research

Long-range surface polaritons

Thin films of almost any material, surrounded by a dielectric with low index of refraction, can guide electromagnetic radiation if they are sufficiently thin compared to the wavelength. The most common waveguide, the dielectric slab waveguide, consists of a lossless dielectric layer with a higher index of refraction than the surrounding. Thin layers of metals can also support long-range surface plasmon polaritons (LRSPPs), which are guided modes arising from the coupling of surface plasmon polaritons at the opposite sides of a metallic thin film.

There is a third kind of thin film waveguide, which is based on layers of strongly absorbing materials. At first sight, absorbing materials seem to be incompatible with guiding radiation over distances of many wavelengths. However, guided modes can be supported in thin layers of strongly absorbing materials due to the coupling of surface evanescent waves at the opposite sides of the thin film. These modes are called long-range surface exciton polaritons if the real component of the permittivity of the thin film is zero. More generally, they are called long-range surface polaritons (LRSPs) if the real component of the permittivity of the thin film does not vanishes and the imaginary component is larger than the real. Contrary to what can be naively expected, optical absorption improves the propagation of the guided modes. These surprising phenomena have motivated us to investigate LRSPs in thin layers of amorphous silicon (a-Si) and phase change materials (GST) at optical and UV frequencies.

We are interested in the excitation and propagation of LRSPs in thin films. We also investigate the use of these surface guided modes to enhance optical absorption in thin layers and to detect small changes of refraction in the surrounding medium.