Abstract
The sensitivity of second and third generations of interferometric gravitational wave (GW) detectors will be limited by the 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 that is optimized for high reflectivity for a wavelength of 1064 nm. The absorption was measured at 1530 nm, where the coating stack transmits approximately 70% of the laser light. The measured absorption was lower than ppm, which is equivalent to ppm for a coating stack that is highly reflective at 1530 nm. 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 ppm for future GW detectors.
Highlights
Gravitational wave (GW) detectors are the most sensitive displacement measurement devices in the history of physics
In this paper we report absorption measurements at room temperature on a gallium arsenide (GaAs)/aluminium gallium arsenide (AlGaAs) coating stack at a wavelength of 1530 nm as a first step towards cryogenic crystalline coated mirrors
The AlGaAs coating stack is optimized for high reflectivity at 1064 nm while at 1530 nm we measured a transmission of approximately 70%
Summary
Gravitational wave (GW) detectors are the most sensitive displacement measurement devices in the history of physics. The requirements on the mirror-substrate material and the coatings are low mechanical loss to minimize Brownian thermal noise, low optical absorption to maintain the cryogenic base temperature and reduce background heating, as well as to avoid thermal lensing, and very accurately specified high reflectivity [6]. At 1064 nm an absorption of 12.5 ppm and a reduction of the Brownian thermal noise by a factor of three (corresponding to a tenfold reduction in the mechanical loss angle) compared to SiO2/Ta2O5 coatings has been reported [15]. These crystalline coatings have to be grown on bulk crystalline substrates with matching coating and substrate crystal structure and lattice parameters. The measured absorption is scaled to allow comparison with the highly reflective case
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