Thomas Søndergaard

A generalized non-local optical response theory for plasmonic nanostructures

Metallic nanostructures exhibit a multitude of optical resonances associated with localized surface plasmon excitations. Recent observations of plasmonic phenomena at the sub-nanometre to atomic scale have stimulated the development of various sophisticated theoretical approaches for their description. Here instead we present a comparatively simple semiclassical generalized non-local optical response theory that unifies quantum pressure convection effects and induced charge diffusion kinetics, with a concomitant complex-valued generalized non-local optical response parameter. Our theory explains surprisingly well both the frequency shifts and size-dependent damping in individual metallic nanoparticles as well as the observed broadening of the crossover regime from bonding-dipole plasmons to charge-transfer plasmons in metal nanoparticle dimers, thus unravelling a classical broadening mechanism that even dominates the widely anticipated short circuiting by quantum tunnelling. We anticipate that our theory can be successfully applied in plasmonics to a wide class of conducting media, including doped semiconductors and low-dimensional materials such as graphene.N.A. Mortensen, S. Raza, M. Wubs, T. Søndergaard, & S. I. Bozhevolnyi Department of Photonics Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark Center for Nanostructured Graphene (CNG), Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark Department of Physics and Nanotechnology, Aalborg University, DK-9220 Aalborg, Denmark Institute of Technology and Innovation, University of Southern Denmark, DK-5230 Odense, Denmark

Plasmonic black gold by adiabatic nanofocusing and absorption of light in ultra-sharp convex grooves

Excitation of localized and delocalized surface plasmon resonances can be used for turning excellent reflectors of visible light, such as gold and silver, into efficient absorbers, whose wavelength, polarization or angular bandwidths are however necessarily limited owing to the resonant nature of surface plasmon excitations involved. Nonresonant absorption has so far been achieved by using combined nano- and micro-structural surface modifications and with composite materials involving metal nanoparticles embedded in dielectric layers. Here we realize nonresonant light absorption in a well-defined geometry by using ultra-sharp convex metal grooves via adiabatic nanofocusing of gap surface plasmon modes excited by scattering off subwavelength-sized wedges. We demonstrate experimentally that two-dimensional arrays of sharp convex grooves in gold ensure efficient (>87%) broadband (450-850 nm) absorption of unpolarized light, reaching an average level of 96%. Efficient absorption of visible light by nanostructured metal surfaces open new exciting perspectives within plasmonics, especially for thermophotovoltaics.

General properties of slow-plasmon resonant nanostructures: nano-antennas and resonators

General properties of retardation-based resonances involving slow surface plasmon-polariton (SPP) modes supported by metal nanostructures are considered. Explicit relations for the dispersion of SPP modes propagating along thin metal strips embedded in dielectric and in narrow gaps between metal surfaces are obtained. Strip and gap subwavelength resonant structures are compared with respect to the achievable scattering and local-field enhancements lending thereby their distinction as nano-antennas and nano-resonators, respectively. It is shown that, in the limit of extremely thin strips and narrow gaps, both structures exhibit the same Q factor of the resonance which is primarily determined by the complex dielectric function of metal.

Waveguidance by the photonic bandgap effect in optical fibres

Photonic crystals form a new class of intriguing building blocks to be utilized in future optoelectronics and electromagnetics. One of the most exciting possibilities offered by photonic crystals is the realization of new types of electromagnetic waveguides. In the optical domain, the most mature technology for such photonic bandgap (PBG) waveguides is in optical fibre configurations. These new fibres can be classified in a fundamentally different way to all optical waveguides and possess radically different guiding properties due to PBG guidance, as opposed to guidance by total internal reflection. In this paper we summarize and review our theoretical work demonstrating the underlying physical principles of PBG guiding optical fibres and discuss some of their unique waveguiding properties.

Extraordinary Optical Transmission Enhanced by Nanofocusing

We demonstrate that the phenomenon of extraordinary optical transmission (EOT) through perforated metal films can be further boosted up by utilizing nanofocusing of radiation in tapered slits. For one-dimensional arrays of tapered slits in optically thick suspended gold films, we show that the maximum transmission at resonance is achieved for taper angles in the range of 7-10 degrees increasing significantly in comparison with the transmission by straight slits. Transmission spectroscopy of fabricated 500 and 700 nm period tapered slits in a 180 nm thick gold film on a glass substrate demonstrates the enhanced EOT with the resonance transmission being as high as approximately 0.18 for the filling ratio of approximately 0.13 and showing good correspondence with theoretical results. It is also shown that the enhanced transmission can be achieved with either weak (2.5%) or strong (43%) reflection depending on the direction of light (normal) incidence.

