Scaling up antennas to the visible frequency regime, while scaling down to the nanometer scale, opens up the unique opportunity to interface such photonic nano-antennas with single photon emitters: individual molecules, quantum dots, color centers, proteins, etc. The fabrication, losses and dispersion of metals at optical frequencies offer major challenges, yet once surmounted, new avenues in the fields of active photonic circuits, bio-sensing and quantum information technology are opened up.In this presentation the optical analogue of monopole, dipole, multipole and multi element antennas will be presented, focusing on nanoscale field concentration, directionality, femtosecond response, spectral resonances and phase shaped excitation.In receiving mode, single molecules are ideal probes of the local antenna field and here we show optical fields spatially localized within 25 nm in the near field of an optical monopole antenna. In transmitting mode, the single photon emitter locally drives the antenna and the emission pattern is determined by the antenna mode; here we show controlled directed emission of single photons sources by various types of photonic antennas, incl. Yagi-Uda design.Beyond spatial confinement and directivity, the excitation and emission of single photon emitters (molecules, quantum dots), can be controlled also in time on fs scale. Using broad band excitation (~ 120 nm bandwidth) in combination with a pulse shaper we control individual photonic nano-antennas, adapt to the spectral phase development of the antenna and optimize the driving efficiency or generate local spatial hotspots at the antenna.Finally recent advances in our research will be presented.

Modification of the fluorescence close to metallic nanostructures has been a topic of great interest because the antenna-like behaviour of these structures modifies the radiative decay and the emission pattern of an emitter in its vicinity. Strong local field enhancement and near field confinement make plasmonic antennas also very attractive for increasing imaging resolution. Some years ago, we showed experimentally that a single spherical gold nanoparticle can act as a nanoantenna for enhancing the fluorescence of an emitter by more than an order of magnitude. Here, we present the results of our recent experimental and theoretical studies on enhancements of spontaneous emission up to 10000 times using nanostructures made of various material and geometries. If time permits, we also discuss very high resolution near-field imaging.

Strong interactions between light and matter can be engineered by confining light to small volumes. Nanoscale plasmonic structures are capable of confining light well below the diffraction limit; however, building resonant cavities in these devices has proven difficult due to large material losses. We report the design and fabrication of one-dimensional plasmonic crystals utilizing patterned dielectric surrounding low-loss, highly crystalline silver nanowires to make distributed bragg reflectors (DBR). Introduction of a defect in the DBR causes a resonant feature to appear in the stopband. These plasmonic cavities have a Q of up to 100 in a sub-diffraction limit mode volume. Quantum dots coupled to these devices show modified fluorescence spectra, as well as emission enhancement at the cavity resonance.

We show that plasmonic whispering gallery cavities doped with dye molecules show enhanced spontaneous emission that is resonant with the cavity modes; calculations show Purcell enhancements of over a factor 2000.Circular V-grooves were made by focused ion beam milling into a high-quality crystalline Au surface obtained by template stripping a thick (>2 μm), thermally evaporated Au layer off a mica substrate onto a silicon wafer. Ring diameters of 200-1000 nm were studied, with groove depths in the range 100-500 nm. We excite the plasmonic ring resonances using a 30 keV electron beam in a scanning electron microscope equipped with a parabolic mirror in combination with a CCD imaging detector, that enables – for the first time – the angle-resolved collection of cathodoluminescence radiation from the sample.Resonances in the ring cavities originate from circulating V-groove plasmons for which an integer number of plasmon wavelengths fits the cavity circumference. We measure these resonances and their azimuthal order by spectrally-, spatially- and angle-resolved cathodoluminescence. Resonances with quadrupolar charge distribution (ring with a two-wavelengths circumference) cannot efficiently be excited by free space light; we do however observe their cathodoluminescence emission. The smallest ring cavities (diameter ~200 nm) fit only a single wavelength in their circumference.Boundary-element-method calculations are in excellent agreement with the measurements. The calculations show that the resonant electric field in the ring cavities is confined to a very small volume. We have calculated that ring resonators based on 100-nm deep, 10-nm wide grooves have mode volumes that are smaller than λ₀/1000. The ring resonances have a Q factor of 10-50, leading to Purcell factors of over 2000.We studied the interaction of emitters with the resonant cavity modes by embedding ATTO 680 dye molecules in the groove voids. Arrays of rings with different diameters and groove depths were fabricated and a thin polymer layer with dye molecules was applied. The collected fluorescence emission spectrum shows a strong spectral reshaping. The reshaping fits the cavity resonances measured using both cathodoluminescence and white-light scattering measurements. Fluorescence intensity enhancements of over a factor 10 were found at the ring resonance wavelength. Time-correlated single photon counting spectroscopy shows a clear reduction of the fluorescence lifetime concomitant with the emission enhancement.The results show that plasmonic whispering gallery resonators allow for a strong and tunable interaction with optical emitters, paving the way for ultra-small low-threshold plasmonic ring lasers.

