Mid-infrared crystalline mirrors with ultralow optical losses

Abstract

Substrate-transferred crystalline coatings are a groundbreaking new concept for the fabrication of ultralow-loss mirrors. The low defect density single-crystal nature of these semiconductor supermirrors enables the lowest mechanical losses and hence unmatched Brownian noise performance [1], which currently limits the stability of precision optical interferometers. Another outstanding feature of these coatings is the wide spectral coverage of the GaAs/AlGaAs material platform. Limited by interband absorption at short wavelengths and the reststrahlenband at long wavelengths [2], crystalline coatings can be employed as low-loss multilayers from approximately 900 nm up to 5 μm and beyond. Excellent optical performance has been demonstrated in the near-infrared with excess optical losses (scatter + absorption) as low as 3 parts per million (ppm) [3], enabling cavity finesse values up to 360, 000 at 1.55 μm at room temperature and as high as 400, 000 at cryogenic temperatures for the same wavelength. Our first attempts at applying crystalline coatings in the mid-infrared has resulted in mirrors with excess optical losses (scatter + absorption) of 159 and 242 ppm at 3.3 and 3.7 μm, respectively. Remarkably, these results are already on par with current state-of-the-art amorphous mirror coatings, with the first use of mid-infrared crystalline mirrors in a cavity-enhanced spectroscopy setup enabling a detailed investigation of the reaction kinetics of the deuterated hydroxyl molecule, OD, and carbon monoxide, CO [4]. Absorption measurements based on photothermal common-path interferometry (PCI) [5] reveal that the optical losses are largely dominated by optical scatter. Via, PCI, we have confirmed absorption losses below 10 ppm at 3.7 μm, showing the enormous potential of GaAs/AlGaAs Bragg mirrors at mid-infrared wavelengths. An optimized fabrication process, which is currently under development, can efficiently suppress optical scatter due to accumulated growth defects on the surface. Ultimately, we foresee excess losses significantly less than 50 ppm in the mid-infrared spectral region.