Revisiting Thermal Charge Carrier Refractive Noise in Semiconductor Optics for Gravitational-Wave Interferometers

TL;DR

This paper reevaluates the thermal charge carrier refractive noise in semiconductor test masses for gravitational-wave detectors, incorporating standing wave effects and using the Fluctuation-Dissipation theorem, finding the noise is lower than previously estimated but still non-limiting.

Contribution

It introduces a refined calculation of TCCR noise using FDT and standing wave effects, updating previous estimates for next-generation interferometers.

Findings

TCCR noise amplitude is up to √2 times greater at 10 K than previous estimates.

At 77-300 K, TCCR noise is 5-7 orders of magnitude lower than earlier claims.

Standing wave effects reduce the estimated TCCR noise amplitude.

Abstract

The test masses in next-generation gravitational-wave interferometers may have a semiconductor substrate, most likely silicon. The stochastic motion of charge carriers within the semiconductor will cause random fluctuations in the material's index of refraction, introducing a noise source called Thermal Charge Carrier Refractive (TCCR) noise. TCCR noise was previously studied in 2020 by Bruns et al., using a Langevin force approach. Here we compute the power spectral density of TCCR noise by both using the Fluctuation-Dissipation theorem (FDT) and accounting for previously neglected effects of the standing wave of laser light which is produced inside the input test mass by its high-reflecting coatings. We quantify our results with parameters from Einstein Telescope, and show that at temperatures of 10 K the amplitude of TCCR noise is up to a factor of $\sqrt{2}$ times greater than what was previously claimed, and from 77 K to 300 K the amplitude is around 5 to 7 orders of magnitude lower than previously claimed when we choose to neglect the standing wave, and is up to a factor of 6 times lower if the standing wave is included. Despite these differences, we still conclude like Bruns et al. that TCCR noise should not be a limiting noise source for next-generation gravitational-wave interferometers.