Multiple-quantum two-dimensional electronic spectroscopy (2DES) can provide direct access to
correlated electronic excited states, revealing their presence and their energetics and dynamics. In
the measurements, coherent light-matter interactions are used to generate multiple-quantum
coherences of biexcitons, triexcitons, etc., in which oscillations of the electronic charge
distribution take place at the multi-exciton frequency. The oscillations are observed by using
additional, time-delayed light-matter interactions to generate single-quantum polarizations that
radiate coherent signals whose amplitudes and phases are modulated as a function of the delay at
the multiple-quantum frequency. The modulation frequency and decay directly reveal the multiexciton
energy and dephasing rate. We developed a method for conducting multiple-quantum
measurements using spatiotemporal femtosecond pulse shaping through which multiple light
beams and multiple pulses can be directed to a sample with full phase coherence among all the
light fields, all controlled by spatial light modulators without the need for interferometric
measurement or feedback [1]. Our initial measurements showed the presence of biexcitons [2] and
triexcitons [3] in GaAs quantum wells and indicated that no higher-order multi-exciton
correlations were present in significant densities. Subsequent measurements in quantum well
microcavities [4] revealed quad-exciton-polaritons, whose high-order correlations were mediated
through the light field so that physical proximity of the electronic excitations was not necessary.
These measurements have been extended into the regime of Bose-Einstein condensation (BEC),
which we explored through separate cw studies as well. Finally, we have observed biexcitons in
Cu2O which are likely responsible for the absence of BEC in that material.
1. “Invited Article: The coherent optical laser beam recombination technique (COLBERT) spectrometer:
Coherent multidimensional spectroscopy made easier,” D.B. Turner, K.W. Stone, K. Gundogdu, and K.A.
Nelson, Rev. Sci. Instrum. 82, 0813301 (2011).
2. “Two-quantum 2D FT electronic spectroscopy of biexcitons in GaAs quantum wells,” K.W. Stone, K.
Gundogdu, D.B. Turner, X. Li, S.T. Cundiff, and K.A. Nelson, Science 324, 1169-1173 (2009).
3. “Coherent measurements of high-order electronic correlations in quantum wells,” D.B. Turner and K.A.
Nelson, Nature 466, 1089-1092 (2010).
4. “Influence of multi-exciton correlations on nonlinear polariton dynamics in semiconductor
microcavities,” P. Wen, G. Christmann, J. J. Baumberg, and K. A. Nelson, New J. Phys. 15, 025005 (2013).

ABSTRACT
Singlet fission has now been observed in a number of molecular systems. However, details regarding the static and dynamical factors governing the process are only beginning to emerge. In this talk, our recent efforts utilizing pump-probe and two-dimensional electronic spectroscopy to investigate the mechanism of singlet fission in nanoparticles of several pentacene derivatives will be described. We find that pentacene derivatives are versatile singlet fission chromophores capable of undergoing singlet fission in the presence of extensive structural disorder. Nanoparticles comprised of weakly coupled chromophores, for example, exhibit non-monoexponential singlet fission kinetics predominantly associated with the migration of energy to sites where singlet fission occurs. In contrast, nanoparticles consisting of more strongly coupled, excitonically shifted chromophores exhibit near monoexponential singlet fission kinetics with rate constants approaching the adiabatic limit. The role of excitonic delocalization in facilitating the highest observed singlet fission rates will be discussed. The nature of vibrational coherences in two-dimensional electronic spectroscopy experiments will be described, and monitoring their time evolution is hypothesized to provide additional insight into the mechanism of singlet fission.

Solution processed organic-inorganic perovskite solar cells are presently the forerunner in the next generation photovoltaic technologies, with power conversion efficiencies approaching 20%. In this talk, I will review the developments in this field and distil the recent findings on the photophysical mechanisms of this remarkable material. I will also highlight some of our latest charge dynamics studies and other investigations on the novel properties of this amazing material.

Hybrid halide perovskites (eg, CH₃NH₃PbI₃) facilitate high power conversion efficiencies in a varierty of solar cell architectures^1,2. The existence and stability of bound electron-hole pairs (excitons) in these materials, and their role in the exceptional photovoltaic performance, remains a controversial issue³. Here we demonstrate, through a combination of femtosecond transient absorption spectroscopy and multiscale modeling as a function of crystal size and temperature, that the electron-hole interaction is sensitive to the microstructure of the material.
We find that by the control of the material processing during fabrication both free carrier and Wannier excitonic regimes are accessible, with strong implications for optoelectronic devices. We elucidate the key role played by the organic cation that creates a dipolar field within the lattice and thus influencing the Coulomb interaction between the electron and hole, eventually dictating the stability of the exciton. We demonstrate that the long range order of the dipole field is disrupted by the polycrystalline disorder that introduces domain walls where dipole twinning breaks down. The resultant variations in electrostatic potential found for smaller crystallites suppresses exciton formation, while larger crystals of the same composition demonstrate an unambiguous excitonic state that is populated in ultrafast timescales.
In addition, within the free carrier regime, we also observe a reduction in the dielectric permittivity of the material after the photo-excitation. We attribute this effect to the saturation of the molecular dipole response induced by the polaronic effects (a result of Frohlich electron-phonon coupling) as the carrier concentration is increased. We show that such a reduction in the dielectric constant results in the quenching of the excitonic screening and thus favouring the formation of a photo-induced meta-stable excitonic transition.
References:
1. Im, J.-H., Jang, I.-H., Pellet, N., Grätzel, M. & Park, N.-G. Growth of CH3NH3PbI3 cuboids with controlled size for high-efficiency perovskite solar cells. Nat. Nanotechnol. (2014). doi:10.1038/nnano.2014.181
2. Ball, J. M., Lee, M. M., Hey, A. & Snaith, H. J. Low-temperature processed meso-superstructured to thin-film perovskite solar cells. Energy Environ. Sci.6, 1739 (2013).
3. D&’Innocenzo, V. et al. Excitons versus free charges in organo-lead tri-halide perovskites. Nat. Commun.5, 3586 (2014).

Solution processing is required for the large-scale manufacture of small-molecule organic semiconductors, and can result in crystalline domains with high charge mobility. However, the interfaces between these domains impede charge transport, degrading device performance. Although understanding these interfaces is essential to improve device performance, their intermolecular and electronic structure is unknown: they are smaller than the diffraction limit, are hidden from surface probe techniques, and cannot be directly resolved using X-ray methods. We have used transient absorption microscopy to inspect a drop-cast thin film of 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene, a material cited for high hole mobility and singlet fission. The crystal and electronic structure of the domains has been characterized, but analogous information for the domain interfaces is unknown. Using a judicious selection of light polarization, we isolate a signal at the interface that is not observed in either of the adjacent bulk domains, exposing the exciton dynamics and inter-molecular structure of this hidden interface. Surprisingly, instead of finding an abrupt grain boundary, we reveal that the interface can be composed of nanoscale crystalline aggregates interleaved by a web of interfaces that compound decreases in charge mobility. The impact of doping and annealing on the interface dynamics will also be reported.

Indium phosphide (InP) quantum dots (QDs) are attractive for hybrid organic/inorganic solar cells due to their excellent electronic properties and widely size-tunable absorption in the visible range.^1-2 The interfacial energy between InP and low-bandgap polymers like P3HT is ideal to form type-II hybrid heterojunctions and promote charge separation.³ In this work we combine femtosecond transient absorption spectroscopy with density functional theory (DFT) to characterize the effects of organic ligands and sizes (2.5 and 4.5 nm) on charge carrier photogeneration and transfer dynamics in a hybrid P3HT/InP QDs system. DFT calculations show that the LUMO energy of the QD-ligand complexes decreases by increasing the size of InP QDs and increases proportionally to the surface coverage of organic ligands. By substituting the original oleylamine stabilizing ligands with a pyridine capping layer, we observe notable quenching of the photoluminescence intensity of InP/P3HT thin films compared to pure P3HT and InP QDs films, an indication of efficient photoinduced charge transfer across the InP/P3HT heterointerface. Photoinduced absorption measurements confirm an instant creation of singlet excitons subsequently dissociate into polarons on an ultrafast time scale (t<1ps) and the yield of polaron formation is significantly enhanced by adding pyridine-InP(4.5 nm) QDs as electron acceptor. Fast electron transfer into the InP QD acceptors is in good agreement with the strong coupling (~15.2 meV) between the lowest unoccupied molecular orbitals of P3HT and InP QDs in the ground state predicted by DFT, demonstrating that our organic/inorganic model accurately captures the key charge transfer dynamics. The yield of polaron formation is found to decrease in magnitude with deceasing QD size from 4.5 nm to 2.5 nm. In 2.5 nm QDs, electron injection is inefficient but reverse energy transfer takes place due to large overlap between absorption of P3HT and QD emission. Thus, both InP QD size and length of the organic ligands can be used as tuneable parameters to optimize performance of hybrid polymer/InP QDs solar cells.
References
1. Xie, R.; Battaglia, D.; Peng, X. J Am Chem Soc, 2007,129, 15432.
2. Dung, M. X.; Mohapatra, P.; Choi, J. K.; Kim, J. H.; Jeong, S.; Jeong, H. D. B Korean Chem Soc, 2012,33, 1491.
3. Selmarten, D.; Jones, M.; Rumbles, G.; Yu, P. R.; Nedeljkovic, J.; Shaheen, S. J Phys Chem B, 2005,109, 15927.

