Birefringence of commercially available silicon samples, measured with our polarimeter.
Research › Detectors and simulation
Gravitational waves
Silicon optics for the Einstein Telescope
The cold silicon optics of the Einstein Telescope must not change the polarisation of light: we map how much they do.
The physics.
The Einstein Telescope will detect gravitational waves down to a few hertz. Its low-frequency interferometers are planned to run at cryogenic temperature, with optics of crystalline silicon instead of fused silica, read with infrared light at which silicon is transparent.
A perfect cubic crystal is optically isotropic, but stress, defects and impurities make silicon slightly birefringent: the two polarisations of light travel at slightly different speeds. In the arm cavities and the beam splitter this turns part of the light into the wrong polarisation, which is lost from the interferometer.
We built a polarimeter that maps birefringence across a substrate. A polarised laser beam crosses the sample, and the change in its polarisation, measured point by point, gives the size of the birefringence and the direction of its axes.
What we have shown.
The low-frequency interferometers of the Einstein Telescope will use cryogenic silicon optics, where birefringence turns into optical loss. We map birefringence in silicon samples and explain it through the crystal's anisotropy, and are extending the method to quartz and sapphire.
In (100)-oriented silicon the birefringence varies with position, in both size and axis direction: a single number is not enough to qualify a substrate.
Read first.
- Apparatus for the measurement of birefringence maps of optical materials: the case of crystalline silicon for Einstein TelescopeSoflau A.M., Della Valle F., Cescato F. … Malagutti L., Mazzolari A., Romagnoni M. et al. · Applied Physics B: Lasers and Optics 131 (2025)