At the nanoscale, metallodielectric structures simultaneously embody characteristics of waveguides, antennas and cavities and lead to strong enhancements in the radiative emission rate relative to free space emission, characterized by the Purcell factor. In this talk I will describe nanoscale semiconductor-metal core-shell cavities for dramatic (> 3000x) enhancement of radiative emission rate and also discuss the role of Purcell factor enhancement by metal-dielectric structures on the open circuit voltage and quantum efficiency of thin film solar cells.

Surface plasmons offer great promise for developing subwavelength optical components such as waveguides, photodetectors, and lasers [1-3]. In particular, highly confined surface plasmons inside a metal nanocavity provide an electromagnetic field enhancement, which can be engineered to control the electronic transitions of light emitters [4]. In this work, we report the first observation of nonthermal hot excitonic emission from single CdS nanowires integrated with a nanoscale plasmonic cavity. A clear size-dependence of hot exciton emission is observed below 200 nm lengthscales, which is absent for simple CdS nanowires or for larger plasmonic cavities. The hot excitonic emission is attributed to the large exciton-longitudinal optical phonon coupling and the huge electromagnetic field enhancement by Ag nanocavity plasmons, leading to a nonthermal emission channel of hot excitons by drastically reducing the recombination lifetime. At the resonance condition of thermalized free excitons with the nonthermal hot excitons, the emission rate of free excitons is greatly enhanced and the linewidth is significantly decreased below the thermal energy. This observation indicates that the intrinsic emission properties of semiconductors can be engineered by means of integrating with nanocavity plasmons and is important for understanding and designing nanoscale emitters with novel properties. [1] W. L. Barnes et al., Nature 424, 824 (2003).[2] A. L. Falk et al., Nature Phys. 5, 475 (2009).[3] R. F. Oulton et al., Nature 461, 629 (2009).[4] Z. C. Dong et al., Nature Photon. 4, 50 (2010).

The dielectric properties of single metal nanoparticles and their surrounding are reflected in the spectral position of the plasmon resonance. In this way, information from the nanoscale can be transmitted into the macroscopic world. Nonlinear pump-probe spectroscopy reveals acoustic oscillations of the metal particles that are triggered by the heating pump pulse, leading to a temporal variation in the electron density and this the plasma frequency. However, the influence of a single nanoparticle on the transmitted light field scales with the third power of the particle's radius. The smaller the particle becomes the more difficult it is to perform nonlinear spectroscopy. We demonstrate how the concepts of optical nano-antennas can be used to couple passive, i.e., not emitting, nanoobjects to the light field to increase the influence a nanoparticle has on the transmitted beam. It will be shown that plasmon hybridization of coupled metallic nanoparticles not only shifts the resonances, but that the variation of one particle's dielectric properties has an increased effect on the total absorption cross section.We prepare by electron beam lithography plasmonic nanoantennas coupled to small metal discs. The acoustic oscillations of individual disc-antenna pairs are investigated by pump-probe spectroscopy and compared with numerical simulations. We find an enhancement of the transient signal of about a factor of 2 that is caused by the optical nanoantenna. This passive use of the antenna - witthout an active quantum emitter - opens the way towards antenna-enhanced sensing and spectroscopy.

Noble metals exhibit high intrinsic optical nonlinearities, but they are usually not employed for frequency conversion because they are reflective and absorptive. However, these limitations can be overcome with nanostructured metal surfaces or with structures of subwavelength scale. We demonstrate that metalnanostructures can be effectively employed for efficient index modulation, two-photon excited luminescence, harmonic generation, and wave mixing. We discuss and review recent results and applications.

One of the key features of interest of plasmonic nanostructures is their ability to confine and enhance light fields on a scale that is (much) smaller than the wavelength in the surrounding medium. The strong interplay between the geometry and the light results in highly structured light fields on the nanoscale.In this presentation I will show recent progress in the visualization of such light fields beyond the diffraction limit. Surface plasmons launched from periodic arrays of holes exhibit the Talbot effect. By exploiting a recent advance in the measurement of vector fields with near-field microscopy, we were able to determine the symmetry of MIM modes in plasmonic slot waveguides and of plasmonic nanowire modes. In addition we use dedicated near-field probe geometries to home in on individual vector components of either the electric or the magnetic component of confined light fields.

