Special Seminar

Semiconductor supermirrors for precision spectroscopy in the near- and mid-IR

Garrett Cole, Thorlabs Crystalline Solutions
Location: P8445.2

Thursday, 30 May 2024 12:00PM PDT

*Pizza lunch will be provided.


Substrate-transferred crystalline coatings represent a new paradigm in optical interference coatings, combining semiconductor materials and microfabrication techniques with super-polished bulk optics. Their production entails a unique process, with separate epitaxial growth, microfabrication, and direct bonding that yields single-crystal films on arbitrary—including curved—optical surfaces. These “semiconductor supermirrors” were first demonstrated in 2013 with the key advantage being the ability to simultaneously achieve ultralow levels of optical and elastic losses.

With continuous refinement in manufacturing and metrology, we have reduced the total optical losses below 5 parts per million (ppm) for near-IR wavelengths spanning 1-1.6 µm, enabling a cavity finesse > 700,000 (reflectance > 99.9995%). Beyond their excellent optical performance, the near structural perfection of these monocrystalline multilayers yields a significant reduction in Brownian noise. This fundamental noise source is a key limitation in the stability of laser-based precision measurement systems, including cavity-stabilized lasers for optical atomic clocks and the high-reflectivity test masses employed in gravitational wave detectors. For the latter application, we have recently supplied low-loss crystalline mirrors up to 10 cm in diameter and are currently producing 20 cm diameter prototypes.

An additional advantage of these novel monocrystalline coatings is the ability to maintain ppm-levels of optical losses in the mid-IR. We marked a major milestone in our long-wavelength development efforts in 2022, demonstrating a cavity finesse exceeding 400,000 at a center wavelength of 4.5 µm (mirror absorption < 5 ppm). These ultralow-loss mid-IR mirrors are poised to revolutionize the performance of spectrometers in this key wavelength range for trace gas sensing.

Looking ahead, we see a bright future for coatings in applications requiring the ultimate levels of optomechanical performance.


Garrett D. Cole obtained his Ph.D. in electronic materials from UC Santa Barbara in 2005. Since completing his doctorate, Dr. Cole has held positions including the first employee of a high-tech startup (Aerius Photonics LLC, now FLIR Electro-Optical Components), a postdoctoral staff member at Lawrence Livermore National Laboratory, a Marie Curie Fellow of the Austrian Academy of Sciences Institut für Quantenoptik und Quanteninformation (headed by Anton Zeilinger, Physics Nobel Laureate 2022), and a university assistant at Universität Wien Fakultät für Physik, together with Markus Aspelmeyer. As a spin-off of fundamental quantum optomechanics research, Dr. Cole, together with Prof. Aspelmeyer, founded Crystalline Mirror Solutions (CMS) in 2012, an international photonics start-up commercializing semiconductor supermirrors for laser-based metrology and manufacturing. CMS was acquired by Thorlabs in December 2019 and rebranded as Thorlabs Crystalline Solutions. Dr. Cole has co-authored 2 book chapters and published more than 60 journal articles including papers in Science, Nature, Nature Physics, Nature Photonics, and Nature Communications. In addition to numerous startup prizes, Dr. Cole received the LIGHT2015 Young Photonics Entrepreneur Award, a Berthold Leibinger Innovationspreis in 2016, an SPIE Prism Award in 2017, and was selected by Unesco as a Trust Science Champion for the International Day of Light in 2021. In the fall of 2023 he was selected as a member of the Photonics100 and elected Fellow of Optica.