Optical absorption measurement at 1550 nm on a highly-reflective Si/SiO2 coating stack

Abstract

Future laser-interferometric gravitational wave detectors (GWDs) will potentially employ test mass mirrors from crystalline silicon and a laser wavelength of 1550 nm, which corresponds to a photon energy below the silicon bandgap. Silicon might also be an attractive high-refractive index material for the dielectric mirror coatings. Films of amorphous silicon (a-Si), however, have been found to be significantly more absorptive at 1550 nm than crystalline silicon (c-Si). Here, we investigate the optical absorption of a Si/SiO2 dielectric coating produced with the ion plating technique. The ion plating technique is distinct from the standard state-of-the-art ion beam sputtering technique since it uses a higher processing temperature of about 250 °C, higher particle energies, and generally results in higher refractive indices of the deposited films. Our coating stack was fabricated for a reflectivity of R = 99.95% for s-polarized light at 1550 nm and for an angle of incidence of 44°. We used the photothermal self-phase modulation technique to measure the coating absorption in s-polarization and p-polarization. We obtained and . These results correspond to an absorption coefficient which is lower than literature values for a-Si which vary from 100 cm−1 up to 2000 cm−1. It is, however, still orders of magnitude higher than expected for c-Si and thus still too high for GWD applications.