et al., Phys. Rev. B, 56, pp. 1893-1904, 1997.[3] P. Andrew et al., Science, 306, pp. 1002-1005, 2004.
9:00 PM - GG14.6
Surface Plasmon Resonances in Nano-metallic Particle Arrays, and Their Sensitivity to Superstrate Layers.
Martina Mohr 1 , Matthew Ng 1 , Paul Stavrinou 1 , Donal Bradley 1
1 EXSS, Physics Department, Imperial College London, London United Kingdom
Show AbstractOptical extinction spectra and time resolved photoluminescence are used to examine a series of structures, which comprise nano-metallic particle arrays, waveguiding layers and polymer superstrate films. Extinction spectra from the structures are spectrally very rich and may be used to highlight an number of spectrally localized features arising from the various interacting mechanisms present in these structures. Regions of selective suppression and enhancement are both present and arise due to the coupling between surface plasmon resonances and waveguiding properties [1].The grating structures in the present study are based on sub-wavelength gold particles, with different diameters (100 to 120 nm) and varying spatial periods (260 to 420 nm), patterned by e-beam techniques onto an ITO waveguiding layer. Various materials were used for the superstrate layer, and typically involved either air (i.e., the bare grating) or spin-coated polymer films, which included conjugated light-emitting polymer films.Extinction spectra recorded from the bare grating structures show a characteristic surface plasmon resonance peak and waveguide dip. In agreement with existing reports [1, 2] we find the spectral positions of these features may be controlled through changing the physical grating parameters or, in the case of diffractive-based phenomena, changing the angle of incidence. Further spectral changes are expected when the superstrate material is varied – particularly in the case of surface plasmon resonances. Using a number of polymer materials, the resulting extinction spectra show clear red shifts of the main features in comparison with the response from a bare structure. In the case of the plasmon resonance we find the red shift is significantly larger than that found for the waveguide dip. This behaviour reflects the strong surface nature of the plasmon resonance and its sensitivity to the optical properties of the superstrate layer. Examining several polymer materials, where the refractive indices have previously been assessed using ellipsometry, we find the red-shift of the plasmon resonance follows an almost linear relationship with the change in refractive index of the films (around 14 nm for an index change of ~0.1). This suggests that such structures would be suitable for sensor applications.[1] S. Linden et al., Phys. Rev. Lett. 86, pp 4688-4691 (2001).[2] N. Felidj et al., J. Chem. Phys. 123, 221103 (2005).
9:00 PM - GG14.7
Nanoscale Optical Transport in Sub-wavelength Plasmon Chains.
Femius Koenderink 1 , Rene de Waele 1 , Albert Polman 1
1 Center for Nanophotonics, FOM Institute AMOLF, Amsterdam Netherlands
Show AbstractThe resonant optical response of plasmon materials may allow to overcome challenges in realizing subwavelength photonic devices that are due to the limited range of dielectric constants available in standard optical materials. The resonant optical response of free electrons provides plasmon particles with maximum polarizability in a small volume. By arranging nanoparticles in closely spaced arrays, resonances of adjacent particles can couple, thereby providing a channel for nanoscale optical guiding. Thus nanoparticle arrays provide a route to waveguides of sub-wavelength width, that are easily fabricated in any arrangement by lithography. So far, experimental demonstrations of these effects are limited and theory mostly concentrated on quasistatic approximations.Here, we present experiments on Ag and Au nanoparticle arrays that are made using electron beam lithography on a silica substrate. Particle diameters range from 50 - 110 nm, and pitches are varied between 75 and 150 nm. Using a white light source (400-900 nm) we perform angle-dependent far-field polarization spectroscopy to determine the dispersion relation for longitudinal and transverse plasmon propagation along chains. The experimentally determined dispersion relation differs remarkably from the initial simple, quasistatic, calculations that were based solely on near-field coupling between particles. We present dispersion calculations that include coupling via far-field radiation and show this has a dramatic effect on the optical behavior of resonant arrays, even for array pitches as small as 50 nm. For transverse waves the dispersion relation is dominated by a polariton anticrossing of the array dispersion at the light line, near the middle of the Brillouin zone. The upper branch (above the light line) suffers large losses due to radiation. The lower branch on the other hand promises much lower loss than predicted in the quasistatic model, essentially because radiation loss is suppressed by destructive interference. As a result, the damping times for these chain modes are much longer than for isolated particle resonances. Furthermore the anticrossing is associated with a bandwidth for signal propagation that is two-fold enhanced compared to the quasistatic prediction. Since our experiments are in excellent agreement with the calculated dispersion, we provide the first proof that plasmon particle interactions are dominated by far-field coherent interactions, even for very small spacings. The calculated propagation lengths for modes below the light line are as large as 10 µm, demonstrating the potential of metal nanoparticle arrays for nanoscale light guiding in nanophotonic integrated circuits. In our presentation, we will also address the effect of disorder on propagation loss, and will present progress with near-field excitation and probing of Ag nanoparticle waveguides.
9:00 PM - GG14.8
Nanoscale Multi-element Optical Antennas for Localization and Directional Emission.
René de Waele 1 , A. Femius Koenderink 1 , Albert Polman 1
1 Center for Nanophotonics, FOM Institute AMOLF, Amsterdam Netherlands
Show AbstractWe show that metal nanoparticle arrays, composed of nanometer-sized plasmonic scatterers with subwavelength spacing, can act as nanoscale antennas for localization and redirection of light.Electron-beam lithography in combination with lift-off was used to fabricate arrays of 110 nm-diameter silver particles with 150 nm pitch on a glass substrate. The samples were embedded in an index matching fluid and then illuminated by a supercontinuum light source (450-850 nm) in a direction along the array axis. Light that was scattered from the arrays was collected using a scanning confocal microscope. We observe that the array response depends strongly on the frequency and direction of the incident light. For wavelengths below 650 nm, light scattering, and hence also the field concentration on the array, is concentrated on the first particle of the array; for longer wavelengths it is centered at the end of the array. Concentration of light on the particle array can thus be controlled by frequency. The bandwidth of the transition is 40 nm. Especially at frequencies where the wave vector of the plasmon wave on the array almost matches that of free photons the localized response becomes very sensitive to frequency. Possible applications in sensing will be discussed. The data are compared with a model that takes into account retardation along the array; excellent agreement is observed, especially regarding the frequency of the transition of light concentration along the array. Metal nanoparticle arrays are the nanophotonic analog of conventional radiowave antenna arrays, that consist of polarizable antenna elements with sub-wavelength spacing: these function as a directional receiver by concentrating a signal at a single dipole element on the antenna chain. An important difference with radio-wave antennas however is that the resonance properties of the nanoparticle antenna array are mainly due to the materials property (i.e. plasmon resonance frequency) of the scatterer material and not solely to shape and size.By reciprocity, the nanoparticle antenna arrays should also act as directional emitters of incident light, with the redirection tunable by frequency. Similary, nanoscale light sources (quantum dots, dyes) should show strongly directional emission when coupled to these arrays. Preliminary data will be shown.
9:00 PM - GG14.9
Radiative Surface Plasmon Resonant Coupling in a Linear Chain of Noble Metal Nanoshells.
Anatoliy Pinchuk 1 , George Schatz 1
1 Chemistry, Northwestern University, Evanston, Illinois, United States
Show Abstract