Photo-initiated charge-transfer processes play a central role in natural systems such as human vision and photosynthesis. While researchers have successfully modified these processes to control simple isolated systems, our understanding of photon-to-electronic mechanisms in realistic and complex environments is still in its infancy. In particular, recent experiments have shown that simple descriptions of solvent interactions (either via classical force fields or effective solvent models) are unable to accurately capture the electron dynamics in even relatively simple systems. These ongoing observations open an entirely new field of research in the properties of light-activated processes in complex environments, with the opportunity to deeply understand the real-time electron dynamics between complex interfaces.
To this end, we have developed a new real-time time-dependent density functional tight binding (RT-TDDFTB) approach to calculate the electron dynamics of donor-acceptor complexes in the presence of explicit solvent molecules - all treated at the quantum mechanical level. Our approach significantly differs from previous linear-response TD-DFT methods in that we directly propagate the one-electron density matrix in the presence of a non-perturbative external field. Furthermore, and most importantly, our implementation in the TD-DFT code allows us to calculate the electron dynamics of large solvated systems (~10,000 atoms), whereas conventional approaches are computationally limited to only hundreds of atoms. Using this new capability, we are able to understand and rationalize electron-hole recombination effects as a function of solvent polarity, configuration, and energy transfer. Furthermore, this new capability gives us mechanistic insight into the electron dynamics of new systems in complex environments with the goal of guiding experiments in the exploration of charge-transfer dynamics driven by time-dependent external fields.

Exciton diffusion in molecular aggregates, films or crystals is mainly controlled by the excitonic coupling between the electronic excited states localized in the molecular units. The excitonic coupling can be decomposed into different short-range (exchange, charge-transfer mediated and overlap) and long-range (Coulombic) contributions. The Coulombic term is the only term effective when the molecules are not in contact and this used for example in the Förster theory of excitation energy transfer. However, in organic crystals, which are held together by weak non-covalent intermolecular interactions, there is a large number of low frequency intermolecular vibrations that cause a relatively large displacement of one molecule respect to their neighbors (dynamic disorder). In this context, short intermolecular contacts can be present and the short-range excitonic interactions may be of great relevance to provide a proper description of the modulation of the excitonic coupling owing to the combined thermal motions of the nuclei.
In this contribution, we assess the magnitude and timescale of the fluctuation of the excitonic coupling due to nuclear thermal motions. To do so, we adopt a combined molecular dynamic and quantum chemical approach, and we derive a general diabatization scheme to compute excitonic couplings, in which both short and long-range effects are accounted for. We present the results for the two archetype anthracene and tetracene molecular crystals, which have been widely studied in the context of organic electronics. We show that the inclusion of short-range excitonic interaction is responsible for the large fluctuation of the excitonic coupling introduced by the thermal motions. The large fluctuation of the excitonic coupling in organic crystals is essential for the proper description of exciton transport in molecular materials.

Semiconductor nanowires are featured in many proposed optoelectronic devices, including photovoltaics and photodetectors. The dynamics of photoexcited carriers, which critically affect device performance, in these devices can be affected by the size, shape, surface passivation and crystalline structure of the nanowires. Variations of these parameters between nanowires in ensembles studied using macroscopic optical probes can obscure their influence on carrier trapping and recombination rates. We report here correlated, sub-micron resolution mapping of the photoluminescence (PL) and ultrafast carrier dynamics along the lengths of individual CdS nanowires produced by physical vapor deposition. Diameters of the nanowires ranged from 50 nm to 150 nm. PL mapping showed significant variations in intensities of emission at the band edge and emission below the bandgap energy attributed to defects. Ultrafast carrier dynamics were characterized by broadband transient reflection (TR), using the same microscope system as the PL measurements. Time resolution, limited by the lengthening of pulses due to dispersion in the microscope objective, was ~ 250 fs. TR spectra exhibit a peak at the bandgap energy that is well fitted by a bi-exponential decay function, and a broad peak at energies below the gap. The bandgap peak is attributed to population of excited states, and decays with fast (< 10 ps) and a slower time constants (~100 ps). The lower energy peak can arise from population of defect states, or from free carrier absorption and is well described by a single exponential decay in the 100 ps range. Lifetimes of both bands varied significantly along the length of the nanowire, with times well correlated with PL intensities, suggesting that carrier trapping plays a major role. Lifetime generally increased with increasing nanowire diameters, in accordance with expected results from trapping primarily at surface defects. Carrier diffusion lengths extracted from transient reflection measurements performed with spatially separated pump and probe beams also exhibit significant variation within individual nanowires.

We employ femtosecond time-resolved photoelectron spectroscopy to directly uncover the time-domain signatures of electron-boson interactions in a cuprate superconductor. We discover an abrupt decrease of the electron lifetime above a characteristic energy of 55 ± 8 meV. Remarkably, the lifetime at this energy is independent of excitation density. By fundamental considerations of electron-boson interactions, these behaviors are explained by coupling with a 55 meV mode which scatters nodal electrons to the nodal region in momentum space. Our experiment complements equilibrium photoemission by providing sensitivity to sub-dominant mode coupling, and suggests a pathway towards the full description of bosonic mode coupling in cuprates.

Coherent Multi-Dimensional Spectroscopy (CMDS) is a powerful interferometric four-wave mixing technique that that measures the phase and amplitude of the emitted signal using spectral interferometry. The technique produces multidimensional spectra, which correlate the light absorbed with the light emitted by the sample of interest, allowing the identification of electronic states that are coherently coupled, as well as which pathways are involved in energy relaxation.
We have developed a variation of this experiment that exploits the polarization selection rules of excitons in semiconductor quantum wells to isolate specific quantum pathways in multi-dimensional spectra. Using this approach we explore the mechanisms behind the coherent coupling of excitons localised to the same quantum well, as well as excitons localised to spatially separated quantum wells.

Graphene and carbon nanotubes provide a variety of new opportunities for fundamental and applied research. Here, we describe results of our recent terahertz and ultrafast studies of carriers and phonons in these materials. Time-domain terahertz spectroscopy is a powerful method for determining the basic properties of charge carriers in a non-contact manner. We show how one can modulate the transmission of terahertz waves through graphene by gating and how one can improve the modulation performance by combining graphene with apertures and gratings. In carbon nanotubes, we demonstrate that the terahertz response is dominated by plasmon oscillations, which are enhanced by collective antenna effects when the nanotubes are aligned. Finally, ultrafast spectroscopy of carbon nanotubes allow us to excite and probe coherent phonons, both in the low-energy radial breathing mode and high-energy G-mode, which are strongly coupled with excitonic interband transitions.

When two sheets of graphene stack, the interlayer electronic states hybridize, resulting in a prominent visible absorption peak that is tunable with the stacking angle. Recent first principle Bethe-Salpeter simulations of twisted bilayer graphene, predict that interlayer electronic orbitals interfere destructively to form stable enabling the formation of stable, strongly bound, exciton states [1]. While this excitonic model corroborates the experimental absorption lineshape observed [2], the actual existence of strongly-bound excitons in an otherwise metallic system remains controversial and untested.
We directly probe the electronic dynamics of twisted bilayer graphene for the first time by developing a unique three-tiered ultrafast confocal microscopy approach that combines (i) transient absorption [3], (ii) ultrafast photocurrent [3, 4] and (iii) transmission electron microscopies. This novel approach enables us to correlate diffraction limited electron relaxation dynamics with the precise local atomic composition of the twisted bilayer grain boundaries. Using these transient microscopy maps of multilayer graphene we definitively isolate the interlayer electronic dynamics. We find resonantly excited twisted bilayer regions display distinct, long-lived dynamics that are not present in 0^o stacked bilayers. We further map out the electronic structure using one and two-photon transient absorption microscopy to observe signatures of both bright and dark strongly-bound excitonic states predicted by many-body electronic calculations [1]. The probable existence of these stable excitons opens up the possibility of efficient carrier extraction by exploiting the unusual hybrid metallic-excitonic nature in twisted bilayer graphene systems.
References
[1] Y. Liang, R. Soklaski, S. Huang, M.W. Graham, R.W. Havener, J. Park and L. Yang, "New mechanism for strongly bound excitons in gapless two-dimensional structures” PRB: 90. 115418 (2014)
[2] R.W. Havener, L. Brown, Y. Liang, L. Yang, and J. Park, "Van Hove Singularities and Excitonic Effects in the Optical Conductivity of Twisted Bilayer Graphene," Nano Letters14, 3353 (2014)
[3] M.W. Graham, S. Shi, Z. Wang, D. C. Ralph, J. Park, and P.L. McEuen ,“Transient Absorption and Photocurrent Microscopy Show Hot Electron Supercollisions Describe the Rate-Limiting Relaxation Step in Graphene,” Nano Letters13, 5497-5502 (2013)
[4] M.W. Graham, S. Shi, D. C. Ralph, J. Park and P.L. McEuen, "Photocurrent Measurements of Supercollision Cooling in Graphene," Nature Physics9, 103-108 (2013)

We investigate the transient photoconductivity of graphene at various carrier densities by optical-pump terahertz-probe spectroscopy. A consensus is yet to be reached in the literature regarding origin and nature of the photoconductivity. Indeed, recently there have been several papers [1,2,3,4,5] contributing to the debate. The main observation in graphene is a photo-response that varies from semiconductor-like (positive photoconductivity) to metal-like (negative photoconductivity) as a function of increasing carrier concentration. There are a number of competing explanations which each explain some of the features observed: as arising from a competition between stimulated emission and induced absorption [1], or indeed through a competition between photo induced changes in chemical potential and scattering, described using either the Boltzmann transport equation [3,5] or a Modified Drude model [2,4].
In this contribution, we investigate the THz photoconductivity of several samples in which the mobility of the graphene has been altered by plasma etching. We demonstrate that, while the negative photoconductivity region displays a striking decrease with decreasing mobility, the positive photoconductivity region is only weakly dependent on the sample mobility. This suggests that the two photoconductivity regions have different behavioural origins. Comparison to existing conductivity models in the literature suggests that the positive photoconductivity does not arise because of additional carriers due to photoexcitation, as previously suggested [5], but because of a subtle interplay of the energy dependences of carrier scattering mechanisms.
[1] C. J. Docherty et-al, Nature Communications 3, 1228, 2012
[2] S.F Shi et.al, Nano Lett. 14, 1578-1582, 2014
[3] S.A Jensen et.al, Nano Lett. 14, 5839-45, 2014
[4] A.J Frenzel et.al, PRL 113, 056602 (2014)
[5] K. J. Tielrooij, Nature Physics,vol 9, 2013