Optical antennas improve the interaction of quantum emitters with light by creating strong local electromagnetic fields. Electric dipole transitions in for example fluorescent molecules and quantum dots are enhanced by local electric fields, and have been widely used to characterize local electric fields at antennas and other nanostructures. In a similar way, magnetic transitions in Lanthanide ions are expected to be enhanced by local magnetic fields, and thus have the potential to probe local magnetic fields. However, unlike the linear pure electric dipole transitions of single molecules, the magnetic dipole transitions of lanthanide ions are accessible only in ensemble experiments, are degenerate with electric transitions, and are isotropic. As a result it is challenging to identify and quantify the electric and magnetic contributions, in particular because their ensemble emission is identical in a homogeneous medium. In this talk, we show that within inhomogeneous environments the different symmetries of electric and magnetic dipoles distinguish the electric and magnetic components of transitions. We experimentally study the angular emission of thin layers of lanthanide ions near dielectric interfaces by conoscopy. The obtained frequency-resolved momentum images contain a wealth of information. Electric and magnetic transitions are readily identified, and the components of mixed transitions, which contain both electric and magnetic components, are quantified. The results provide a direct experimental visualization of the different symmetry of electric and magnetic transitions. Finally, we identify transitions that are ideal to simultaneously study both the local electric and magnetic fields at optical antennas and in other nanostructures, such as metamaterials.

Plasmonic nanostructures such as antennas, metal-insulator-metal stacks or tapered wires have been designed to confine light in truly sub-wavelength (sub-λ) volumes opening new opportunities to enhance the interaction of light with small quantities of matter down to the molecular level. Beyond confining light at fixed locations, imposed by the structure geometry, there is a need for dynamical spatial control of such hot-spots, for instance to achieve selective optical addressing of different nearby nano-objects. Several strategies borrowed from the field of coherent control have recently been suggested to reach this goal. A first approach relies on temporally shaping the phase and amplitude of an ultrashort laser pulse illuminating the nanostructures [1]. By combining pulse shaping with a learning algorithm, Aeschlimann et al. have recently demonstrated experimentally the feasibility of generating user-specified optical near field response of a star-like silver object [2]. Experimental control of the local optical response of a metal surface was also achieved by adjusting the temporal phase between two unshaped ultrashort pulses [3]. Alternatively, the idea of time reversal has been lately proposed by Li and Stockman [4]. In this approach, a femtosecond optical nano-source is locally coupled to the surface plasmon oscillations of a complex plasmonic system leading to the subsequent radiation of electric field in the far zone. Time-reversing the later and sending it back to the system as an excitation wave thus provides the right illumination conditions for concentrating light at the initial local source location.
Here we propose a novel approach based on continuous light flows which aims at achieving a deterministic control of plasmonic fields by using the spatial polarization inhomogeneities of high order beams such as Hermite-Gaussian (HG) [5]. We show both experimentally and numerically that spatial phase shaping of the illumination field provides an additional degree of freedom to drive nano-optical antennas and consequently control their near field response. Furthermore, the potential of this approach to deterministically confine light at specific locations of a more complex metallic nanostructure is also demonstrated [6].

References:

[1] Stockman, M., Faleev, S. V., Bergman, D. J., Phys. Rev. Lett. 88, (2002) 067402.
[2] Aeschlimann, M., et al., Nature 446, 301 (2007)
[3] Kubo, A., et al., Nano Lett. 5, 1123 (2005)
[4] Li, X., Stockman, M., Phys. Rev. B. 77, 195109 (2008)
[5] G. Volpe, et al., submitted (2010)
[6] G. Volpe, et al., Nano Lett. 9, 3608-3611 (2009)

