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

TL;DR

This study measures optical loss in crystalline silicon test masses at 1550nm, revealing significant surface absorption that could impact future gravitational wave detectors operating at cryogenic temperatures.

Contribution

It provides the first detailed measurement of surface versus bulk absorption in silicon test masses at relevant intensities for GW observatories.

Findings

Non-linear absorption observed at high intensities.

Surface absorption estimated at approximately 800 ppm per surface.

Potential challenges for cryogenic operation due to surface absorption.

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.