Improvement in THz light extraction efficiencies with antireflection subwavelength gratings on a silicon prism

An antireflection (AR) structure that incorporates a subwavelength grating (SWG) is a promising candidate for suppressing the Fresnel reflection of a silicon prism used as a component of an injection-seeded THz wave parametric generator (is-TPG) to improve the efficiency in extracting THz waves. Here, a two-dimensional binary AR-SWG with a 20 mu m period is designed and then numerically and experimentally realized. The measured transmittance of the AR-SWG is discovered to be greater than 80% from 0.5 to 4.0 THz and to exceed 90% from 1.25 to 2.83 THz. Moreover, the AR-SWG is tightly attached to the Si prism of the is-TPG by direct-bonding technology. Compared to the is-TPG without AR-SWGs, the output increases by 1.3-1.5 times from 1.09 to 2.52 THz. The improvement in THz light extraction efficiency is thus successfully demonstrated.