Amorphous-Si Transmission Gratings Prepared by Ga+-Focused-Ion-Beam Milling and Their Polarization Characteristics

Abstract

Amorphous-Si (a-Si) transmission gratings of ∼ 0.1 µm thickness with four-type periods (Λ= 0.4 µm and 0.6 µm for sub-wavelength and Λ=1.0 µ and 1.4 µm for above-wavelength) have been fabricated on quartz substrates using 50 keV Ga+-focused-ion-beam (FIB) milling and CF4-reactive-ion etching (RIE) methods. Some optical properties of the fabricated gratings have been investigated using a linear polarized HeNe laser beam (λ=632.8 nm). Maximum diffraction efficiencies (ηMAX), measured at an incident angle (θB) to satisfy Bragg's condition with an effective index Neff=1.803–1.853, were very high. In particular, for gratings with Λ=0.6 µm (linewidth=0.25 µm, linespace=0.35 µm), the ηMAX at θB=17.0° is estimated to be about 96%. In addition, transmittance for two polarizations (TE and TM) was measured using a simple optical system, and the total phase difference (ΔΨTE-TM) and birefringence (Δn) were approximately calculated. The transmittance difference between TE and TM polarizations is the largest in the case of gratings with Λ=0.4 µm (approximately∼λ/2) among the gratings fabricated by us. That is, the TM/TE ratio is about 10.38, and both ΔΨTE-TM and Δn exhibit maximum values in the case of Λ=0.6 µm, which are about -76.7° and -1.35, respectively. The a-Si transmission gratings with a relatively high refractive index (∼ 2.9 at 632.8 nm) are successfully fabricated by FIB milling. In addition, we believe that this technique is one of the potential candidates for use in various optical devices and elements and, in particular, compatible with other optoelectronic devices for monolithic planar integration.