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Future laser-interferometric gravitational wave detectors (GWDs) willpotentially employ test mass mirrors from crystalline silicon and a laserwavelength of $1550\,\rm{nm}$, which corresponds to a photon energy below thesilicon bandgap. Silicon might also be an attractive high-refractive indexmaterial for the dielectric mirror coatings. Films of amorphous silicon (a-Si),however, have been found to be significantly more absorptive at $1550\,\rm{nm}$than crystalline silicon (c-Si). Here, we investigate the optical absorption ofa Si/SiO$_2$ dielectric coating produced with the ion plating technique. Theion plating technique is distinct from the standard state-of-the-art ion beamsputtering technique since it uses a higher processing temperature of about250$^\circ$C, higher particle energies, and generally results in higherrefractive indices of the deposited films. Our coating stack was fabricated fora reflectivity of $R=99.95\,\%$ for s-polarized light at $1550\,\rm{nm}$ andfor an angle of incidence of 44$^\circ$. We used the photothermal self-phasemodulation technique to measure the coating absorption in s-polarization andp-polarization. We obtained $\alpha^{\rm coat}_{s}=(1035 \pm 42)\,\rm{ppm}$ and$\alpha^{\rm coat}_{p}=(1428 \pm 97)\,\rm{ppm}$. These results correspond to anabsorption coefficient which is lower than literature values for a-Si whichvary from $100\,\rm{/cm}$ up to $2000\,\rm{/cm}$. It is, however, still ordersof magnitude higher than expected for c-Si and thus still too high for GWDapplications.

Optical Absorption Measurement at 1550 nm on a Highly-Reflective Si/SiO$_2$ Coating Stack