The first direct detection of gravitational waves (GW) of cosmic origin (2015) and the ensuing multi-messenger observation of a neutron-star binary-coalescence event (2017) are recent milestones in Astroparticle Physics.
The visibility-range, and event-rate of existing (LIGO, Virgo, KAGRA) and future (Cosmic Explorer, ET) Earth-bound interferometric Detectors of Gravitational Waves are critically dependent on the optical finesse and the optical path-length fluctuations in the Fabry-Perot cavities making the interferometer arms.
The development of mirrors with extremely high reflectivity and extremely low thermal noise has evolved during the last decades into a whole new chapter of Applied Optics, leading to intensive studies along different directions, from multi-objective, multi-material optical coating design optimization, to optical coating material science, including artificially engineered optical materials (e.g., mixtures, metasurfaces and nanolaminates), and metrology (both classical and quantum of the relevant optical and visco-elastic/noise-affecting properties). These efforts resulted in spectacular improvements, from the earliest data-taking detectors (TAMA-300, 1999-2004) to the three-pronged LIGO-Virgo-KAGRA world-scale GW observatory presently in operation, and beyond. The related scientific and technological advances are relevant to a wide range of different applications, including ultrastable lasers and optical clocks. This special Session is aimed at presenting to the wide audience of PIERS Optics/Photonics experts a glimpse of the status, trends and challenges of this art, and stimulating further interactions. The Special Session agenda will focus on:

● Optical coating requirements for present/planned GW Detectors;

● Coating materials, amorphous : status and trends;

● Coating materials, crystalline : status, promise and challenges;

● Microscopic modeling and numerical simulation of coating material properties;

● Nanolaminated coating materials and bandgap engineering;

● Multi-material coatings and multi-objective coating-design optimization;

● Deposition technologies (IBS, ALD, CVD, etc), annealing  protocols and their impact  on relevant coating properties;      

● Coating optical metrology : ellipsometry, photothermal common-path interferometry, etc.;

● Direct measurement of coating thermal noise : HR stacks and monolayers;

● Indirect (ringdown-based) measurement of coating  thermal noise;

● More coating noises : thermooptic, photothermal, birefringence, etc.;

● Metasurface-mirrors : optical properties and noise