Graphene exhibits intriguing electronic properties that originate from massless Dirac dispersion of electrons. A striking example is the half-integer quantum Hall effect (QHE) which endorses the presence of non-zero (π) Berry&’s (topological) phase associated with the Dirac cone. Then we can pose a question as to whether such an exotic character of Dirac electrons emerges in the optical responses or not. For example, it is unclear how such a QHE sustains in the optical response, because the topological protection that accounts for the quantized value of the Hall conductivity no longer exists in the AC responses. Recently, a progress has been made in the theoretical study based on the exact diagonalization method, and the emergence of a plateau-like behaviour in the Hall conductivity even in the Landau-level transition energy, i.e., in the terahertz (THz) frequency regime has been predicted both in a conventional 2-dimensional electron gas system in GaAs and in graphene.
In this talk, we will report on our recent experimental study on the optical quantum Hall effect. Experimentally, the AC (optical) Hall conductivity can be measured through the magneto-optical effect such as Faraday effect and Kerr effect. By combining the THz time-domain spectroscopy (THz-TDS) with the polarization sensitive detection scheme, we have investigated the optical quantum Hall effect or equivalently the quantum Faraday effect in a modulation-doped GaAs/AlGaAs single heterojunction and in a monolayer graphene epitaxially grown on a SiC substrate. In the GaAs/AlGaAs single heterojunction sample, a plateau-like structure is identified in the magnetic-field dependence of the Faraday rotation angle around the Landau-level filling of nu;=2, whereas the plateau-width was much narrower than that of dc regime. In contrast, the graphene sample exhibits a robust plateau-like structure and the quantum steps between the Landau-level filling of nu;=2 and nu;=6. The rotation angle was in the order of fine structure constant, and was consistent with the half-integer QHE.

Ideal topological insulators are bulk insulators with metallic surfaces that are protected by time reversal symmetry. However, in real materials residual bulk conductivity hinders the properties of the topologically protected surfaces and prevents their characterization by common transport measurements. Alternatively, terahertz (THz) spectroscopy has recently been shown to be capable of discriminating between the conductivity due to bulk and surface carriers. We use optical pump--THz probe spectroscopy at low temperatures to study the hot carrier response in thin Bi₂Se₃ films of several thicknesses, allowing us to separate the bulk from the surface transient response. We find that for thinner films the photoexcitation changes the transport scattering rate and reduces the THz conductivity, which relaxes within 10 picoseconds (ps). For thicker films, the conductivity increases upon photoexcitation and scales with increasing both the film thickness and the optical fluence, with a decay time of approximately 5 ps as well as a much higher scattering rate. These different dynamics are attributed to the surface and bulk electrons, respectively, and demonstrate that long-lived mobile surface photo-carriers can be accessed independently below certain film thicknesses for possible optoelectronic applications.

We report ultrafast electron diffraction studies on the lattice dynamics in a photoexcited topological insulator Bi₂Te₃ and a new finding of transient short-range lattice disordering in ~5 ps. With recent developments in ultrafast electron and x-ray sources, it is now possible to directly observe the atomic motions during the chemical reactions or phase transitions using pump-probe protocols. The new information forthcoming from this approach is ideally suited to study one of the most interesting new developments in in material science and solid state physics, i.e., the ultrafast electron and lattice dynamics of topological insulators. Topological insulators are a new quantum phase of matter. The material behaves as an insulator with a small band gap in bulk; while it has metal-like conductivity at its surface. The electronic excitation and relaxation processes in k-space of photoexcited topological insulators were explored by time- and angle-resolved photoemission spectroscopy (tr-ARPES) and found that the excited electrons far-above band gap energy relax via inter- and intra-band phonon mediated scattering processes ion the timescale of ~10 ps. In this study, we employed ultrafast time-resolved electron diffraction methodolgoy and investigated the atomic dynamics in Q-space of the photoexcited topological insulator Bi₂Te₃. The excitation used 400 nm, 100 fs pulses. The electron structural probe pulses were generated by irradiation of a gold photocathode using 266 nm pulses, accelerated to 75 keV, and focused to a spot size of 100 um. The duration of electron pulse was measured with the plasma method to be <1.8 ps. The six symmetric electron diffraction spots perpendicular to c-axis are obtained from the thin film (30-nm thickness) of a Bi₂Te₃ single crystal. The evolution of the diffraction intensity of the [11-20]-type spots showed rapid decrease in ~5 ps, slight recovery in 20 ps with modulation (~35 GHz) until 100 ps, and a successive slow decrease until 500 ps. These dynamics provide insight into the electron-lattice interaction. The 400 nm excitation and subsequent ballistic electron transport brought the system to a highly excited state, which then decays to a non-equilibrium state with short-range lattice disordering in ~5 ps. The atoms in this non-equilibrium state start to relax in ~20 ps with the acoustic phonon oscillations, being fully thermalized in 100-500 ps. These results revealed that atomic dynamics are well consistent with the electronic dynamics obtained with tr-ARPES and the potential surface of the concept of a non-equilibrium state in which the Q-space shifts slightly from that of equilibrium state. In this work, we see the lattice coupling directly. More detailed studies combined with time-resolved optical measurements in near-ultraviolet to infrared range and the theoretical calculations will be presented in the conference.

Laser terahertz (THz) emission microscope (LTEM) is a unique imaging system which provides two-dimensional (2-D) map of the THz pulse emission from a variety of electric materials and devices[1]. LTEM directly reflects local dynamic behavior of optically excited carriers. In this presentation, we review recent progress of LTEM development and apply it to study gas absorption to graphene[2]. Electrical and optical properties of graphene are known to be affected by the adsorption of gas molecules. We fabricate THz emission plate by coating graphene on semi-insulating InP wafers, and observe the THz emission waveforms. The results indicate that they sensitively change with the type of the atmospheric gas and the laser illumination time. The change of the terahertz waveforms in different environmental gases can be explained by modification of the surface depletion-layer potential of InP due to the surface dipole induced by the adsorbed gas molecules. [1] J. Phys. D: Appl. Phys. 47, 374007(2014), [2] SCIENTIFIC REPORTS 4, 6046(2014).

The photoactive material of organic solar cells commonly consists of a conjugated polymer blended with a fullerene derivative, yielding a complex network of intermixed and phase-pure domains. The charges that are photo-generated in the blend create an electric field in their vicinity, which can affect neighboring molecules and shift their absorption spectrum (Stark effect). The corresponding electro-absorption signature is a powerful tool to understand charge generation in the organic materials. We have investigated pBTTT:PCBM samples with a variety of well-defined microstructures, exploiting the Stark effect in two complementary ways. First, we have studied the evolution of the electro-absorption signal present in ultrafast transient absorption data (no external bias). This has allowed for the first time to directly visualize the migration of charges from intermixed to phase-pure regions, leading to significant insight to the still poorly understood mechanism by which the neat domains favour spatial separation of charges. Second, we have looked at the field-dependent generation and transport of charges in full solar cell devices with externally applied reverse bias, where the photo-generated charged cause a time-resolved reduction of the electro-absorption induced by the external field.

The promise of organic electronics lies in the synthetic and mechanical flexibility of their constituent materials and in the cost-effectiveness and energy efficiency of their printability. Yet, printing organic films from a solution can lead to complex, heterogeneous physical structure. With the heterogeneity in physical structure comes heterogeneity in the electronic structure, whose weakest local attributes often determine or limit device performance. A cross-cutting theme in my lab is to correlate the nanoscale physical properties in organic semiconducting thin films to their local optical properties in order to inform and improve the non-equilibrium processes used to deposit the materials to make effective devices. I will first describe how we have measured ultrafast exciton dynamics in small-molecule polycrystalline TIPS-pentacene thin films used in transistors by using polarized transient absorption microscopy. In so doing, we not only determine the particulars of excited-state dynamics in the absence of spatial averaging—we also more unusually infer a complex nanoscale structural motif at domain interfaces in the films. Our findings provide an explanation for the surprisingly high observed resistivity of domain interfaces in small-molecule polycrystalline films, and further investigations on annealed films suggest how solution processing could be altered to eliminate it.
I will also describe how we have conceived of and implemented a completely new form of nano-optical imaging, one of whose utilities is to uncover heterogeneity in the optical properties of more disordered conjugated polymer films used in photovoltaics. Our near-field approach is unusual in that it leverages the focus and rapid scanning of a keV electron beam with the non-invasiveness and spectral selectivity of optical fields. I will show how we image nanoscale features in luminescent polymer blends through electron-beam-induced (cathodoluminescence) activation of an oxide thin film scintillator that locally excites the adjacent sample through resonant energy transfer.

We present measurements of the coupled electronic and vibrational dynamics of polaron formation using femtosecond wavepacket techniques. The experiments are carried out on the halide-bridged mixed-valence transition metal linear chain complex [Pt(en)₂][Pt(en)₂Cl₂].(ClO₄)₄ (en = ethylenediamine, C₂H₈N₂), or PtCl(en), a Peierls insulator with strong electron-phonon coupling. Earlier studies in this and similar materials on longer time scales have shown that excitation well above the optical gap energy can result in the formation of charged polarons in addition to the self-trapped excitons (STEs) that form following excitation near the band edge, though the formation mechanism for polarons has not previously been established. In this work, we address the mechanism of polaron formation by impulsively exciting the PtCl(en) complex well above the peak of the intervalence charge transfer band, and probing the response within the subgap photoinduced absorption band. We find that the response is modulated by vibrational wavepacket oscillations that damp as the induced absorbance associated with the nonlinear excitations forms on a time scale of ~ 200 fs. Oscillations are observed at two optical phonon frequencies, ~176 cm^-1 and ~240 cm^-1, both of which are significantly lower than the ground state Raman frequency of 312 cm^-1. The ~176 cm^-1 frequency essentially matches that observed in our previous studies of STE formation in PtCl(en), in which we excited near the low energy onset of the absorption band, a condition that is expected to yield only STEs, and is assigned to the lattice motions that create the lattice distortion that stabilizes the self-trapped state. We assign the new component at ~240 cm^-1 observed under high energy excitation to vibrational motions associated with the self-trapping process that leads to the polaron formation. To our knowledge, this is the first observation of this process. We note that the rapid formation of the self-trapped polaron state, on the time scale of a single vibrational period following photoexcitation, together with the observation of accompanying vibrational coherence strongly suggests that the polarons form directly from the initial photoexcitation, rather than by dissociation of primary excitons. We relate the difference in excited state optical phonon frequencies associated with the formation of polarons vs. STEs to the difference in charge distribution and local structure associated with each type of nonlinear excitation.
This work was supported by the National Science Foundation under grant DMR-1106379. We thank J. A. Brozik (WSU) for preparing the samples used in these studies.

