Redefining Precision With Optical Interference Coatings: A Five Minute Review With Assoc. Prof. Garrett Cole

A cross-disciplinary team of researchers led by Associate Professor Garrett Cole recently published a comprehensive mini-review in Optica exploring how advances in optical interference coatings are transforming precision measurement technologies. Drawing on more than 200 references and decades of research, the review, "Redefining precision interferometry and spectroscopy with high-performance optical interference coatings," examines how state-of-the-art mirror coatings are enabling breakthroughs in optical atomic clocks, gravitational-wave detection, ultra-sensitive molecular sensing, and emerging quantum technologies.

In this Five Minute Review, Cole discusses the history behind the publication, explains how these remarkable coatings work, and shares why quieter, or in other words—less wiggly—mirrors can unlock entirely new frontiers in science and technology.

Three years ago I was contacted by then editor-in-chief of Optica, Prof. Prem Kumar of Northwestern University, together with Christophe Dorrer, Deputy Editor, from the Laboratory for Laser Energetics at the University of Rochester. Prem and Christophe were interested in a review article covering the development arc of crystalline coatings, a high-performance optical coating technology that I invented about 15 years ago at the Institute for Quantum Optics and Quantum information, or IQOQI in Vienna, Austria. The seed was planted for this paper, but it would take time to grow…

In early 2025, I contacted Prof. Oliver Heckl, a longtime collaborator and now a Prof. in Physics at the University of Vienna, and we decided to work together on this and expand the scope to cover relevant coatings for precision measurement systems. Basically, coatings that allow for very high sensitivity measurements of time or frequency and distance.

Most people are unaware of the optical coatings in their lives although they interact with them many times a day. Examples are the coatings employed in camera systems in smart phones or coatings on glass picture frame covers to reduce unwanted or ghostly reflections. Optical coatings can also be used as protective “Blockers” to eliminate harmful UV rays from bleaching your car’s interior. And so much more. This technology is used everywhere, from the lab to even appearing in the fashion industry for colorful iridescent objects.

Optical interference coatings are made by combining individual, nanometer-sized, layers stacked up to many micrometers in thickness. Keep in mind that a layer that is 10 nm thick corresponds to a thickness of roughly 50 atoms. Typical layers are at the tens to hundreds of nanometers individually and the stack would then have from 20 to on order 100 of these layers of alternating materials, with an overall thickness that is ultimately less than a tenth the thickness of a human hair. Think of it as a microscopic layered pastry such as a croissant or Baklava.

These layers have to be so tiny because they are manipulating light directly at the scale of the physical wavelength (in other words the color) of the light. Appropriate designs leverage interference (hence “optical interference coatings”), which is to say how we overlap the peaks and valleys of the light wave so we can manipulate the light itself.

If you were to do an accounting exercise and physically track the individual particles of light, photons, that bounce off your bathroom mirror, what you would find is that for every 100 photons you throw toward the mirror you would get back maybe 96% of them, so you would lose 4 out of 100, which seems pretty good. 

Up that number to 1 million photons sent to the mirror, and you would find that about 40,000 were lost upon reflection. For some applications that is absolutely unacceptable. We want to push that number down as low as possible and get to where if a million photons are sent to the mirror you lose at most a handful. That can be very challenging to properly and confidently quantify when you are now talking not 96% reflectivity, but more than 99.999% reflectivity. 

At that level we have pushed the optical properties to their ultimate extreme, basically the mirror reflectivity is about as high as it will ever be, and this is where we are today. With optical losses at their limits, for many cutting-edge applications now, it turns out that the mirror “stability” is the limiting factor. By stability I mean unavoidable atomic-scale “jiggling” of the mirror surface. That sounds incredible or ridiculous depending on your perspective, but it turns out in these precision measurement systems, this is a key limitation!

