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Physics, Optics, physics.optics
Abstract:
The sensitivity of future gravitational wave (GW) observatories will be
limited by thermal noise in a wide frequency band. To reduce thermal noise, the
European GW observatory Einstein GW Telescope (ET) is suggested to use
crystalline silicon test masses at cryogenic temperature and a laser wavelength
of 1550nm. Here, we report a measurement of the optical loss in a prototype
high-resistivity crystalline silicon test mass as a function of optical
intensity at room temperature. The total loss from both the bulk crystal and
the surfaces was determined in a joint measurement. The characterization window
ranged from small intensities below 1W/cm^2, as planned to be used in ET, up to
21kW/cm^2. A non-linear absorption was observed for intensities above a few
kW/cm^2. In addition we have observed an intensity-independent offset that
possibly arises from absorption in the crystal surfaces. This absorption was
estimated to approx. 800 ppm/surface, which might be too high for a cryogenic
operation of a fibre-suspended silicon test mass. Such an offset was not
observed in other recent measurements that were insensitive to surface
absorption. Finally, a set of further characterization measurements is proposed
to clearly separate the contributions from the surfaces and the bulk crystal.
