Optical absorption measurements on crystalline silicon test masses at 1550 nm

Crystalline silicon is currently being discussed as test-mass material for future generations of gravitational wave detectors that will operate at cryogenic temperatures. We present optical absorption measurements on a large-dimension sample of crystalline silicon at a wavelength of 1550 nm at room temperature. The absorption was measured in a high-intensity monolithic cavity setup using the photo-thermal self-phase modulation technique. The result for the absorption coefficient of this sample with a specific resistivity of 11 k Omega cm was measured to be alpha(A) = (264 +/- 39) ppm cm(-1) for an intensity of 700 W cm(-2).