Optical interferometers, such as those used in gravitational wave detectors, are currently the most sensitive measurement devices. Their sensitivity and stability are limited by thermodynamically induced length fluctuations, or Brownian noise, of high-reflectivity mirror coatings. As this fundamental noise is determined by the material’s mechanical loss, optical coatings based on crystalline materials with low mechanical loss have been developed. However, the spectral noise of such coatings has yet to be accurately measured, as this would require measurement devices with an unprecedented level of precision in the zeptometer regime. A team of researchers led by Jialiang Yu and Uwe Sterr from the Physikalisch Technische Bundesanstalt in cooperation with the group of Jun Ye at JILA/NIST in Boulder, USA has now carried out the first detailed noise characterization of crystalline coatings at cryogenic temperatures. They have published the results of the work, which was supported by QuantumFrontiers and DQ-mat, in the current issue of Physical Review X.
In their experiments, the researchers used crystalline coatings in two independent cryogenic silicon optical resonators. They confirmed the expected low Brownian noise from the mechanical loss, but also unexpectedly discovered novel noise mechanisms that are much larger than the Brownian noise limit. These noise sources pose an obstacle to a straightforward improvement of the next generation of cryogenic gravitational wave detectors and ultrastable lasers. The observed spectral and spatial properties of the noise provide an important basis to develop an understanding and subsequent optimization of these semiconductor-based high-reflectivity mirror coatings.
Further investigations on crystalline coatings will be performed over a wider range of temperature and wavelength to help expose the underlying physical mechanisms. With an improved understanding, methods for noise reduction may be developed to finally fulfill the high expectations placed on these coatings for future ultrasensitive interferometers.
Original article:
Excess Noise and Photoinduced Effects in Highly Reflective Crystalline Mirror Coatings
Jialiang Yu, Sebastian Häfner, Thomas Legero, Sofia Herbers, Daniele Nicolodi, Chun Yu Ma, Fritz Riehle, Uwe Sterr, Dhruv Kedar, John M. Robinson, Eric Oelker, and Jun Ye
Phys. Rev. X 13, 041002
