Terahertz-wave antireflection coating on Ge wafer using optical lapping method

Antireflection (AR) coatings for optical devices at terahertz wavelengths have much greater thickness (several tens of micrometers) than those at optical wavelengths. Hence, fabrication and control of the film thickness are difficult. In order to resolve this problem, we describe a new procedure to fabricate AR coatings at terahertz wavelengths with a wavelength of several hundred micrometers (far infrared and submillimeter wavelengths), using film thickness control by high-precision mechanical lapping after a crystal or amorphous thin plate with an appropriate index of refraction is applied to the surface of a terahertz optical device with optical adhesive. By means of this approach, an AR coating is fabricated on an undoped Ge wafer. A fused quartz thin plane with an appropriate index of refraction in the terahertz region (close to the square root of the index of Ge) is attached on the Ge wafer with an optical adhesive and the optical thickness of the fused quartz plate is lapped to one-quarter of the terahertz wavelength. The reflectance of this coating surface is reduced to one several hundredths of the one without coating, as computed. Also, with a view to confirm the reproducibility of the lapping thickness accuracy, an AR coating on both surfaces is attempted. It is confirmed that the interference spectrum disappeared and a high transmission coefficient is obtained at the desired wavelength, as designed.