In solar cells that incorporate semiconductor polymers as electron donors and fullerene derivatives as acceptors, a number of reports based on ultrafast optical probes reveal that charges can be generated on timescales significantly faster than ~100 fs in certain solid-state microstructures. Techniques that have been applied in these studies include variants of visible transient absorption and photoluminescence spectroscopy, terahertz spectroscopy, time-resolved infrared spectroscopy, and femtosecond stimulated Raman spectroscopy. These probes allow measurement of population dynamics of relevant photoexcitations (excitons, polarons) but do not reveal directly how these interact to produce photocarriers. Here, we present a non-linear coherent spectroscopy, photocurrent-detected two-dimensional spectroscopy (2DPC), which is an ultrafast optical thechnique belonging to a family of 2D Fourier-domain spectroscopies that allows measurement of correlations between optical transitions induced by short optical pulses. In our implementation, spectral correlations are detected via the time-integrated photocurrent produced in a photovoltaic diode. Four collinear ultrashort laser pulses (10 fs, centered at 600 nm in our experimental setup) excite the semiconductor polymer in the solar cell, with a variable delay that is independently controlled between each pulse in the sequence. Each pulse separately excites a quantum wavepacket with spectral phase and amplitude imparted by that pulse, while the effect of the pulse sequence is to collectively excite multiple quantum coherences. Interferences between the various combinations of the wavepackets determine linear and non-linear contributions to the material optical response. The fourth-order signal terms of the detected photocurrent are read using phase-sensitive detection schemes with reference waveforms corresponding to a modulation of specific phase combinations of the four femtosecond excitation pulses. By scanning the time delay between the pulses 1 and 2, as well as that between pulses 3 and 4 (coherence times), at a fixed delay between pulses 2 and 3 (population waiting time), one measures a two-dimensional coherence decay function that is Fourier transformed to produce a 2D photocurrent correlation excitation spectrum. Measurement of such spectra at different population waiting times provides insight into the role of spectral correlations and state coherence in photocurrent generation in such complex functional materials. We focus on solar cells produced by blends of a common carbazole-thiophene-benzothiadiazole polymer, PCDTBT (the donor polymer), and PCBM (the fullerene acceptor), in which we analyse the dynamics of total photocurrent generation via the time evolution of diagonal and off-diagonal spectral correlations. We address the role of vibronic coherence as well as resonant tunneling in charge separation pathways on ultrashort timescales.

Photovoltaic diodes based on blends of semiconductor polymers and fullerene derivatives now produce power conversion efficiencies exceeding 10% under standard solar illumination. This indicates that photocarriers can be generated efficiently in well-optimized organic het- erostructures. Generally speaking, the dynamics of electron transfer and charge separation in polymer-based photovoltaic systems is dictated by a sequence of elementary steps following photo-excitation: an exciton created within the bulk of the material must diffuse to an region of the material where there is an electronic driving force for charge-separation. Recent evidence, however, suggests that free-polarons produced by dissociation of excitons appear within the first 35-50 fs following photoexcitationlsaquo;suggesting that the process occurs by direct long-ranged tunneling. We will present here our theoretical models and discuss recent experiment experimental evidence that support this notion.

Controlling charge transfer (CT), charge separation (CS), and charge recombination (CR) at the donor-acceptor interface is extremely important to optimize the conversion efficiency in solar cell devices. In general, ultrafast CT and slow CR are desirable for optimal device performance. In this talk, I will present new experimental results on the CT, CS and CR at PbS QDs/PCBM,¹ porphyrin/CdTe QDs,² porphyrin/oligomer³ and porphyrin/graphene⁴ interfaces using femtosecond transient absorption spectroscopy with broad-band capability and 120 fs temporal resolution. For the first two systems, the time-resolved results reveal that the quantum confinement is the key element for efficient electron injection and charge separation processes. For the last two systems, on the other hand, turning the on/off CT has been shown to be possible by controlling the charge density on the nitrogen atom of the porphyrin meso unit and the charge localization on the porphyrin cavity. In these cases, a specific ground-state interaction that brings the donor and acceptor components into close molecular proximity, allowing ultrafast photoinduced CT to occur, giving rise to a CT state that is probed by femtosecond transient absorption spectroscopy. Real-space imaging a donor-acceptor interfaces using four-dimensional electron imaging⁵ will also be presented.
References
1- Alaa O. El-Ballouli, Erkki Alarousu, Marco Bernardi, Shawkat M. Aly, Alec P. Lagrow, Osman M. Bakr and Omar F. Mohammed, J. Am. Chem. Soc.,2014, 136, 6952-6959.
2- Ghada A. Hamdi, Shawkat M. Aly, Anwar Usman, Mohamed S. Eita, Vasily Melnikov, Omar F. Mohammed, Scientific Reports, Submitted.
3- Shawkat M. Aly, Subhadip Goswami, Qana A. Alsulami, Kirk S. Schanze and Omar F. Mohammed, J. Phys. Chem. Lett.,2014, 5, 3386-3390.
4- Shawkat Aly, Manas Parida, Erkki Alarousu, and Omar F. Mohammed, Chem. Commun., 2014, 50, 10452-10455.
5- Omar F. Mohammed, D-S. Yang, Samir K. Pal, and Ahmed. H. Zewail, J. Am. Chem. Soc.,2013, 133, 7708-7711.

The ability to coherently rearrange atoms and chemical bonds at will is among the grand challenges of materials science and chemistry. A primary component of this challenge is to drive a desired reaction without adding excessive residual energy. One common method is to use photons to excite the electronic system, attempting to guide ionic relaxations towards an intended state. While effective in some systems, this approach may incur additional heating or less desirable conformational results. Meanwhile, slower processes without electronic excitations may be thwarted by intramolecular vibrational relaxation (IVR), where anharmonic coupling between vibrational modes leads to irreversible heating of the target.
Motivated by recent advances in the generation and control of strong terahertz (THz) single-cycle pulses, we have investigated the theoretical potential of such pulses to drive chemistry while circumventing both of these problems. This may be possible with single-cycle THz pulses because their spectral content is well separated from electronic excitation frequencies in most molecules, but their timescales may be fast enough to add and remove energy from the ionic system without allowing IVR to take place.
Here we discover that it is, in fact, possible to switch a molecule between several isomers using a strong THz pulse while depositing sufficiently little energy that the molecule stays in the isomerized state. We establish the feasibility of the method we present in this work by addressing the following questions: 1.) How accurately must we know the potential energy surface of the molecule in order to design an effective isomerizing pulse? 2.) How large an activation barrier can be surmounted before the fields become strong enough to ionize the molecule? 3.) How well must the target alignment be prepared in order for the desired trajectory to occur? In this work, we take a first look at these questions by way of density functional theory (DFT) calculations and semi-classical time-dependent DFT (TDDFT)-based Ehrenfest molecular dynamics (MD). TDDFT-Ehrenfest MD offers a way to include a time-varying electric field in the TDDFT Hamiltonian within the adiabatic exchange and correlation (XC) approximations. LiNC is one of the simplest molecules with multiple stable conformations, allowing a relatively thorough study of the DFT potential surface and efficient TDDFT-Ehrenfest simulations. We find that LiNC can be isomerized to either of its metastable conformations in TDDFT-Ehrenfest MD with very low residual heating and ionization rates. This work points the way toward THz coherent control of chemical bonds in materials and biological systems, and provides guidance and limiting factors.

Semiconductor nanostructures possess a number of applications in solar energy conversion. This includes using colloidal quantum dots in solar cells and, more recently, using analogous nanostructures such as nanowires (NWs) in photocatalytic applications. Here we describe recent work to understand the photocatalytic response of solution-synthesized CdSe NWs within the context of hydrogen generation. Various CdSe NW-based systems such as core/shell structures and hybrid metal nanoparticle/semiconductor NW hybrid systems have been studied. In all cases, femtosecond transient differential absorption spectroscopy has been used to reveal relevant carrier relaxation processes in these materials as well as the flow of charges across the different heterointerfaces that are present. By correlating these transient absorption results to results from accompanying hydrogen generation efficiency measurements, we have, in turn, rationalized the response of these materials, clarifying the role that different heterojunctions play in establishing both charge separation and hydrogen generation efficiencies. Ongoing efforts seek to probe these processes at the single wire level on a wire-by-wire basis.

