Fabrication and properties of visible-light subwavelength amorphous silicon transmission gratings

Subwavelength amorphous silicon transmission gratings with periods from 100nm to 800nm and a thickness of 180nm on silica substrates have been fabricated and examined. For normal-incidence light, strong polarization effects are observed along with large birefringence. Both effects are found to greatly depend on the ratio of grating period to wavelength, with largest variation when the period is near half of the wavelength. The largest transmission difference for the polarization is 12dB; the largest phase difference is 170°. The behavior of the gratings deviates significantly from the previous experiments and on simple form birefringence theory and effective medium theory, except with rigorous modal expansion theory.