Electromagnetic resonance absorption in metallic gratings

Abstract

Reflection of electromagnetic waves with the 1.5 μm length from a metallic grating (silver) with rectangular groove profile was analyzed using the method of coupled waves. Appearance of the waveguide effect in a dielectric film and, accordingly, of electromagnetic field resonance is a necessary condition of resonance absorption at presence of dielectric thin film on the grating. The electromagnetic field resonance phenomenon confirms a dependence of reflection on the wavelength, which is described by the Lorenz function. When the film is absent, strong absorption is possible at appearance of the waveguide effect between both grating metallic vertical walls. Due to plasma resonance, both the waveguide effect and, accordingly, high absorption arise at any distance between metallic walls of grating (slot width) for TM polarized waves. For this polarization in the slot of 0.478 μm width filled with dielectric with permittivity 9ε = , the two waveguide modes propagate. That fact is confirmed by a dependence of reflection logarithm on the grating depth. For TE polarized waves in the slot of 0.378 μm width, the waveguide effect is possible on one mode only. The spectral dependence of reflection for TE polarized waves is well described by the Lorenz function, and due to excitation of two waveguide modes some deviation from this function is observed for TM polarized waves. The dependence of the reflection index on the grating depth completely agrees with the constant of waveguide mode propagation in the grating slot.