Recent demonstrations of GaAs nanowire-based lasers [1] and solar cells [2] have highlighted the remarkable potential of GaAs nanowires for future optoelectronic devices. To fully realize the technological potential of GaAs nanowires, tight control over their electronic properties is imperative. The main obstacle facing GaAs nanowires is their high density of midgap surface states, which results in a high surface recombination velocity, extremely short charge carrier lifetimes of only a few picoseconds, and low charge carrier mobilities [3]. Previous work demonstrated that passivating GaAs nanowires by overcoating with AlGaAs shells improved the charge carrier lifetimes, but unexpectedly also reduced the electron mobility [4].
Here we investigate means of optimizing the AlGaAs shells to improve both charge carrier lifetime and mobility in the GaAs cores [5]. We employed optical pumpminus;terahertz probe (OPTP) spectroscopy to measure carrier transport and dynamics in GaAs/AlGaAs core-shell nanowires with sub-picosecond temporal resolution. As a noncontact technique, OPTP spectroscopy circumvents the problems associated with traditional contact based electrical measurements, and is ideally suited to studies of GaAs nanowires at room temperature. We also performed time-resolved photoluminescence (PL) measurements, using time-correlated single photon counting and PL up-conversion, to provide complementary information on the charge carrier dynamics.
Notably, the carrier lifetimes and mobilities both improved significantly with increasing AlGaAs shell thickness. Optimized GaAs/AlGaAs coreminus;shell nanowires with 34 nm thick AlGaAs shells exhibited electron mobilities up to 3000 cm²V^minus;1s^minus;1. Throughout the measured range of carrier densities, between 10¹⁶ and 10¹⁸ cm^-3, the measured electron mobilities were over 65% of the electron mobilities typical of high quality intrinsic bulk GaAs at equivalent photoexcited carrier densities. This suggests that the quality of the GaAs/AlGaAs interface is high and that the levels of ionized impurities and lattice defects are low. The photoconductivity lifetime of the optimized GaAs/AlGaAs coreminus;shell nanowires was 1.6 ns, which is almost three orders of magnitude longer than the lifetime of bare GaAs nanowires. The long lifetimes observed in both the photoconductivity and PL measurements indicate that midgap GaAs surface states are significantly reduced in these high quality nanowires. The long photoconductivity lifetimes and high electron mobilities indicate the immediate suitability of these high quality GaAs/AlGaAs coreminus;shell nanowires for optoelectronic devices.
[1] D. Saxena et al., Nat. Photon. 7: 963-968 (2013); B. Mayer et al., Nat. Commun. 4: 2931 (2013)
[2] P. Krogstrup et al., Nat. Photon. 7: 306-310 (2013)
[3] H. J. Joyce et al., Nanotechnology, 24: 214006 (2013)
[4] P. Parkinson et al., Nano Lett. 9: 3349-3353 (2009)
[5] H. J. Joyce et al., Nano Lett. 14: 5989minus;5994 (2014)

A detailed understanding of the factors that govern the recombination, diffusion and drift of mobile charge carriers in nanostructures is critical to many emerging nanotechnologies in electronics, optoelectronics and solar energy conversion. We have combined ultrafast pump-probe spectroscopy with optical microscopy to directly image the charge carrier dynamics in individual Si nanowires (NWs) with both spatial and temporal resolution. In these experiments, an individual NW is excited by a 425 nm femtosecond pump pulse that has been focused to a diffraction limited spot (350 nm) by a microscope objective, producing photogenerated carriers (electrons and holes) in a localized region of the structure. After a well-defined delay, pump-induced changes to the transmission of an 850 nm probe pulse are detected, providing the time evolution of the carrier population at a specific point within the structure. By correlating optical images with scanning electron microscopy images obtained from the same structures, we are able to correlate recombination behavior with specific structural features. Experiments performed on bent NWs reveal the emergence of a strain-induced recombination mechanism. Motion of the photogenerated carriers is observed using a spatially-separated pump-probe configuration, in which carriers are created in one location and detected in another, allowing direct imaging of charge carriers as move away from the excitation spot. In this configuration the pump beam is held fixed and the position of the probe beam is scanned by varying the angle of the probe beam as it enters the objective, resulting in a spatial map of the photoinduced transparency (and the free carriers) at a specified pump-probe delay. Images collected at a series of delays shows the spatial-temporal evolution of the charge cloud following excitation, providing a direct visualization of carrier diffusion in undoped NWs and charge separation in a Si NW encoded with a p-type/intrinsic/n-type (p-i-n) junction.

Owing to their widely tuneable optical properties and strong light-matter interaction, colloidal quantum dots (QDs) are considered for next-generation photonic devices such as all-optical wavelength conversion. Using QDs for this application is limited by either slow interband dynamics (10⁶-10⁹ s^-1) or energy consuming multi-exciton dynamics (10⁹-10¹¹ s^-1). Here we show, using white light pump-probe spectroscopy, that the interplay between two intrinsic material properties of PbS QDs, intraband absorption and interband bleach can lead to a very strong modulation of near-infrared light on an ultrafast, picosecond, timescale. After a few picoseconds upon non-resonant photo-excitation with a femtosecond pulse, hot carriers have thermalized to the band edges and a strong reduction of the QD absorption A (ΔA<0) is observed around the band gap due to well-known occupation of the band-edge states by electrons and holes. Adversely, at energies below the band gap, a photo-induced absorption (ΔA>0) is observed showing the same dynamics as the band gap bleach, attributed before to intraband transitions of the cooled electrons and holes. As both bleach and intraband show a distinct spectral dependence and scale with the exciton density, a wavelength range exists - called the matching wavelength - where they cancel out independent of pump fluence, leaving the excited dots with a similar cross section as the unexcited dots, i.e. ΔA = 0. In the first picoseconds after photo-excitation however, no bleach is observed (as the carriers are not occupying the band edge) and the strong photo-induced absorption of the hot exciton cooling down is dominating the transient response ΔA, leading to a net positive ΔA at the matching wavelength. As such, a peculiar dynamic arises at the matching wavelength where a strong burst of absorption induced by the pump vanishes within 1-2 picoseconds, leaving the dots as they were before the pump pulse arrived. This provides an excellent platform to convert datastreams between different wavelengths with high conversion efficiency, picosecond speed and without residual absorption. To characterize the conversion, the normalized absorption change (ΔA/A₀) is chosen as a figure-of-merit (FOM). It reaches up to 23 for a single exciton population, both in colloidal solution and film, and we deduce the strength of the absorption burst as 5200 cm^-1. To shown the potential for high speed conversion, a pump pulse sequence of up to 4 femtosecond pulses ,separated by 2.2 orr 4.4 ps, is converted to a probe wavelength while preserving the intrinsic strength, speed and zero background of the single pulse case, showing the ability for handling 450 and 225 Gb/s datastreams. Combining the QDs with on-chip or fiber based devices is shown viable with conversion energies as low as 10-100 femtojoule per bit. Moreover, due to the strong light-matter interaction of QDs, the device footprints of 10-100 hundred micron are much smaller than existing approaches.

Silicon nanocrystals (NCs) and their optical and conductive properties are intensively studied to enable Si-based optoelectronics. On the one hand, the quantum confinement deeply modifies the energy structure of Si, leading to a quasi-direct band gap in the visible region, which is of fundamental importance for many applications. On the other hand, the photoconductivity and long-range charge carrier transport often exploited in the devices can be deeply modified by the NC boundaries and surface states and by the connectivity within the NC network.
In this contribution we carry out (contact-free) ultrafast terahertz photoconductivity measurements in silicon nanocrystal superlattices [1]. Silicon NCs are prepared by thermal decomposition of Si-rich 2-5 nm thick SiO_x layers with 0.64 le; x le; 1. The control of the layer thickness and composition allows us to tune independently the NC size and concentration. Moreover, nanometer thick SiO_x layers are sandwiched between isolating SiO₂ barriers, which makes the arrangement of the NCs truly two-dimensional.
The high (terahertz) frequency of the probing radiation allows sensing the charge motion directly on nanometer scales with picosecond time resolution. Our analysis of the measured time-resolved THz photoconductivity spectra, supported by structural TEM characterization of the samples, is carried out within the recently developed microscopic framework. (i) The interaction of mobile carriers with NC boundaries is calculated by Monte-Carlo simulations of the quasi-classical carrier thermal motion [2]. (ii) The depolarization fields, which inherently develop in any inhomogeneous structure and screen the incident THz field, are evaluated thanks to the scaling of the NC photoconductivity by the optical pump fluence over two orders of magnitude [3,4].
We characterize and describe both intra-NC charge transport and charge transport within aggregates of NCs in samples with various compositions close to the percolation threshold. In addition, control of the depolarization fields by means of the excitation fluence allows us to asses the morphology, namely the degree of percolation of the photoconducting component.
[1] M. Zacharias, J. Heitmann, R. Scholz, U. Kahler, M. Schmidt, and J. Bläsing, Appl. Phys. Lett. 80, 661 (2002).
[2] H. Necaron;mec, P. Ku#382;el, and V. Sundström, Phys. Rev. B 79, 115309 (2009).
[3] H. Necaron;mec, V. Zajac, I. Rychetský, D. Fattakhova-Rohlfing, B. Mandlmeier, T. Bein, Z. Mics, and P. Ku#382;el, IEEE Trans. Terahertz Sci. Technol. 3, 302 (2013).
[4] P. Ku#382;el, and H. Necaron;mec, J. Phys. D: Appl. Phys. 47, 374005 (2014).

Quantum confined semiconductor nanorods have emerged as a new family of materials for solar driven water reduction. In the photoreactions, light induced excitons are dissociated by transferring the electron and hole to the hydrogen generation catalyst and the electron donors. The overall efficiency of these charge transfer processes strongly depends on the transient conductivity of the electrons and hole that, however, remain poorly understand. In this paper, we investigated the transient conductivity of electron-hole bounded pairs and the unbounded holes by time resolved THz and transient absorption spectroscopies. We found that the complex conductivity of the CdSe nanorods was mostly imaginary throughout the probing frequency, which was characteristic of bounded electron hole pairs (excitons). The transient absorption measurement revealed that the excitons could be dissociated within 20ps via electron transfer to the methylene blue and the separated charges could last longer than 5 ns. The THz measurement was also applied to the nanorod methylene blue solution. At early delay, the complex conductivity of the sample contained both real and imaginary parts that could be fit by the Drude-Smith model, indicating the dispersive free holes in the nanorods. After ~100 ps, the complex conductivity decayed to negligible small, suggesting that the hole was completely localized.

