Thermorefringent noise in crystalline optical materials

TL;DR

This paper investigates thermorefringent noise in crystalline optical materials, revealing a fundamental thermal fluctuation source due to birefringence anisotropy that impacts high-precision optical systems.

Contribution

It establishes the theory of thermorefringent noise caused by birefringence fluctuations in crystalline materials, highlighting its significance for optical precision experiments.

Findings

Thermorefringent noise arises from birefringence fluctuations in crystals.

The theory links thermal fluctuations to depolarization effects.

Conditions to mitigate this noise are discussed.

Abstract

Any material in thermal equilibrium exhibits fundamental thermodynamic fluctuations of its mechanical and optical properties. Such thermodynamic fluctuations of length, elastic constants, and refractive index of amorphous materials -- like dielectric mirror coatings and substrates -- limit the performance of today's most precise optical instruments. Crystalline materials are increasingly employed in optical systems because of their reduced mechanical dissipation, which implies a reduction of thermo-mechanical fluctuations. However, the anisotropy of the crystalline state implies a fundamental source of thermal noise: depolarization induced by thermal fluctuations of its birefringence. We establish the theory of this effect, elucidate its consequences, discuss its relevance for precision optical experiments with crystalline materials, and hint at the conditions under which it can be evaded.