Argon and Other Defects in Amorphous SiO2 Coatings for Gravitational-Wave Detectors

Abstract

Amorphous SiO2 thin films are one of the two components of the highly reflective mirror coatings of gravitational-wave detectors. For this study, layers of amorphous SiO2 on crystalline Si substrates were produced by ion-beam sputtering (IBS), using accelerated neutralized argon ions as sputtering particles, as is the case for the actual mirror coatings of gravitational-wave detectors. The aim of this study is to investigate the possible presence of various defects in the materials in order to improve the coating quality. We provide evidence that, due to the synthesis method, about 0.2 wt.% of Ar is present in the coatings, and it can be released by means of thermal treatments, starting around 400 °C. The time and temperature to obtain the total release of Ar increases with the coating thickness; for a thickness of 100 nm, all argon is released below 600 °C, while an isotherm of one hour at 900 °C is necessary for a coating 5 μm thick. Besides the Ar atoms left from the synthesis, other defects, such as Si clusters and silicon dangling bonds, are present in the coatings. The concentration of both of them is strongly reduced by thermal treatments either in vacuum or in air. The overall thickness of the coating is slightly increased after thermal treatments, as witnessed by the change of the period of interference fringes.

Photon Bose-Einstein condensation in arbitrary potential landscapes

“…Bubbles with sizes in the 10-100 nm range are visible in the bottommost two Ta 2 O 5 layers, but not in the SiO 2 . It has been shown [70,71] that argon incorporation is far lower in SiO 2 than Ta 2 O 5 , which matches our findings. Additional SEM/FIB scans of 40 nm high structures written at a -40 µm focus reveal no visible bubbles -the cross section is visually indistinguishable from a non-structured mirror.…”

Mirror Surface Nanostructuring via Laser Direct Writing—Characterization and Physical Origins

“…These amorphous thin films are widely used and have been studied frequently. [20,[25][26][27][28][29][30] In particular, it is known that amorphous tantalum oxide layers produced by IBS store a high degree of mechanical energy, which can be released by annealing processes, whereby the layer thickness of the films increases by a few percent. [31,32] Seen microscopically, this growth comes about through an enlargement of pore structures within the amorphous film.…”

“…[ 14 ] Furthermore, the study of polaritonic quantum gases based on open cavities [ 15,16 ] may benefit from this method. Beyond studying optical condensation phenomena, nanostructuring of high‐reflectivity mirrors via laser direct writing may also find novel applications in areas such as cavity ring‐down spectroscopy, [ 17 ] wavefront shaping, [ 18 ] and precision interferometry [ 19,20 ] (and references therein).…”

“…[14] Furthermore, the study of polaritonic quantum gases based on open cavities [15,16] may benefit from this method. Beyond studying optical condensation phenomena, nanostructuring of high-reflectivity mirrors via laser direct writing may also find novel applications in areas such as cavity ringdown spectroscopy, [17] wavefront shaping, [18] and precision interferometry [19,20] (and references therein).In addition to the permanent modification of mirror surfaces, it is also desirable to perform time-dependent control over the transverse flow of light in microcavities. Mirrors with integrated piezo arrays, [21] heater arrays, [22] and electrostatic membranes [23] may be used for this purpose.…”

Mirror Surface Nanostructuring via Laser Direct Writing -- Characterization and Physical Origins

“…The amorphous oxide films used in the quarter-wave stack of our mirrors, Ta 2 O 5 and SiO 2 , are produced via IBS using argon ions. These amorphous thin films are widely used and have been studied frequently [23][24][25][26][27][28][29]. In particular, it is known that amorphous tantalum oxide layers produced by IBS store a high degree of mechanical energy, which can be released by annealing processes, whereby the layer thickness of the films increases by a few percent [30,31].…”