Crystalline Mirrors

Highly reflective optical interference coatings are indispensable tools for modern scientific and industrial efforts. Systems with ultralow optical losses, namely, parts per million (ppm) levels of scatter and absorption, were originally developed for the construction of ring-laser gyroscopes by Litton Systems in the late 1970s. Stemming from this breakthrough, ion beam sputtering (IBS) is now firmly established as the gold standard process technology for generating ultralow-loss reflectors in the visible and near infrared (NIR).

Though exhibiting phenomenal optical properties, limitations of these amorphous coatings include excess Brownian noise, negatively impacting the ultimate performance of precision optical interferometers, poor thermal conductivity, as well as significant levels of optical absorption for wavelengths beyond 2 μm.

Crystalline interference coatings offer vastly reduced Brownian noise, the highest thermal conductivity (about 30 times higher than conventional mirrors) and achieve optical performance that rival those of IBS multilayers in both the NIR and MIR.

We fully characterize mid-IR mirrors which includes their reflectivity, transmission and losses (scatter and absorption) as well as their group delay dispersion. Together with CMS we work on new and perfected mirror designs.


Previous Relevant Work:

High-performance near- and mid-infrared crystalline coatings

G. D. Cole, Z. Wei, B. J. Bjork, D. Follman, P. Heu, C. Deutsch, L. Sonderhouse, J. Robinson, C. Franz, A. Alexandrovski, M. Notcutt, O. H. Heckl, J. Ye, and M. Aspelmeyer, “High-performance near- and mid-infrared crystalline coatings,” Optica, vol. 3, no. 6, pp. 647–656, 2016.

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 now realized significant improvements in the limiting optical performance of these novel single-crystal GaAs/AlxGa1-xAs multilayers. In the near-infrared (NIR), for coating center wavelengths spanning 1064–1560 nm, we have reduced the excess optical losses (scatter + absorption) to levels as low as 3 parts per million (ppm), enabling the realization of a cavity finesse exceeding 3 x 105 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-infrared (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 multilayer coatings, while our first prototype MIR optics have already reached state-of-the-art performance levels for reflectors covering this portion of the fingerprint region for optical gas sensing. Mirrors fabricated with our crystalline coating technique exhibit the lowest mechanical loss, and thus the lowest Brownian noise, the highest thermal conductivity, and, potentially, the widest spectral coverage of any “supermirror” technology 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.