Speaker
Description
Coating thermal noise limits the sensitivity of gravitational-wave detectors in their most sensitive frequency band. As third-generation observatories such as the Einstein Telescope advance toward cryogenic operation, current coating materials, Ta$_{2}$O$_{5}$ and SiO$_{2}$, become inadequate due to their strongly increased mechanical loss at low temperatures. Alternative materials such as a-Si and SiN$_{x}$ are promising candidates for low thermal noise coatings, but the high optical absorption of a-Si and the impurity-driven absorption of conventionally deposited SiN$_{x}$ remain critical limiting factors.
We propose the use of ion implantation to form highly reflective multilayer structures directly inside crystalline silicon mirror substrates, by creating buried low-refractive-index layers of SiO$_{2}$ or SiN$_{x}$ at controlled depths within c-Si. This approach preserves the excellent optical and mechanical properties of high-purity crystalline silicon in the high-index layers, while the implantation of pure nitrogen into high-resistivity c-Si is expected to significantly reduce the absorption of the SiN$_{x}$ layers compared to conventionally deposited material. Using a dedicated ion implanter, we report the first successful fabrication of a multilayer structure in the context of mirrors for gravitational-wave detectors. Simulations of the implantation schedule are in good agreement with Rutherford Backscattering Spectrometry and ellipsometry measurements, confirming the depth and uniformity of the implanted layers. Preliminary optical and mechanical loss results are also presented. The continued development of SiN$_{x}$-implanted structures represents a promising path toward coatings that meet the strict requirements of third-generation gravitational-wave observatories.