arXiv subjects

I. W. Martin

Publications and source records attributed to I. W. Martin.

4 recordsLinked to original sources

arXiv2025

Low thermal noise mirror coatings utilising titanium dioxide and germanium dioxide mixtures

Upgrades to ground-based gravitational-wave observatories will require mirror coatings with reduced thermal noise, enabling improved detector sensitivity and extended astrophysical reach. Recent studies have shown that optical coatings utilising amorphous materials that exhibit a larger fraction of corner-sharing between adjacent structural units of metal-centered polyhedra are a promising route for reducing mechanical dissipation and thus thermal noise at room temperature. We report on multilayer optical coatings that are fabricated using germanium dioxide mixed with titanium dioxide (TiO$_2$:GeO$_2$) for the high index layers, and silicon dioxide (SiO$_2$) for the low index material. Single layers of TiO$_2$:GeO$_2$ are characterised to optimise the mixture proportion and based on that highly reflective multilayer stacks were deposited. Exceptional optical absorption at 1064 nm below 1 part-per-million (ppm) is observed in the multilayer stacks after heat treatment. The annealing process also induces the formation of blisters which leads to increased optical scattering. However, there is indication that blisters can be suppressed by decreasing the water partial pressure in the deposition chamber. Direct thermal noise measurements provide experimental verification of a significant 25\% reduction of thermal noise over the mirrors currently employed, which combined with sub-ppm levels of optical absorption show the potential of TiO$_2$:GeO$_2$ to improve the sensitivity of gravitational-wave observatories.

physics.ins-det↗

arXiv2021

Proposed large scale monolithic fused silica mirror suspension for 3rd generation gravitational wave detectors

Thermal noise from the suspension fibres used in the mirror pendulums in current gravitational wave detectors is a critical noise source. Future detectors will require improved suspension performance with the specific ability to suspend much heavier masses to reduce radiation pressure noise, whilst retaining good thermal noise performance. In this letter, we propose and experimentally demonstrate a design for a large-scale fused silica suspension, demonstrating its suitability for holding an increased mass of 160 kg. We demonstrate the concepts for improving thermal noise via longer suspension fibres supporting a higher static stress. We present a full thermal noise analysis of our prototype, meeting requirements for conceptual 3rd generation detector designs such as the high frequency interferometer of the Einstein Telescope (ET-HF), and closely approaching that required for Cosmic Explorer (CE).

astro-ph.IM↗

arXiv2021

Exploration of co-sputtered Ta$_2$O$_5$-ZrO$_2$ thin films for gravitational-wave detectors

We report on the development and extensive characterization of co-sputtered tantala-zirconia thin films, with the goal to decrease coating Brownian noise in present and future gravitational-wave detectors. We tested a variety of sputtering processes of different energies and deposition rates, and we considered the effect of different values of cation ratio $η=$ Zr/(Zr+Ta) and of post-deposition heat treatment temperature $T_a$ on the optical and mechanical properties of the films. Co-sputtered zirconia proved to be an efficient way to frustrate crystallization in tantala thin films, allowing for a substantial increase of the maximum annealing temperature and hence for a decrease of coating mechanical loss. The lowest average coating loss was observed for an ion-beam sputtered sample with $η= 0.485 \pm 0.004$ annealed at 800 $^{\circ}$C, yielding $\overlineφ = 1.8 \times 10^{-4}$. All coating samples showed cracks after annealing. Although in principle our measurements are sensitive to such defects, we found no evidence that our results were affected. The issue could be solved, at least for ion-beam sputtered coatings, by decreasing heating and cooling rates down to 7 $^{\circ}$C/h. While we observed as little optical absorption as in the coatings of current gravitational-wave interferometers (0.5 parts per million), further development will be needed to decrease light scattering and avoid the formation of defects upon annealing.

physics.ins-det↗

arXiv2010

Investigation of mechanical losses of thin silicon flexures at low temperatures

The investigation of the mechanical loss of different silicon flexures in a temperature region from 5 to 300 K is presented. The flexures have been prepared by different fabrication techniques. A lowest mechanical loss of $3\times10^{-8}$ was observed for a 130 $μ$m thick flexure at around 10 K. While the mechanical loss follows the thermoelastic predictions down to 50 K a difference can be observed at lower temperatures for different surface treatments. This surface loss will be limiting for all applications using silicon based oscillators at low temperatures. The extraction of a surface loss parameter using different results from our measurements and other references is presented. We focused on structures that are relevant for gravitational wave detectors. The surface loss parameter $α_s$ = 0.5 pm was obtained. This reveals that the surface loss of silicon is significantly lower than the surface loss of fused silica.

cond-mat.mtrl-sci↗