Plasmons in metal nanostructures

Plasmon excitations, i.e. collective oscillations of the conduction electrons, strongly influence the optical properties of metal nanostructures and are of great interest for future photonic devices. Here, plasmons in metal nanostructures are investigated by nearand far-field optical microscopic techniques. Emphasis is placed on the study of the linear interaction of light with individual nanostructures. Specifically, the light transmission through individual nanometer-sized holes in opaque metal films is investigated using a scanning near-field optical microscope (SNOM). It is shown unambiguously that excitation and lateral propagation of surface plasmons support the light transmission through these nanoholes. This process has been under discussion recently. The propagation direction is given by the light polarization, thus allowing controlled addressing of individual holes — a process that may figuratively be described as “nanogolf”. This polarization controlled addressing of specific holes may be applicable for de-multiplexing purposes in future optical systems. Furthermore, the dephasing times and local field enhancement factors of plasmons in noble metal nanoparticles are determined by spectrally investigating the light-scattering from individual nanoparticles in a dark-field microscope setup. It is found that radiation damping limits the plasmon dephasing time, T2, in the gold nanospheres studied here to 25 fs, whereas suppression of interband damping in gold nanorods leads to surprisingly long particle plasmon dephasing times and large local field enhancement factors. The record value of T2=18 fs approaches the theoretical limit given by the free-electron relaxation time. The results of this first systematic and conclusive experimental study on individual particles answer the long debated question of the amount of plasmon damping in gold nanoparticles. Good agreement with calculations using the bulk dielectric function of gold shows that purely collective dephasing and surface effects contribute negligibly to the overall damping in the particles under investigation. The experimental results allow to deduce the relative contributions of radiation, interand intraband damping in gold nanoparticles. Also, the “true” particle plasmon dephasing time is determined in the sense of a pure plasmon oscillation without coupling to photons or interband excitations. The spectroscopic investigation of plasmons in single nanoparticles can also be used to determine the refractive index of the medium surrounding the particles. It is demonstrated for the first time how this can be used to build optical nanosensors based on particle plasmons. Applications for local concentrations, binding events and redox reactions are shown. The sensing volume of such a nanosensor is on the order of attoliters. First experiments towards increasing the sensitivity show the prospect of this technique. Actively changing the environment around metal particles by electrically aligning liquid crystal molecules allows the shift of the resonance frequency over a wide spectral range. The experiments presented here show that this novel effect can be used for electrically controlled light scattering with high contrast and spectral selectivity.