High-efficiency transmission gratings buried in a fused-SiO_2 glass plate

Binary gratings were fabricated with high first-order diffraction efficiency by conventional electron-beam drawing and subsequent inductive coupled-plasma dry etching upon the surfaces of SiO2 glass plates. The gratings were covered with a thin SiO2 film by plasma-enhanced chemical-vapor deposition without the grooves' being filled in. The buried gratings exhibited first-order diffraction efficiencies of 84% for transverse-electric and 87% for transverse-magnetic polarized light at a wavelength of 1.55 microm when the period and the depth were 1.5 and 2.8 microm, respectively.