Nano-Plasmonics for Bio-Photonics

Abstract

One of the major aims in the emerging field of nano-plasmonics is to obtain complete control over electromagnetic light waves by means of tailoring optical materials on the nanometer scale – for example aiming at squeezing light into very tight spaces for the purpose of microscopy or lithography. Yet, isn’t the wavelength of light hundreds of nanometers in size or, talking about the important telecom regime, even exceeding 1 µm? What’s “nano” here? Clearly, one carefully has to distinguish between the vacuum wavelength and the effective material wavelength of light. While the frequency and the photon energy of light are generally conserved quantities in linear optics, the wavelength of light is not. For example, the special type of light waves that propagates on metal surfaces and that is commonly referred to as surface plasmons (or, more precisely, as surface plasmon polaritons) can have wavelengths that are two orders of magnitude smaller than the vacuum wavelength due to the special dispersion relation of surface plasmons. This possibility of having X-ray wavelengths at visible or telecom frequencies is one of the keys to the entire field of plasmonics. The usual diffraction limit of visible light remains to be a valid and important concept, but it has to be applied to wavelengths of just a few nanometers. As a result, metals are suddenly becoming a key ingredient in nanophotonics, whereas they have previously often been quite unwanted in optics as they do not transmit light in their bulk form.