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Physics, Optics, physics.optics
Abstract:
Future laser-interferometric gravitational wave detectors (GWDs) will
potentially employ test mass mirrors from crystalline silicon and a laser
wavelength of $1550\,\rm{nm}$, 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 $1550\,\rm{nm}$
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 $1550\,\rm{nm}$ 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 $\alpha^{\rm coat}_{s}=(1035 \pm 42)\,\rm{ppm}$ and
$\alpha^{\rm coat}_{p}=(1428 \pm 97)\,\rm{ppm}$. These results correspond to an
absorption coefficient which is lower than literature values for a-Si which
vary from $100\,\rm{/cm}$ up to $2000\,\rm{/cm}$. It is, however, still orders
of magnitude higher than expected for c-Si and thus still too high for GWD
applications.
