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 $1550\mathrm{nm}$, 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 $1550\mathrm{nm}$ 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${}^{\circ }$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 $1550\mathrm{nm}$ and for an angle of incidence of 44${}^{\circ }$. We used the photothermal self-phase modulation technique to measure the coating absorption in s-polarization and p-polarization. We obtained ${\alpha }_s^{\mathrm{coat}}=(1035\pm 42)\mathrm{ppm}$ and ${\alpha }_p^{\mathrm{coat}}=(1428\pm 97)\mathrm{ppm}$. These results correspond to an absorption coefficient which is lower than literature values for a-Si which vary from $100/cm$ up to $2000/cm$. It is, however, still orders of magnitude higher than expected for c-Si and thus still too high for GWD applications.

References in corpus (2)

• Titania-doped tantala/silica coatings for gravitational-wave detection
• Measurements of a low temperature mechanical dissipation peak in a single layer of Ta2O5 doped with TiO2

Cited by in corpus (1)

• Thermal Noise Reduction and Absorption Optimisation via Multi-Material Coatings