Hydrogen-Induced Ultralow Optical Absorption and Mechanical Loss in Amorphous Silicon for Gravitational-Wave Detectors

M. Molina-Ruiz, A. Markosyan, R. Bassiri, M. M. Fejer, M. Abernathy, T. H. Metcalf, X. Liu, G. Vajente, A. Ananyeva, and F. Hellman

Phys. Rev. Lett. 131, 256902 – Published 21 December 2023

Abstract

The sensitivity of gravitational-wave detectors is limited by the mechanical loss associated with the amorphous coatings of the detectors’ mirrors. Amorphous silicon has higher refraction index and lower mechanical loss than current high-index coatings, but its optical absorption at the wavelength used for the detectors is at present large. The addition of hydrogen to the amorphous silicon network reduces both optical absorption and mechanical loss for films prepared under a range of conditions at all measured wavelengths and temperatures, with a particularly large effect on films grown at room temperature. The uptake of hydrogen is greatest in the films grown at room temperature, but still below 1.5 at.% H, which show an ultralow optical absorption (below 10 ppm) measured at 2000 nm for 500-nm-thick films. These results show that hydrogenation is a promising strategy to reduce both optical absorption and mechanical loss in amorphous silicon, and may enable fabrication of mirror coatings for gravitational-wave detectors with improved sensitivity.

Received 16 June 2023
Revised 31 October 2023
Accepted 22 November 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.256902

Physics Subject Headings (PhySH)

Hydrogen bonds Internal friction Light propagation, transmission & absorption Silicon vacancies

Amorphous semiconductors Thin films

Gravitational wave detectors Optical absorption spectroscopy

Condensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

Authors & Affiliations

M. Molina-Ruiz ^1,*, A. Markosyan ², R. Bassiri ², M. M. Fejer ², M. Abernathy ^3,†, T. H. Metcalf ³, X. Liu ³, G. Vajente ⁴, A. Ananyeva⁴, and F. Hellman ¹

¹Department of Physics, University of California, Berkeley, California 94720, USA
²Edward L. Ginzton Laboratory, Stanford University, Stanford, California 94305, USA
³Naval Research Laboratory, Code 7130, Washington, DC 20375, USA
⁴LIGO Laboratory, California Institute of Technology, Pasadena, California 91125, USA

^*Corresponding author: manelmolinaruiz@gmail.com
^†Present address: The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland 20723, USA.