Photothermal and thermo-refractive effects in high reflectivity mirrors at room and cryogenic temperature

TL;DR

This paper experimentally investigates photothermal and thermo-refractive effects in high reflectivity mirrors at room and cryogenic temperatures, providing insights crucial for enhancing interferometric measurement sensitivity.

Contribution

It presents a comprehensive experimental and theoretical analysis of mirror effects, including cryogenic conditions, to better understand and quantify noise sources in high-precision measurements.

Findings

Quantified photothermal and thermo-refractive effects across frequency range

Validated theoretical model with experimental data

Extracted effective thermo-mechanical parameters at cryogenic temperatures

Abstract

Increasing requirements in the sensitivity of interferometric measurements is a common feature of several research fields, from gravitational wave detection to quantum optics. This motivates refined studies of high reflectivity mirrors and of noise sources that are tightly related to their structure. In this work we present an experimental characterization of photothermal and thermo-refractive effects in high reflectivity mirrors, i.e., of the variations in the position of their effective reflection plane due to weak residual power absorption. The measurements are performed by modulating the impinging power in the range 10 Hz $\div$ 100 kHz. The experimental results are compared with an expressly derived theoretical model in order to fully understand the phenomena and exploit them to extract useful effective thermo-mechanical parameters of the coating. The measurements are extended at cryogenic temperature, where most high sensitivity experiments are performed (or planned in future versions) and where characterizations of dielectric film coatings are still poor.