The direct detection of gravitational waves by ground-based interferometric gravitational wave detectors in recent years has opened a new window of the universe, allowing the astrophysical observations of previously unexplored phenomena, such as the collisions of black holes and neutron stars. However, small thermodynamic fluctuations of the density of the thin films that compose the mirrors used within the gravitational wave detectors, such as the LIGO and Virgo detectors, give rise to noise which limits these instruments at their most sensitive frequencies. This "Brownian Thermal Noise" can be related to the inherent internal friction of the mirror materials through the fluctuation-dissipation theorem. Therefore, the improved sensitivity of gravitational wave detectors depends, to some extent, upon the development of optical thin films with low internal friction. The past two decades have therefore seen the growth of internal friction experiments undertaken within the gravitational wave detection community. This article attempts to summarize the results of these investigations and to highlight current research directions in order to foster a stronger dialogue with the larger internal friction and mechanical spectroscopy community.

We evaluate two methods to improve the background internal friction of the Double Paddle Oscillator (DPO), which has a nominal low temperature value of Q-1 ≈ 2×10-8. We find that annealing the DPO in vacuum at 300°C for 3 hours systematically reduced Q-1 at all temperatures and revealed a 100K internal friction peak by permanently removing it. We also find a striking decrease of low temperature Q-1 as the DPO geometry is altered through extended etching. This decrease is evaluated via Finite Element Method modeling in terms of mode mixing and attachment loss.

We have studied the thickness and quench-rate dependent internal friction of amorphous selenium (a-Se) thin films deposited at room temperature. The internal friction of a-Se films exhibit a temperature independent plateau below 1 K followed by a broad maximum at 10 K. The plateau, which is seen in almost all amorphous solids, is caused by dissipation by two-level tunneling systems (TLS), whose origin is still unknown. The maximum is caused by thermal relaxation over the same energy barrier that induces TLS. The internal friction and shear modulus are almost thickness independent from 100 nm to 10 µm. Unlike other elemental amorphous materials, the sufficiently low glass transition temperature (Tg) of a-Se (only about 10 K above room temperature) allows in-situ quench-rate dependent study of TLS. The amorphous structure resets itself by a thermal equilibration cycle above Tg. We show that a faster quench rate freezes a-Se to a lower density structure with a higher TLS density and vice versa. The changes are reversible supporting a relationship between different quenched states and the density of TLS. Our study shows that a-Se can be a simple monatomic amorphous system to constrain models for the origin of TLS in amorphous solids.