Material scientists and chemists have been using intentional incorporation of impurities and defects as powerful tools for modification of electronic and optical properties of host materials. In semiconductor nanostructures, this approach can be used to introduce new optical transitions that enable wide range of functionalities. For example doping of quantum dots and quantum wells with Mn²⁺ enables spin-electronics and spin-photonics. Introduction of vacancy centers in diamond and SiC makes quantum computing with defects possible. Recent studies have shown that such introduction of new optical transitions is also possible in single wall carbon nanotubes (SWCNTs) through the incorporation of low-level oxygen or diazonium covalent dopants on their side walls [1,2]. By capturing the excitons that would have recombine non-radiatively through defects and allowing efficient radiative recombination, the covalent dopant states of SWCNTs dramatically enhance the photoluminescence (PL) emission efficiency of the SWCNTs (from < 1% to ~28%) and shift the PL of SWCNT deeper into near infrared spectral regime (1.1 to 1.3 µm) [3,4]. This enhancement and red-spectral shift not only make envisioned applications of SWCNTs in optoelectronic, sensing and imaging technologies more feasible but also open the way for new applications of SWCNTs requiring single photon emission and optical gain. However at this moment, fundamental understandings essential for realization of these potentials are still lacking. In this talk I will present a review on our recent low temperature single tube PL study that provides a first look into the electronic structure and chemical nature of oxygen doped SWCNTs [5]. I will then report pico-second carrier dynamics and quantum optical behaviors of solitary dopant states measured using state-of-the-art superconducting nanowire single photon detector.
[1]. Piao, Y. M.; Meany, B.; Powell, L. R.; Valley, N.; Kwon, H.; Schatz, G. C.; Wang, Y. H., Brightening of Carbon Nanotube Photoluminescence through the Incorporation of Sp³ Defects. Nat. Chem. 2013,5, 840-845.
[2]. Ghosh, S.; Bachilo, S. M.; Weisman, R. B., Advanced Sorting of Single-Walled Carbon Nanotubes by Nonlinear Density-Gradient Ultracentrifugation. Nat. Nanotechnol. 2010,5 (6), 443-450.
[3]. Wang, Q. H.; Strano, M. S., Carbon Nanotubes: A Bright Future for Defects. Nat. Chem. 2013,5, 812-813.
[4]. Miyauchi, Y.; Iwamura, M.; Mouri, S.; Kawazoe, T.; Ohtsu, M.; Matsuda, K., Brightening of Excitons in Carbon Nanotubes on Dimensionality Modification. Nat. Photon. 2013,7, 715-719.
[5]. Ma, X.; Adamska, L.; Yamaguchi, H.; Yalcin, S. E.; Tretiak, S.; Doorn, S. K.; Htoon, H., Electronic Structure and Chemical Nature of Oxygen Dopant States in Carbon Nanotubes. Acs Nano 2014.

Quantum coherence has long been investigated using transient four-wave mixing experiments, but has recently become the subject of conjecture in a number of fields, including semiconductor nanostructures[1]. The development of coherent multidimensional spectroscopy (CMDS) has opened up a range of additional capabilities, which promise to help answer newly posed questions. These capabilities have enabled identification of many body effects and separation of correlated and uncorrelated broadening[2]. Additionally, CMDS configurations utilizing short pulses and broad spectral bandwidth have been used to detect quantum coherence between energetically distant electronic states while maintaining high time resolution. However, the same broad spectral bandwidth that allows these studies simultaneously excites many pathways, which can lead to congested spectra and ambiguity in interpretation. Worse, in semiconductor nanostructures, excitation of transitions at one energy can affect the dynamics at other energies[3].
To overcome these limitations we have developed pathway selective CMDS (PS-CMDS) in which each of the pulses is spectrally tuned to different transitions, and pulse sequences are generated to drive specific signal pathways in isolation. PS-CMDS removes ambiguity in the interpretation of cross peaks and can be compared quantitatively with CMDS spectra. Using PS-CMDS we can detect and isolate coherence signals in double quantum wells where the excitons are separated and localized but close enough that dipole interactions are not negligible. This is a regime in which the mechanism for coherent coupling is not well understood[4]. The isolated coherence signal provided by PS-CMDS allows us to extract quantitative details such as dephasing rates, linewidths and peak strengths of the coherently coupled excitons. Using these tools, we focus on studying the mechanisms underlying long range coherent interactions and the role of phonons and free carriers in enhancing or diminishing excitonic coherence.
In separate experiments, we have also shown this technique is more generally applicable. First, we apply PS-CMDS to a second double quantum well system in which transitions are spectrally more closely spaced and therefore more difficult to isolate. The intended pathways are significantly enhanced relative to all others, and peaks are revealed which are not accessible with standard CMDS experiments. We also demonstrate that spectral shaping of excitation beams in the two quantum coherence (2QC) pulse ordering alleviates ambiguities in interpretation of cross peaks. This pathway selective 2QC provides a complimentary tool for studying coherent coupling of excitons.
[1] doi:10.1038/nphoton.2010.284
[2] doi:10.1109/JSTQE.2011.2123876
[3] doi:10.1088/1367-2630/15/4/045028
[4] doi:10.1364/OE.22.006719

Two dimensional electron/hole gases (2DEG/2DHG) in high mobility semiconducting quantum wells have been the focus of intense research due to the fascinating quantum phenomena they exhibit, including the quantum Hall effect (QHE), 2D metal-insulator transitions, composite fermions, and high magnetic field-induced Wigner crystal phases. Although 2DEGs have been the focus of most research, 2DHGs have been preferred for applications such as quantum computing and spintronics, as holes have longer spin coherence times owing to the decreased hyperfine interaction of the p-type hole valence band with the nuclei. These novel phenomena make it particularly important to explore 2DHG physics in depth. To date, static (DC) transport measurements have been most widely used, followed by investigations using infrared, microwave and more recently, terahertz (THz) electromagnetic waves. In particular, recent theoretical predictions regarding the existence of the QHE at THz frequencies has made it even more important to understand these phenomena in this regime, since the THz energy is on the order of the cyclotron frequency in these systems.
Here, we have investigated the cyclotron resonance in high mobility two-dimensional electron and hole gas systems using non-contact magneto-optical terahertz (THz) spectroscopy. Since we observe cyclotron oscillations directly in the time domain, it is possible to directly extract the cyclotron resonance frequency and the scattering time of the majority carriers. For a 2DEG, the cyclotron frequency varies linearly from 0.11-1.55 THz with an applied magnetic field from 0.5-6 T (perpendicular to the sample), as expected. However, for a 2DHG the cyclotron frequency (0.12-0.53 THz) and effective mass (0.20_-0.35 m_e) vary nonlinearly with the applied magnetic field (0.5-6 T). This nonlinear variation is due to the complex non-parabolic valence band structure in a 2DHG (compared to the single parabolic conduction band in a 2DEG), as shown using Hartree-Fock calculations. This is the first time that this has been observed with THz magneto-optical spectroscopy, motivating further studies of these unique 2D nanosystems.

Nanostructures have attracted much attention as one of the promising candidates for realizing ultrafast optical switching devices with large nonlinearity. In nanostructures, the optical response is described by the long wavelength approximation (LWA) and increases with the system size, while this enhancement has been believed to be saturated by the excitonic coherent length or the breakdown of the LWA. On the other hand, the size-resonantly enhancement of optical response in a system with high crystalline quality is theoretically proposed, where the self-consistent interaction between the internal field and the induced polarization causes an enhancement of the response field in the size region beyond the LWA regime. In the present work, we investigated the optical effects of high-quality semiconductor thin films with thicknesses of a few hundreds of nanometers.
Degenerate four-wave mixing (DFWM) and resonant optical Kerr spectra were measured with the second harmonic of a mode-locked Ti:sapphire laser with a pulse duration below 110 fs. The wavelength was tuned around the exciton energy. DFWM and Kerr spectra of CuCl thin films exhibit a peculiar structure with several peaks. The photon energies are in good agreement with the calculated eigenenergies including the radiative shift in the coupled system of light and multinode-type excitons [1]. Similar structures are observed in photoluminescence spectra under both exciton resonant and band-to-band excitations [2]. DFWM spectra of ZnO thin films also show peculiar peak structures below exciton energies. These results suggest that peculiar optical phenomena induced by long-range coupling between light and excitons such as the radiative shift can be observed in any material with a large exciton effect. This long-range coupling effect causes the increase of the radiative width as well as the radiative shift. We measured radiative decay profiles by three-pulse DFWM (transient grating configuration), and succeeded in observing 100 fs-class ultrafast radiative decay of excitons in CuCl and ZnO thin films. In addition, 100 fs-class Kerr response corresponding to the large radiative width is also observed in CuCl films. Exceptionally high speed radiative decay may avoid the dephasing by phonons at high temperatures. The DFWM signal can be observed from 5 K to room temperature in CuCl and ZnO thin films. The radiative width shows complicated size dependence and the enhancement of optical response depends on the consistency between the radiative width and the spectral width of the excitation pulse. Efficient nonlinear optical signal at temperatures exceeding room temperature and ultrafast radiative decay below 10 fs will be achieved by improving the control accuracies of film thickness and excitation pulse width.
[1] M. Ichimiya, M. Ashida, H. Yasuda, H. Ishihara, and T. Itoh, Phys. Rev. Lett. 103, 257401 (2009).
[2] L. Q. Phuong, M. Ichimiya, H. Ishihara, and M. Ashida, Phys. Rev. B 86, 235449 (2012).

