Optical absorption measurement at 1550 nm on a highly-reflective Si/SiO ₂ 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/SiO 2 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 α coat s = (1035 ± 42) ppm and α coat p = (1428 ± 97) ppm. 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.

Optical absorption of ion-beam sputtered amorphous silicon coatings

“…The absorption for an HR coating based on the IBS aSi single layer results, at the optimum heat treatment temperature, is comparable to previous measurements on an aSi/SiO 2 HR coating which was deposited via ion plating showing an absorption of approximately 1000 ppm at 1550 nm [16]. The IBS bilayer absorption is approximately a factor of two higher, but more similar to a multilayer and therefore more comparable to an actual HR coating.…”

“…The optical absorption of aSi-based HR coatings deposited by ion plating has been shown to be approximately 1000 ppm at 1550 nm [16] and therefore too high for use in mirror coatings using a design of two alternating high and low refractive materials. However, a multimaterial design, in which the outermost bilayers are made of Ta 2 O 5 and SiO 2 to significantly reduce the incident laser power, while in the lowest bilayers the Ta 2 O 5 is replaced by aSi, enables exploitation of the low mechanical loss and high refractive index of aSi to reduce the thermal noise at 20 K by 25 % while keeping the optical absorption as low as 5 ppm [17,18].…”

Ion-beam sputtered amorphous silicon films for cryogenic precision measurement systems

“…While the optical absorption of amorphous silicon (deposited by ion-plating) is typically significantly higher than required for use in gravitational wave detectors [47], recent work has suggested the use of a-Si as a replacement for the lower layers of tantala in a coating stack, where the light power is so low that the absorption requirements are significantly relaxed [48,49].…”

Effect of Stress and Temperature on the Optical Properties of Silicon Nitride Membranes at 1,550 nm

“…In addition, the very high refractive index (n Si = 3.5 at 1,550 nm) would reduce the numbers of layers required to achieve high reflectivity as the reflectivity per pair of high and low refractive layers increases with the contrast between the refractive indices. However, the optical absorption of aSi is too high for application in GW detectors at the envisioned wavelength of 1,550 nm (Steinlechner et al, 2014(Steinlechner et al, , 2017 and there has been research over the past years to reduce the absorption and to find ways to exploit the good mechanical properties while keeping the absorption low (Steinlechner et al, 2015;Yam et al, 2015;). An aSi/SiO 2 multilayer coating would be dominated by the mechanical loss of the SiO 2 layers, but Si 3 N 4 would be a suitable low-index material with a similarly low mechanical loss as aSi.…”