Multiple surface gratings on azopolymers for generating surface plasmons

摘要

Surface relief gratings are optically inscribed on azobenzene polymer films. Multiple gratings can be inscribed on the same spot either simultaneously or sequentially by projecting a holographic pattern onto the surface. The grating pitch can range from 350 nm to 1.5 μm and depths can easily reach 400 nm. The inscription is done at room temperature, in air using a 100 mW/cm~2 beam for 1 to 5 minutes. In the present study we inscribe two sinusoidal gratings and coat the surface with a 20 nm thin film of gold. Light incident onto this structured surface is coupled into a surface plasmon when the appropriate angle of incidence and wavelength are selected. The coupling condition can be observed in the reflection spectrum from the surface. A dispersion curve for the plasmon propagation can then be produced by studying the reflection spectrum as a function of the angle of incidence. By using two gratings as couplers, a surface plasmon standing wave can be produced for a selected incident light wavelength.