Optical absorption of Si at 1550 nm at cryogenic temperatures

The new generation of gravitational wave interferometers are conceived to work at cryogenic temperatures to improve the signal-to-noise ratio by reducing unwanted noise sources associated to thermal excitations in the mirror test masses. Due to its favorable behavior at low temperatures, Silicon (Si) is nowadays considered as the most promising candidate to fabricate the suspended masses. However, another key requirement that needs to meet the design specifications is that the Si substrates remain very transparent at the operation conditions, with optical losses of the order of 1 ppm or below. In spite of the fact that Si is a widely investigated material, there are very scarce studies of its behavior at low temperature when such extremely small absorption levels are involved. In this thesis we propose to optimize and exploit a newly designed set-up to measure Si optical absorption with very high sensitivity and in cryogenic conditions. The experiment consists in measuring the temperature increase of a Si sample when the latter is illuminated by a high-power laser beam at the target

The new generation of gravitational wave interferometers are conceived to work at cryogenic temperatures to improve the signal-to-noise ratio by reducing unwanted noise sources associated to thermal excitations in the mirror test masses. Due to its favorable behavior at low temperatures, Silicon (Si) is nowadays considered as the most promising candidate to fabricate the suspended masses. However, another key requirement that needs to meet the design specifications is that the Si substrates remain very transparent at the operation conditions, with optical losses of the order of 1 ppm or below. In spite of the fact that Si is a widely investigated material, there are very scarce studies of its behavior at low temperature when such extremely small absorption levels are involved. In this thesis we propose to optimize and exploit a newly designed set-up to measure Si optical absorption with very high sensitivity and in cryogenic conditions. The experiment consists in measuring the temperature increase of a Si sample when the latter is illuminated by a high-power laser beam at the target wavelength (1550 nm).