Phototric Band Structure of Surface Modes

Abstract

Wavelength scale periodic structures are of great current interest since they may be used to modify the interaction between light and matter [I]. In particular photonic band gaps may be used both to suppress [Z] and enhance [3] spontaneous emission. Recently we have studied band gaps for surface rather than bulk modes, and have shown that these may also be used to control spontaneous emission from a thin layer of dye 141. Such gaps occur when electromagnetic modes propagate on periodically modulated surfaces. We report on a detailed study to look at how the surface mode band structure depends on the surface profile. Our system comprised a corrugated metallic swfpce, fabricated using holographic techniques. Such a surface supports the propagation of surface plasmons. For M appropriate grating pitch Bragg scattering of the surface plasmon mode reds m the formation of a band gap. Using a convergent beam technique we are able to directly image the dispersion curve for the surface modes. With this technique we have determined how the band structure depends on the nature ofthe surface profile. The figure below shows how the width and central frequency of the gap valy as a hction of cormgation depth. The experimwtal data is seen to be well modelled by a recent theory [SI, indicated by the solid Line m the figures. The band gap described above only prohibits propagation over a small range of directions about the nod to the grating grooves. We have mersurcd how the band gap depends on the propagation direction and discuss the implications for the desiga of a photonic surface, ie one which prohibits the propagation of surface modes m all directions.