We demonstrate the non-perturbative use of diffraction-limited nonlinear optics and photon localization microscopy to visualize the nanometer-scale controlled shifts of ultraconfined zeptoliter mode volumes within plasmonic nanostructures. Unlike tip-based or coating-based methods, these measurements do not affect the electromagnetic properties of the nanostructure being investigated. The photon-limited localization accuracy of nanoscale mode distributions is determined for many of the measured devices to be within a 95% confidence interval of +/- 2.5 nm. In addition, because of the accuracy of these photon localization microscopy measurements, we were able to observe and characterize the effects of nm-scale fabrication variations and irregularities on local plasmonic mode distributions.As a proof of concept, we image the local energy-dependent changes in near-field distributions within individual gold asymmetric bowtie nano-colorsorters (ABnCs) [1], a class of plasmonic color sorters, based on the “cross” nanoantenna geometry. These devices are specifically engineered to not only capture and confine optical fields, but also to spectrally filter and steer them while maintaining nanoscale field distributions. Their spectral properties and localized spatial mode distributions can be readily tuned by controlled asymmetry, and each of the zeptoliter mode volumes within an ABnC, separated by only tens of nm, can be individually addressed simply by adjusting the incident wavelength. We imaged relative spatial shifts down to 7 nm of distinct modes within the same device, demonstrating the local field manipulation capabilities of ABnCs, in strong agreement with theoretical calculations.[1] Zhang, Z. et al. Manipulating Nanoscale Light Fields with the Asymmetric Bowtie Nano-Colorsorter. Nano Lett. 9, 4505-4509 (2009).

The optical response of a metallic nanoantenna is determined by the interplay between the intrinsic bright and dark plasmon modes supported. A complete characterization hence requires a means to excite dark modes, which do not couple efficiently to far-field optical radiation. We demonstrate that electron energy loss spectroscopy (EELS) is a powerful tool allowing the complete determination of the plasmonic mode spectrum, with nanometre-scale spatial resolution facilitated by the scanning electron beam. Correlations with optical measurements and full-field electrodynamic simulations will be presented for a variety of top-down fabricated nanoantennas supported by ultrathin silicon nitride membranes, as well as for colloidal assemblies based on core/shell structures. A particular focus will lie on different nanofabrication strategies suitable for use with the thin substrates (30-50 nm thickness) required for EELS investigations. Furthermore, we present a comparison of the experimentally acquired data with numerical simulations based on calculations of both optical extinction and electron energy loss. Additionally, the influence of dark modes on the emission properties of nearby single emitters will be discussed.Finally, we will outline a new strategy for the design of optical nanoantennas showing a broadband optical response while maintaining sub-wavelength size, based on the tools of transformation optics.

The visualization of localized plasmon resonances on the nanometer scale in combination with spectral information over the entire visible range is of prime importance in the field of biosensors, surface-enhanced Raman spectroscopy (SERS), apertureless scanning near-field optical microscopy (SNOM), and for the design of metamaterials. With the advent of monochromators and highly dispersive energy filters, energy-filtering TEM has now become available for the study of the optical response of materials well into the infrared range. This technique was applied to the detection of band gaps [1] as well as to the study of surface plasmons on metal particles, like Ag nanoprisms [2–4] or Au nanorods [5]. It offers a spatial resolution in the nanometer range which is well below the resolution of present light-optical techniques.Here, the dielectric response of holes in a Ag film is studied by energy-filtering TEM [6, 7]. Circular holes and rectangular slits were drilled into a 100 nm thick Ag film using a focused ion beam. The arrangement of the circular holes was chosen such that well separated holes, holes that are closely spaced, and interpenetrating holes were present. Slits were cut with different aspect ratios. Taking advantage of the monochromated electron beam (FWHM below 0.1 eV) and the MANDOLINE energy filter of the Zeiss SESAM microscope, energy-filtered images were recorded in the energy range between 0.4 and 4 eV using energy steps of 0.2 eV. Depending on energy loss, we find a number of resonant features that can be ascribed to resonances of single holes, to coupled resonance of several holes, and to Fabry-Pérot-type resonances. Coupling effects are discussed within the hybridization scheme. The experimental results are compared to finite-element calculations. The coupling effects between adjacent holes lead to very strong field enhancements which occur primarily in the infrared range. These results demonstrate the power of the EFTEM technique for the mapping of surface plasmon resonances of complex structures. [8] References[1] L. Gu et al., Phys. Rev. B 75 (2007) 195214.[2] J. Nelayah et al., Proc.14th European Microscopy Congress, Aachen, S. Richter, A. Schwedt (Eds), Springer, Berlin (2008) 243.[3]C.T. Koch et al., Proc. 14th European Microscopy Congress, Aachen, M. Luysberg, K. Tillmann, T. Weirich (Eds.), Springer, Berlin (2008) 447.[4]J. Nelayah et al., Optics Letters 34 (2009) 1003. [5]B. Schaffer et al., Phys. Rev. B 79 (2009) 041401.[6] W. Sigle et al., Optics Letters 34 (2009) 2150.[7]W. Sigle et al., Ultramicroscopy (2010) in press.[8]The authors acknowledge financial support from the European Union under the Framework