In a gravitational wave detector, the mirrors act as the 'test masses' that feel the influence of the gravitational waves. This makes them very important in the detector, and as a result they have to be very precisely engineered in a number of ways.

The mirrors in gravitational wave detectors are made of two parts; the substrate and the coating. The substrate is the main bulk of the mirror, which is essentially one large transparent piece of glass or crystal. In the planned Advanced LIGO detector the substrate will be a cylindrical piece of special glass, called fused silica, 34cm in diameter and 20cm thick! To give a mirror its reflective characteristics, a coating is applied on top of the substrate. The coatings are made up of layer upon layer of alternately low and high refractive index materials. Some light is reflected at each boundary between low and high refractive index, so the more layers there are, the higher the overall reflectivity of the coating.

GEO 600 mode cleaner mirrors

GEO 600 mode cleaner mirrors. Image credits: Albert Einstein Institute Hannover

The mirrors have to be super-smooth so that the laser light isn't scattered in the wrong direction and lost. They should be kept ultra-clean too - even a tiny speck of dust can can ruin a pure beam and reduce the sensitivity of the detector.

Most of the mirrors in the detector have to transmit some light as well as reflecting it, so it is crucial that the substrate is very transparent. Any light absorbed in the substrate will be lost from the interferometer and it will also heat up the middle of the mirror, creating a 'thermal lens' which can create other problems in the detector.

To have low optical losses is one crucial property for the mirrors, but what other properties must we consider when choosing the material to make the mirrors from? It turns out that is also crucial for the materials to have very low mechanical losses too. Having low mechanical losses means that if you 'pinged' the mirror, it should continue to ring for a long time before the sound dies away. This is important for minimising the thermal noise of the mirrors; a noise source which arises from the tiny thermal vibrations of the atoms that make up the mirror! If the mirror has very small mechanical losses, these vibrations can be confined to a very small range of frequencies about the resonant frequency of the mirror, which is outside the frequency range in which we want to see gravitational waves.

Crystalline materials tend to be the materials with the lowest optical and mechanical losses, and so are a good choice for the mirrors. The planned Japanese detector LCGT will use Sapphire mirrors weighing tens of kilos; which would make some engagement ring! Other good crystalline materials include silica and silicon. Of course diamond would be great, but it is very hard to get such a huge diamond!