Mapping the Optical Absorption of a Substrate-Transferred Crystalline AlGaAs Coating at 1.5um

TL;DR

This study measures the optical absorption of a crystalline AlGaAs coating at 1550nm, showing low absorption levels promising for gravitational wave detector mirrors, but further reduction is needed to meet future requirements.

Contribution

It provides the first detailed measurement of optical absorption in a substrate-transferred crystalline AlGaAs coating at 1550nm, demonstrating its potential for low-loss optical applications.

Findings

Absorption measured at 30.2 ppm at 1550nm

Coating transmits approximately 70% of laser light at 1550nm

Further work needed to reduce absorption below 1 ppm

Abstract

The sensitivity of 2nd and 3rd generations of interferometric gravitational wave detectors will be limited by thermal noise of the test-mass mirrors and highly reflective coatings. Recently developed crystalline coatings show a promising thermal noise reduction compared to presently used amorphous coatings. However, stringent requirements apply to the optical properties of the coatings as well. We have mapped the optical absorption of a crystalline AlGaAs coating which is optimized for high reflectivity for a wavelength of 1064nm. The absorption was measured at 1550nm where the coating stack transmits approximately 70% of the laser light. The measured absorption was lower than (30.2 +/- 11.1)ppm which is equivalent to (3.6 +/- 1.3)ppm for a coating stack that is highly reflective at 1530nm. While this is a very promising low absorption result for alternative low--loss coating materials, further work will be necessary to reach the requirements of <1ppm for future gravitational wave detectors.