Low-loss crystalline coatings for the near- and mid-infrared

Substrate-transferred crystalline coatings have recently emerged as a groundbreaking new concept in optical interference coatings. Building upon our initial demonstration of this technology, we have recently realized significant improvements in the limiting optical performance of these novel single-crystal GaAs/AlGaAs multilayers. In the nearinfrared (NIR), for center wavelengths spanning 1064 to 1560 nm, we have reduced the excess optical losses (scatter + absorption) to less than 5 ppm, enabling the realization of a cavity finesse exceeding 300,000 at the telecom-relevant wavelength range near 1550 nm. Moreover, we demonstrate the direct measurement of sub-ppm optical absorption at 1064 nm. Concurrently, we investigate the mid-IR (MIR) properties of these coatings and observe exceptional performance for first attempts in this important wavelength region. Specifically, we verify excess losses at the hundred ppm level for wavelengths of 3300 and 3700 nm. Taken together, our NIR optical losses are now fully competitive with ion beam sputtered films, while our first prototype MIR optics have already reached state-of-the-art performance levels for reflectors covering the important fingerprint region for optical gas sensing. Thus, mirrors fabricated via this technique exhibit the lowest mechanical loss (and thus Brownian noise), the highest thermal conductivity, and, potentially, the widest spectral coverage of any “supermirror” technology, owing to state-of-the art levels of scatter and absorption losses in both the near and mid IR, all in a single material platform. Looking ahead, we see a bright future for crystalline coatings in applications requiring the ultimate levels of optical, thermal, and optomechanical performance.