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The sensitivity of future gravitational wave (GW) observatories will belimited by thermal noise in a wide frequency band. To reduce thermal noise, theEuropean GW observatory Einstein GW Telescope (ET) is suggested to usecrystalline silicon test masses at cryogenic temperature and a laser wavelengthof 1550nm. Here, we report a measurement of the optical loss in a prototypehigh-resistivity crystalline silicon test mass as a function of opticalintensity at room temperature. The total loss from both the bulk crystal andthe surfaces was determined in a joint measurement. The characterization windowranged from small intensities below 1W/cm^2, as planned to be used in ET, up to21kW/cm^2. A non-linear absorption was observed for intensities above a fewkW/cm^2. In addition we have observed an intensity-independent offset thatpossibly arises from absorption in the crystal surfaces. This absorption wasestimated to approx. 800 ppm/surface, which might be too high for a cryogenicoperation of a fibre-suspended silicon test mass. Such an offset was notobserved in other recent measurements that were insensitive to surfaceabsorption. Finally, a set of further characterization measurements is proposedto clearly separate the contributions from the surfaces and the bulk crystal.

Indication for dominating surface absorption in crystalline silicon test masses at 1550nm