A new paradigm for all-optical detection and control of interfacial magnetoelectric coupling on ultrafast timescales is achieved using femtosecond time-resolved second harmonic generation (SHG) to study a ferroelectric/ferromagnet oxide heterostructure. We use femtosecond optical pulses to photoinduce interfacial coupling in a Ba_0.1Sr_0.9TiO₃(BSTO)/La_0.7Ca_0.3MnO₃ (LCMO) heterostructure and selectively probe the ferroelectric response using SHG. In this heterostructure, the pump pulses photoexcite non-equilibrium quasiparticles in LCMO, which rapidly interact with phonons before undergoing spin-lattice relaxation on a timescale of tens of picoseconds. This relaxes the spin-spin interactions in LCMO, applying stress on BSTO through magnetostriction. This in turn leads to a transverse magnetoelectric effect that occurs much faster than laser-induced heat diffusion from LCMO to BSTO. During this presentation, I will demonstrate how an ultrafast interfacial magnetoelectric effect can be mediated through elastic coupling, which could lead to future high-speed optically controlled magnetoelectric devices.

Topologically protected phases in condensed matter systems are a current research topic of tremendous interest due to both the unique physics and their potential in device applications. Skyrmions are a topological phase that manifest in magnetic systems as a hexagonal lattice of vortex-like spin configurations. We use time-resolved resonant soft x-ray diffraction to measure the dynamics of the skyrmion and conical phases in Cu₂OSeO₃. Pumping above the insulating gap gives increasingly fast dynamics at high fluences with a shift in the skyrmion wave-vector, indicating an electronically-driven collapse of the skyrmion phase.

Rare earth manganites, such as HoMnO₃, exhibit coupled ferroelectricity (FE) and antiferromagnetism (AFM) below their critical temperatures (for HoMnO₃, T_N (AFM)= 78 K and T_c (FE) = 875 K). Their complex magnetic structure and multiferroicity make this class of materials both fundamentally interesting and also potentially useful for practical applications involving, for example, four-state memory or magnetoelectric switching. Hexagonal rare earth manganites RMnO₃ (R=Ho, Er, Tm, Yb, or Lu) have their ferroelectric polarization along the c axis, and a 120⁰ antiferromagnetic order in the ab plane due to frustrated exchange of the Mn³⁺ ions which are arranged in a triangular lattice. Such a magnetic order leads to antiferromagnetic spin waves, or magnons, which are circularly polarized in the c-plane and can be magnetic dipole active. The magnetic-dipole active magnons lying in the 200 GHz-3 THz frequency range offer a promising route for both probing and controlling magnetic order using ultrashort THz pulses.
Here, for the first time, we optically excite a sample with 800 nm (1.55 eV) femtosecond pulses and directly observe the resulting changes in antiferromagnetic order by resonantly probing the magnon absorption line with near-single-cycle pulses centered at 1.5 THz. Unlike most optical-pump THz-probe experiments, we do not observe any signatures in the dynamics due to free carriers; in fact, the response is dominated by a broadening and shifting of the 1.2 THz magnon. This change in the magnon absorption occurs within about 30 picoseconds (ps) of optical excitation, though the exact response varies with temperature. Furthermore, above T_N, where there is no magnon resonance, no optically induced changes in the THz absorption are observed, again confirming that free carriers are not involved.
To better understand how the magnon reacts to the photoinduced electronic excitation, and to relate this to other relaxation process such as electron lattice and spin-lattice relaxation, we also performed all-optical pump-probe measurements at 800 nm. These results are similar to previous measurements on rare earth manganites, except that by going to lower temperatures we see the influence of the spin reordering transition that happens in HoMnO₃ at T_SR = 40 K. Overall, our measurements further demonstrate that using THz or MIR pulses to probe low energy excitations, such as magnons, electromagnons, and phonons, offers an interesting route to understanding the couplings between different degrees of freedom in complex materials.

Ferroelectric nanowires represent a fundamental building block of complex functional devices due to their large nonlinear optical and piezoelectric responses, related to the large polarization of the ferroelectric material. The coupling of light to nanoscale ferroelectrics enables novel means of engineering and manipulating their coupled degrees of freedom and inducing new functionality, including photoferroelectric effects and other opto-mechanical responses. Because one-dimensional nanowires with subwavelength diameters exhibit modified light-matter interactions they represent an unexplored means of coupling to these coupled degrees of freedom all-optically. Here we present ultrafast nonlinear-optical measurements of light-induced structural and electronic dynamics within single potassium niobate and barium titanate ferroelectric nanowires driven by above-band-gap excitation, using both optical trapping to define a single oriented nanowire as well as single nanoparticles on substrates. Using the second-order nonlinear susceptibility as a real-time structural probe, we observe large-amplitude, reversible modulations of the ferroelectric polarization and the associated nonlinear optical properties developing within a few picoseconds at megahertz repetition rates.

We present time- and angle-resolved photoemission spectroscopy measurements on the prototypical CeTe₃ charge density wave (CDW) system. Ultrafast optical transitions across the measured CDW gap of 0.59 eV leads to the excitation of collective modes and coherent lattice vibrations. This in turn coherently modulates the dispersion of the Te-5p_x,z CDW band structure [1].
First, a transient tight-binding model dispersion was applied to disentangle coherent changes of the band dispersion, depending on the distinct spatial polarization of the coherently excited modes. DFT frozen phonon calculations identify an out-of-plane optical A_1g phonon mode (at 3.0 THz), which rigidly renormalizes the energy scaling of the measured 5p_x,z band. In contrast, the collective in-plane modes at 2.2 THz and 2.7 THz are found to affect both, the CDW gap size as well as the rigid energy scaling. Periodic modulation of the CDW gap size is attributed to a change in hybridization between the in-plane electronic 5p_x,z bands and their corresponding CDW shadow bands, solely driven by the in-plane lattice modes.
Second, the pump-photon energy has gradually been lowered towards the CDW gap size while keeping a constant optical excitation density. We find the maximum coherent response of the CDW amplitude mode at a pump-photon energy of 1.0 eV. Surprisingly the result deviates from optical conductivity [2], which is dominated by direct interband transitions across the CDW gap between the lower and upper Te-5p_x,z bands. Our measured photon-energy dependence can be reproduced by a calculated joint density of states for optical transitions between the localized Ce-4f states and the upper Te-5p_x,z band. This implies that the coherent response of the amplitude mode is dominated by optical transitions between states with different orbital character.
[1] F. Schmitt et al., Science 321, 1649 (2008)
[2] B. F. Hu et al., PRB 83, 155113 (2011)

Metallic holmium & dysprosium are prototypical antiferromagnets with a very large magnetic moment. The indirect exchange interaction (RKKY) leads to an incommensurate helical spin structure below their Néel temperature of approx. 131K and 178K for holmium and dysprosium, respectively. The strong coupling of magnetism and lattice leads to a strong magnetostriction in both materials.
Resonant soft X-ray scattering at the FemtoSpex facility at BESSY II allows for directly probing the antiferromagnetic order in holmium and dysprosium after photoexcitation with a temporal resolution of 100fs up to microseconds delays. We follow the evolution of the magnetic tau;-peak in reciprocal space upon 800nm femtosecond laser excitation. The rapid decrease of peak intensity (< 1ps) indicates fast loss of magnetic order. By comparing the subsequent peak shift with temperature-dependent static data we have access to the “magnetic” temperature in the system. The loss of intensity and change in position of the magnetic tau;-peak on different timescales reveal the temporal hierarchy in thecoupling between 4f and 5d orbitals and the lattice after the optical excitation. We compare our results for both materials for different film thicknesses and excitation fluences.

Ultrafast and nanoscale phenomena are closely linked in complex materials, motivating experiments that access these extremes to help understand the basic interactions and correlations. In particular, transition metal oxides exhibit an intriguing self-organization of charges into atomic-scale stripes, whose dynamics remain largely unexplored. I will review our recent experiments that track the initial steps of charge ordering in nickelates, which are model compounds for stripe physics. By exploring the transient infrared response of La_1.75Sr_0.25NiO₄, we resolve femtosecond charge localization and transient electron-phonon coupling as precursors of stripe formation. Moreover, the Ni-O bending vibration exhibits a splitting due to long-range order, where transient multi-THz spectroscopy provides access to both short and long-range ordering within a single pulse spectrum. Terahertz pulses can also be applied to track the electronic dynamics in lamellar and 2D materials, and measurements resolving their transient low-energy conductivity over an ultra-broadband frequency range will be presented. Such studies provide new insight into fundamental electronic interactions and correlations in complex materials, and motivate further work e.g. using time-resolved photoemission for complementary insight into the momentum-space signatures of atomic and nanoscale order.

The transition metal dichalcogenide (TMD) crystals in the family of MoS₂, MoSe₂, WS₂, and WSe₂ are layered semiconductors that exhibit a direct gap in the limit of monolayer thickness. These new two-dimensional materials have attracted much recent attention because of their strong light-matter interactions, pronounced excitonic effects, and their ability to provide access to the valley degree of freedom through excitation by circularly polarized light. Recent attention has also turned to understanding the ultrafast dynamics in these materials and the distinctive phenomena that emerge at high excitation densities.
In this paper, we present recent results on the dynamics of electronic excitations in monolayer TMD crystals after ultrafast laser excitation. We highlight the role of many-body interactions in these systems for which exciton binding energies of 100&’s of meV have been established. Our investigations of exciton dynamics by time-resolved pump-probe measurements have revealed the role of exciton-exciton annihilation (or an excitonic Auger effect) for high exciton concentrations. The signature of this process is a decay rate for the exciton population that increases strongly with increasing exciton concentration. In complementary measurements, we show by means photoluminescence and time-resolved photoluminescence that exciton-exciton interactions can also lead to the formation of strongly bound biexcitons. The biexciton state is identified through the emergence of a new, concentration-dependent emission feature. The energy of emission peak implies a biexciton binding energy of 10&’s of meV compared to a pair of separated excitons, an unusually large stabilization energy for such a four-body state in an inorganic solid.

