Abstract

The LANCET project tackles one of the outstanding challenges for the construction of the Einstein Telescope, the future European gravitational-wave detector: reducing light absorption in the mirrors, which will have to operate at cryogenic temperatures. The Einstein Telescope (ET) will enable us to observe the Universe with unprecedented sensitivity. To achieve this, however, it is necessary to develop highly advanced components, including multi-coated mirrors capable of maintaining high performance even at very low temperatures. Amorphous silicon (a-Si) and amorphous silicon nitride (a-SiN) are among the most promising candidates for the construction of the ET’s cryogenic mirrors. However, their optical absorption has so far proved too high for the ET’s extremely stringent requirements. The LANCET project proposes new strategies to reduce this absorption in both materials, with the aim of making them truly suitable for use in ET’s cryogenic mirrors. To achieve this, LANCET will combine advanced thin-film fabrication techniques with post-deposition treatments. In both cases, in-situ diagnostic techniques will enable the monitoring of changes in the relevant parameters throughout the process. This will enable the identification of the most effective conditions for reducing optical absorption, bringing it down to levels compatible with those required by the ET design.

By reducing the optical absorption of a-Si and a-SiN to levels never before achieved, the LANCET project will contribute not only to the construction of the Einstein Telescope’s cryogenic mirrors, but also to opening up new applications for these materials in other sectors of photonics and optoelectronics. Materials characterised by extremely high transparency are, in fact, required, for example, in integrated photonic circuits, photothermal microscopy and multilayer mirrors for high-power lasers.

The project aims to solve the problem of optical absorption in the cryogenic mirrors of the Einstein Telescope, the next-generation gravitational-wave detector to be built in Europe. The Einstein Telescope (ET) will enable a significant improvement and expansion of observations in the field of gravitational astronomy, but its success depends on the availability of key components, such as multi-layer mirrors suitable for cryogenic temperatures.

To date, the search for materials with the right properties to form the ET’s cryogenic mirrors has remained an unresolved issue. Amorphous silicon (a-Si) and amorphous silicon nitride (a-SiN) are among the best materials available, but their optical absorption has so far proved too high compared with the very stringent specifications required by the ET design. The project proposes strategies to reduce the optical absorption in both a-Si and a-SiN, and in doing so, aims to provide a viable solution for the fabrication of ET’s cryogenic mirrors. It will achieve this through an innovative combination of thin-film engineering techniques and post-deposition processes. The materials will be deposited in an ultra-clean environment, where contamination will be reduced to the minimum permitted by current technology. Post-deposition thermal processes will then be optimised using in situ diagnostic techniques, with the aim of minimising optical absorption in both a-Si and a-SiN. By reducing the optical absorption in the two materials under consideration to record-low levels, this will not only provide a solution for the fabrication of cryogenic ET mirrors, but also a new paradigm for utilising these materials in new optoelectronic and photonic applications where extreme transparency is required, such as integrated photonic circuits, photothermal spectromicroscopy, and multi-layer mirrors for high-power lasers.