In reality, everything is moving at the atomic scale–that is completely unavoidable as it is the very definition of temperature. You can cool your optics down to slow the jiggling, but even this is not always an option or has its own limitations. One interesting point is that some materials, or configurations of materials over a given frequency range will inherently jiggle less.

The crux of our review is to explain this—to outline the underlying physics on how to make the best mirrors, even at the atomic level. Optimization of current coating materials has enabled amazing things, but what we have shown is that if you make the coating from an assemblage of a SINGLE CRYSTAL material it will be even quieter and more still.

This research can literally improve our ability to measure time and space. We can now access quantum realms—these delicate states are extremely prone to being disturbed by the random jiggling of the mirrors for example or slight shifts in the color of light of the laser, etc. So we need the mirrors to be as still as possible.

Once we’ve reached this level of sensitivity, we can exploit quantum enhancements (for example, so called squeezing, etc.). The potential applications are enormous—from improved navigation to better time keeping (which has all sorts of applications for example in networking), portable instruments for geology (mapping for petroleum, aquifers and such underground, by directly measuring the local gravitational environment). And at the basic research level allowing for ever more sensitive measurements for tests of fundamental constants of nature (are they really constant?), probing general relativity, quantum gravity, and dark matter searches…and the list just keeps going.

I'm especially excited to continue advancing ultrastable lasers using these types of optical coatings for future optical clock systems. We're approaching a level of performance where laser frequency instability reaches the 10⁻¹⁸ range. In these cavities, that means the surface of the mirrors fluctuate, on average, a thousand times less than the size of a proton (which is femtometer scale).

Bringing that same level of mirror performance to future gravitational-wave detectors would be incredibly exciting. It could dramatically increase the rate at which we detect black hole and neutron star mergers, opening the door to even more precise measurements of our universe.

• Technology Context, "Ultralow-Noise Coatings for Gravitational-Wave Detectors" (Optics & Photonics News, "Year in Optics" Special Issue, Dec. 2023) -- An overview from OPN's annual review of top global optics research, explaining how crystalline coatings minimize thermal noise to enable next-generation precision optical measurement.
• Biographical Context, "Optica Fellow Profile: Garrett Cole" (Optica, Fellow Class of 2024) -- A look at Dr. Cole’s career pioneering substrate-transferred crystalline interference coatings and translating academic breakthroughs into commercial technology.
• Commercial & Industry Impact, "Thorlabs Adds Crystalline Coating Capability" (Photonics Spectra, Dec. 2019) -- Coverage detailing the commercial acquisition of Crystalline Mirror Solutions (CMS), demonstrating how the technology successfully translated from novel research into an industry-standard product line.
• Multimedia, "Berthold Leibinger Innovationspreis: Crystalline Mirror Solutions" (YouTube Video, 2016) -- A visual introduction to the founders of CMS, an overview of the substrate-transfer process, and a discussion of precision optics applications.

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Assoc. Prof. Garrett D. Cole is the Nelson E. Claytor Endowed Chair in Optical Sciences at the Wyant College of Optical Sciences. Cole earned his Ph.D. in Materials Science and Engineering from the University of California, Santa Barbara, in 2005. He began his career at Aerius Photonics and later held research positions at Lawrence Livermore National Laboratory, the Austrian Academy of Sciences, and Universität Wien. In 2012, he co-founded Crystalline Mirror Solutions, a spin-off commercializing semiconductor supermirrors for precision metrology. The company was acquired by Thorlabs in 2019, where Cole served as Managing Director of Thorlabs Crystalline Solutions before joining the faculty at U of A.

His work has earned international recognition, including the LIGHT2015 Young Photonics Entrepreneur Award, the Berthold Leibinger Innovationspreis, the SPIE Prism Award, and election as a Fellow of Optica. Named to the Photonics100 in 2024, Cole continues to advance optical coating technologies, mid-infrared applications, and quantum hardware solutions while mentoring the next generation of optical scientists.

Learn more about this work and the mini-review.