Instituto de Óptica “Daza de Valdés”

Measurement and Simulation of Mechanical and Optical Properties of Sputtered Amorphous SiC Coatings

Óptica para el espacio e iones de alta energía (GOLDION)

  • A critical point in sensitivity of gravitational wave detectors is in their mirror coatings

Madrid / 1 de December de 2022

A large international team made up of research centers and universities in Italy, France and Spain have studied the feasibility of using amorphous silicon carbide (a-SiC) mirrors for use in high-precision optical experiments, such as gravitational wave detectors. current and future.

gravitational wave detectors

In gravitational wave detectors such as the American LIGO interferometer, the European Virgo and JapaneseKAGRA light has to travel enormous distances before it can detect the variation of path caused by gravitational waves.
This is achieved by bouncing light hundreds of times off mirrors. These mirrors are one of the most critical elements of the installation in terms of loss of system sensitivity.

The sensitivity level of the system is such that the noise affecting the measurements is caused by the Brownian motion of the mirror molecules. caused by the temperature of the material. These thermally induced surface fluctuations can be more or less dissipated depending on the elastic properties of the mirror coating.

Amorphous silicon carbide

Crystalline silicon carbide (SiC) is a widely used semiconductor due to its good physical properties and the rich variety of its crystalline forms. Crystalline silicon carbide is part of a family of materials that exhibit a one-dimensional polymorphism, called polytypism. Its crystals are made up of layers of different types stacked one on top of the other, following a different pattern in each polytype.

microscope view of the surface of crystalline silicon carbide

Crystalline silicon carbide surface

An almost infinite number of polytypes of crystalline silicon carbide is possible, and this is reflected in the great variability of the physical properties it can have, such as optical bandwidth, thermal and electrical conductivity, etc.
In the case of the amorphous structure studied here, to this variety of configurations is added the possibility of doping control with other elements that allows adjusting the optical and mechanical properties to achieve a high refractive index, mechanical stability, and low absorption.

For this reason, the research team wanted to study amorphous silicon carbide as a possible coating, manufactured by different physical deposition methods.

However, after successfully culturing several amorphous samples from crystalline silicon carbide thin films, it has been found that the deposited samples are in something of an unstabilized state.
After analysis it has been found that the films show a level of light absorption at least 5 orders of magnitude higher than current coatings used in gravitational wave detectors, such as tantala-titania or silica.
This seems to indicate that a-SiC thin films are intrinsically much more absorbent in the infrared range than their crystalline counterpart.
In addition, the necessary elastic properties are also several times worse than the corresponding value for currently used coatings.

The main conclusion of this work is that amorphous crystalline silicon carbide, despite its favorable properties, appears to be uncompetitive with respect to other materials used in experiments that need ultra-low optical and mechanical losses.
However, interest in amorphous crystalline silicon carbide continues to exist due to its wide range of applications. Therefore, we believe that the data obtained throughout this work could serve as valuable resources for future studies.

This is a collaborative work of the University Of Padova, the Claude Bernard Lyon University, the University of Rome Tor Vergata, the Padova Institute of Photonics and Nanotechnology, the CNRS Advanced Materials Laboratory, the optics for space and high energy ions group of the CSIC, the University of Genoa, the University of Rome “La Sapienza” and the National Institute of Nuclear Physics of Pisa

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