Resonant plasmon nanofocusing by closed tapered gaps.

We study radiation nanofocusing by closed tapered gaps, i.e. metal V-grooves, under normal illumination, and discover that the local field inside a groove can be resonantly enhanced due to interference of counter-propagating gap plasmons. Considering V-grooves milled in gold, we analyze this phenomenon theoretically, deriving an analytic expression for the resonance condition and predicting more than 550-fold intensity enhancements at resonance, and observe it experimentally with two-photon photoluminescence microscopy, demonstrating more than 100-fold intensity enhancements.

Plasmon-polariton nano-strip resonators: from visible to infra-red

Dispersion of the resonant properties exhibited by silver and gold nano-strips in a wide range of wavelengths is considered. The tunability and Q-factor of scattering resonances as well as the field enhancement achieved at strip terminations are analyzed in the wavelength range from visible to near infrared (400-1700 nm), confirming that the resonant behaviour is dominated by dispersion properties of short-range surface-plasmon polaritons (SR-SPPs) propagating along the strip. It is found that, while the Q-factor decreases for longer wavelengths due to the SR-SPP dispersion curve moving closer to the light line, the field enhancement depending also on the metal susceptibility magnitude remains largely unaffected. The results obtained are also used to estimate the phase change involved in the SR-SPP reflection by strip terminations.

Metal nano-strip optical resonators

Rectangular gold and silver nano-strips embedded in glass or water are considered as optical resonators. Their scattering cross section and field enhancements in the case of p-polarized plane-wave illumination are analyzed using a surface integral equation method. Peaks in the scattering spectra are shown to be related to the resonant excitation of forward and backward travelling short-range surface plasmon polaritons in thin metal strips. The influence of angle of incidence and strip thickness on resonances is investigated. Finally, we consider the possibility of obtaining large local fields in a narrow (5nm) gap between two metal strips by taking advantage of the plasmon field distribution and boundary conditions, demonstrating the feasibility of at least 10-fold field magnitude enhancement.

Compact Z-add-drop wavelength filters for long-range surface plasmon polaritons

We design, fabricate and investigate compact Z-add-drop (ZAD) filters for long-range surface plasmon polaritons (LR-SPPs) at telecom wavelengths. The ZAD filter for LR-SPPs consists of two ridge gratings formed by periodic gold thickness modulation at the intersections of three zigzag-crossed gold stripes embedded in polymer. We investigate influence of the grating length and crossing angle on the filter characteristics and demonstrate a 10 masculine- ZAD filter based on 80-microm-long gratings that exhibit a 15-dB dip (centered at ~ 1.55 microm) in transmission of the direct arm along with the corresponding ~ 13-nm-wide transmission peak in the drop arm.

Field enhancement and extraordinary optical transmission by tapered periodic slits in gold films

We investigate field enhancements by one-dimensional periodic arrays of tapered slits fabricated to a high quality (nm precision) using focused ion beam milling in a 180 nm-thick gold film. Tapering of periodic slits in metal was recently shown to boost the extraordinary optical transmission (EOT) exhibited by similar, but non-tapered, plasmonic structures. Here, both simulated and experimental reflection spectra, along with high-resolution two-photon luminescence (TPL) scanning optical images and simulated electric field plots of the metal slits, are compared, revealing good correspondence between spectral dependences and field intensity enhancements (FEs) estimated via the local TPL. Experimentally investigated structures had a fixed taper angle α=20.5° for two different widths, w=80 and 130 nm, having gaps g=25 and 65 nm, respectively, both fabricated at two different periods, Λ=500  and 700 nm. We attributed the obtained FE reaching ~110 to nanofocusing and resonant interference of counter-propagating plasmons by the periodic tapered gaps. As both simulated and experimentally achieved FEs depend on taper angle, gold film thickness, period and gap of the slit arrays, the resonances can actually be tuned in the wavelength range from visible to infrared, making this configuration promising for a wide range of practical applications, e.g. within surface-enhanced spectroscopies.