Precise measurement of optical constants of Si for short-wavelength far-infrared region

Powerful short-wavelength far-infrared lasers (lambda = 40 mum - 100 mum) and the measurement system are developing for plasma diagnostics (NIFS) and a gamma ray production (JASRI). In the wavelength region, the choice of optical materials is very important to construct the efficient optical system. A silicon etalon with high resistivity is one of the useful materials for windows and beam splitters. The optical constants (refractive index, absorption coefficients, transmissivity) of the silicon etalon have been measured by using 119-, 71-, 57- and 48-mum lasers. In order to design the optical elements having any transmissivity and reflectivity with less than +/- 1% uncertainty for the wavelength region, the accuracies of five figures for the refractive index, the wavelength and the thickness are required.