DFG/NSF: Novel Low Loss Coatings-Enabling the Third Generation of Gravitational-Wave Detectors

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A century ago, Einstein predicted the existence of gravitational waves (GWs) which are ripples in the curvature of space-time. On 14th September 2015, the (second-generation) detectors of the LIGO project made the first direct GW detection. This was a ground-breaking event for fundamental physics, observing for the first time a binary black hole system merging to form a single black hole, and has opened a new window into the universe allowing us to 'listen' to its gravitational signatures. The project supported by this award will enable sensitivities envisioned for the next generation GW detectors and new discoveries in gravitational astronomy, as well as an improvement of the detection rates for black hole and neutron star mergers. Gravitational waves cause changes in the separation of LIGO's mirrors so small that the thermal vibration of the mirrors and their coatings, so called Brownian thermal noise, will limit the sensitivity of current and future detectors. This project will develop new materials with low thermal noise to be used in future LIGO upgrades. This work is a collaborative effort between Martin Fejer's group at Stanford University and Jessica Steinlechner's group at Hamburg University, with this award supporting the US contribution to the collaboration. Progress in this area will benefit a broader applied research community, since there are applications beyond LIGO for low thermal noise coatings: atomic clocks, quantum information systems, precision interferometry all demand low-loss coatings. The nature of this multidisciplinary effort makes it ideal for training graduate students.Thermal noise in highly-reflective mirror coatings is directly proportional to the coating thickness, the mechanical loss of the materials and the mirror temperature. To significantly reduce coating thermal noise, future GW detectors will be cryogenically operated. The aim of this project is the development of highly-reflective multilayer coatings for future GW detectors, based on studies of coating properties, composition and structure, to meet the challenging requirements on absorption, reflectivity and mechanical loss. While this project mainly targets future cryogenic detectors, thermal noise reduction for detector upgrades operating at room temperature detectors is also part of the proposed research. A key part of this effort is optimizing coating materials which have demonstrated lower levels of mechanical loss, such as amorphous silicon and silicon nitride; and investigate the use of nano-layers, proven to suppress crystallization enabling higher heat treatment temperatures, which can lead to lower optical and mechanical losses. Hamburg will have a unique ability to deposit coatings using a pulsed laser deposition (PLD) system with heated substrates and variable laser pulse durations. Stanford has had a leading role within the LSC in developing experimental methods to characterize the optical and structural properties of the amorphous coating materials. This work will be underpinned by studies of coating properties to understand links between deposition parameters, coating structure, loss and absorption.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.