Abstract: Gravitational waves are ripples in spacetime that travel at the speed of light and carry information about cataclysmic events such as the mergers of black holes and neutron stars. Ground-based gravitational-wave observatories are complex opto-mechanical systems, and their thermal environment sets a physical limit on the sensitivity. Future gravitational-wave detectors such as Cosmic Explorer (CE, US based) and Einstein Telescope (ET, EU based) are envisioned to operate with better optical coatings and substrates to reduce the impact of thermal noise and optical losses. CE is designed to be a 40km laser interferometer building on LIGO A+ technologies and ET (low frequency) aims to use cryogenically cooled crystalline silicon as mirrors since it has desirable thermal and optical properties. However, optical properties of both A+ coatings and crystalline silicon (cSi) require thorough investigation as these factors influence the overall optical losses in an interferometer. My research focusses on studying temperature dependent scatter in A+ coatings and cSi in separate experiments. Understanding these properties is essential for enabling next-generation detectors such as CE and ET to extend the reach of gravitational-wave astronomy to redshifts of 10 and beyond.