Thermal noise and mechanical loss of SiO$_2$/Ta$_2$O$_5$ optical coatings at cryogenic temperatures

TL;DR

This study investigates the mechanical loss and thermal noise of SiO$_2$/Ta$_2$O$_5$ optical coatings at cryogenic temperatures, providing insights relevant for gravitational wave detectors and optical clocks.

Contribution

It presents a systematic measurement of coating thermal noise from room temperature down to 4 K, directly linking thermal noise to mechanical loss at cryogenic temperatures.

Findings

Thermal noise decreases significantly at cryogenic temperatures.

Measured mechanical loss values are consistent with previous reports.

Results inform the design of low-noise optical coatings for precision instruments.

Abstract

Mechanical loss of dielectric mirror coatings sets fundamental limits for both gravitational wave detectors and cavity-stabilized optical local oscillators for atomic clocks. Two approaches are used to determine the mechanical loss: ringdown measurements of the coating quality factor and direct measurement of the coating thermal noise. Here we report a systematic study of the mirror thermal noise from room temperature to 4 K by operating reference cavities at these temperatures. The directly measured thermal noise is used to extract the corresponding mechanical loss for SiO$_2$/Ta$_2$O$_5$ coatings, which are compared with previously reported values.