In this talk we will present a new take on traditional two pulse desorption measurements. In traditional two pulse desorption measurements a pair of intense femtosecond pulses are incident on the sample interface, pre-prepared with a coverage of adsorbed molecules. Desorption is then characterised using mass spectrometry under ultra-high vacuum conditions. Timescales appropriate to the desorption process are extracted by changing the time delay between the two pulses and monitoring changes in desoprtion.
Our approach [1] uses sensitivity of the graphene sheet conductivity to absorbed species to characterise the desorption dynamics. By directly probing the graphene surface itself, we are able to carry out two pulse correlation measurements on a sample typically considered too “dirty” for ultra high vacuum techniques. Using this method we will present measurements of molecular oxygen absorbed to the graphene surface. We show that the desorption process is characterized by a surprisingly fast timescale, ~100fs, which suggests a strong coupling between electronic and adsorbate thermal baths. By comparing our results to an empirical friction model, we suggest a diabatically driven desorption mechanism with a rather large energy of desorption of several hundred meV. Such large binding energies are in distinct contrast to molecular binding energies of oxygen on graphene predicted from DFT calculations, < 60 meV [2].
[1] S. M. Hornett, M. Heath, D. W. Horsell, and E. Hendry, Phys. Rev. B 90, 081401(R), 2014
[2] M.-T. Nguyen, J. Phys.: Condens. Matter 25, 395301, 2013

Monolayer transition metal dichalcogenides feature Coulomb-bound electron-hole pairs (excitons) with exceptionally large binding energy and coupled spin and valley degrees of freedom. The development of optoelectronic technologies relies on understanding and quantifying the fundamental properties of the exciton. A key parameter is the intrinsic exciton homogeneous linewidth, which reflects irreversible quantum dissipation arising from system (exciton) and bath (vacuum and other quasiparticles) interactions. Using optical coherent two-dimensional spectroscopy, we provide the first experimental determination of the exciton homogeneous linewidth in monolayer WSe₂. The role of exciton-exciton and exciton-phonon interactions in quantum decoherence is revealed through excitation density and temperature dependent linewidth measurements.

Monolayer transition metal dichalcogenides (TMDs) have garnered considerable interest in recent years as a new class of direct band gap, atomically-thin semiconductors. The fundamental optical excitation in TMDs is an exciton - a Coulomb-bound electron hole pair. Remarkably, the exciton binding energy is at least an order of magnitude larger than in quasi-2D systems, such as GaAs quantum wells, making excitons stable even at room temperature and relevant for optoelectronics. Such strong Coulomb interactions arise from the large carrier effective masses, strong quantum confinement, and reduced dielectric screening due to the monolayer material thickness. Monolayer TMDs are thus an ideal platform for investigating excitonic many-body interactions at the ultimate two-dimensional limit. In this work we use a suite of nonlinear optical spectroscopy techniques to examine how many-body effects influence the coherent optical properties of excitons and demonstrate that atomically-thin TMDs exhibit exceptionally strong Coulomb interactions compared to conventional semiconductors.
We first examine interactions between excitons in monolayer WSe₂ using optical two-dimensional coherent spectroscopy (2DCS). 2DCS is an enhanced version of three-pulse four-wave mixing with the addition of interferometric stabilization of the pulse delays. This technique enables unambiguous and simultaneous determination of the homogeneous and inhomogeneous linewidths. Excitation-density dependent measurements of the homogeneous linewidth reveal that interactions between excitons play an essential role in quantum decoherence. We demonstrate that monolayer TMDs exhibit strong exciton-exciton interaction linewidth broadening compared to quasi-2D quantum wells and 3D bulk materials.
Many-body interactions between neutral and charged excitons (trions) are also investigated in doped MoSe₂ using two-color ultrafast pump-probe spectroscopy. In a high quality sample with spectrally well-resolved exciton and trion resonances, we find signatures of electronic coupling between these two types of quasiparticles, which are isolated as cross-peaks in the two-dimensional spectra. Density matrix calculations reveal that the unique lineshape of the peaks arises from coherent exciton-trion many-body interactions resulting in a correlated state with a renormalization energy at least an order of magnitude larger than in modulation-doped quantum wells.

Although magnetic and electric optical resonances of Mie scatterers have been known for decades, it is only recently that these particles (dielectric resonators - DR) have been utilized for the realization of low loss metamaterials at optical frequencies. When considering arrays of these DRs, it is important to use the highest possible permittivity for the constituent material in order to preclude any diffractive or photonic crystal behavior. For optical frequencies, this poses severe limitations as most transparent materials have refractive indices <~5. When working in the IR spectral range, Tellurium is an attractive material for dielectric resonators since its index is ~5 and has low optical loss. In this talk I will describe optical magnetic mirrors created using Tellurium DRs of different shapes in the mid-infrared. The demonstration of optical magnetism was done using ultrafast phase-locked time domain spectroscopy, verifying that the
reflection of an optical wave does not revert its phase. Such optical magnetic mirrors can enhance the radiative rate of dipoles very close to the surface; this is in contrast to the behavior for regular metallic surfaces where dipole emission is strongly quenched. Finally, I will discuss scaling of such DR metasurfaces to the near infrared using other high index materials such as Si or Ge. Such metasurfaces might enable directional emission, phase manipulation, and other optical devices.
This work was performed at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Hot carriers generated by light absorption are challenging to investigate experimentally as they thermalize at subpicosecond time and nanometer length scale. Recent experiments [1] employed hot carriers induced by surface plasmon damping in noble metals such as Au and Ag to catalyze chemical reactions. The energy distribution and transport of the generated hot carriers play a key role in such light-controlled catalysis experiments at room temperature, given that these reactions would normally require very high temperatures when performed with carriers in thermal equilibrium.
In this talk, we present first-principles calculations of the energy distribution of hot carriers generated by surface plasmons in noble metals, and the relaxation time and mean free path of the hot carriers along different crystal directions in an energy range within 5 eV of the Fermi energy. Our calculations show the interplay of the noble metal s and d bands in determining both the damping rate of the plasmon and the mean free path of the generated hot carriers. Clear trends emerge as a function of surface plasmon momentum and frequency, allowing us to define optimal experimental conditions to maximize hot carrier generation and extraction. Our approach combines in novel ways density functional theory, GW, and electron-phonon calculations, and extends methods we recently developed to study hot carriers in semiconductors [2]. Taken together, our work provides microscopic insight into hot carriers in noble metals, and their behavior at ultrafast time scale in the presence of surface plasmons.
[1] S. Mukherjee, et al., Nano Lett. 13, 240-247 (2013).
[2] M. Bernardi, D. Vigil-Fowler, J. Lischner, J. B. Neaton, and S. G. Louie, Phys. Rev. Lett. 112, 257402 (2014).

Nanoscale optical energy focusing using plasmonic structures is crucial for many applications, such as nanoscale imaging, absorption enhancement in solar cells, near-field transducer (NFT) for heat assisted magnetic recording (HAMR) and plasmonic nanolithography. Thermal management in many plasmonic structures is of great importance to maintain their reliability because the intense focusing of optical energy will cause strong local energy dissipations. Most of the focusing applications are featured by nanoscale spatial field distribution, strong photon-electron-phonon coupling and non-equilibrium response. At the scale of a few nanometers, nonlocal electron response becomes prominent because of the Coulomb interaction and the Pauli exclusion principle. Meanwhile, the characteristic sizes of the plasmonic structures can be comparable to or even smaller than the mean free paths of heat carriers. The same is true for the durations of ultrafast laser pulse comparing to the relaxation times of heat carriers. These conditions make the nonlocal ballistic heat transport and locally thermal non-equilibrium significant, which is out of the scope of previous models under the framework of two temperature model (TTM) with local dielectric heating. Those models are limited to the quasi-steady-state response at the length scale of tens of nanometers or larger and are based the assumption that the electron temperature is locally well-defined. In addition, first-principles simulation alone is still impractical at this scale and the classical theories such as the Fourier law for heat transfer may be insufficient when the associated process involve non-continuum small scale heat generation and transport of multiple types of energy carriers.
Here we report a new multiphysics model to study the ultrafast optothermal response in nanoscale plasmonic structures with the considerations of nonlocal electron response and nonlocal thermal transport. First we simulated optothermal energy deposition from surface plasmon polaritons (SPPs) to the electron system using the hydrodynamic Drude model, which incorporates an auxiliary hydrodynamic current density to describe collective motions of electrons. Then we studied the nonlocal thermal transport processes based on the Boltzmann transport equation (BTE) with the relaxation time approximation. Our results showed that nonlocal Joule heating is not only a function of the local electric field, but also depends on the response of the neighboring material because of the Coulomb repulsion and the Pauli exclusion between electrons. Also, the profile of structural heating is not well aligned with the profile of the SPP energy dissipation due to the ballistic nature of heat carriers. In comparison to the previous models, our model can more accurately capture the strong nonlocalities in the optothermal response of the plasmonic nanofocusing structures, which is critical for predicting performance and the lifetime of these devices.

Metamaterials provide a route for creating new functional devices through the judicious design of subwavelength elements that are fashioned into various two- or three-dimensional arrays. This idea has proven particularly fruitful at terahertz frequencies with regards to creating dynamic filters, phase and amplitude modulators, and detectors. Further, metamaterials extend to the nonlinear regime, with a typical approach being the incorporation of a nonlinear element within the electric field-enhanced capacitive region of a split ring resonator (SRR). Coinciding with advances in generating intense terahertz electromagnetic pulses (approaching 1MV/cm), this enables exploration of nonlinear metamaterial devices in the far-infrared. We present our recent efforts in this area, with a focus on obtaining enhanced nonlinear responses using semiconductors as the resonant subwavelength elements. We have patterned doped thin films of InAs into arrays of disks that exhibit a plasmonic response at terahertz frequencies. Further, the plasmonic response exhibits nonlinear damping as a function of the incident terahertz field strength. This arises primarily from field-induced scattering of carriers from the G valley to the lower mobility L valley. In addition, we have incorporated InAs plasmonic disks into various absorber geometries, which enable the creation of terahertz saturable absorbers and optical limiters.