Researchers at Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS) built highly reflective mirrors that direct the beams from high-powered continuous-wave (CW) lasers without incurring damage. The mirrors are made from single-crystal diamond. Conventional mirrors are made from multiple layers of coatings with various optical properties. Thermal loading degrades the optical performance of the layers. A single defect in a layer can cause a high-power laser beam to burn through, leading to irreversible damage to the mirror and to device failure. “Our one-material mirror approach eliminates the thermal stress issues that are detrimental to conventional mirrors formed by multimaterial stacks when they are irradiated with large optical powers,” professor Marko Loncar said. To circumvent the limitations of multilayered, multimaterial optical coatings, the team surface-engineered the optical response of single-crystal diamond, a material known for its strength and exceptional optical and thermal properties. Using an ion beam, the researchers etched nanostructures into the surface of the diamond, sculpting an array of columns on the surface of a 3- × 3-mm diamond sheet. The shape of the columns, wide on top and thin on the bottom, made the surface of the diamond 98.9% reflective. The researchers tested the diamond mirror using 10 kW of CW laser light at 1070 nm. The mirror demonstrated the ability to withstand the heat from the beam, which was strong enough to burn through steel. In contrast, a conventional dielectric mirror, when illuminated by the same beam, was unable to hold up against the high thermal loading exhibited at this power level. The experimental results were supported by beam profile measurements and numerical modeling. “We had a 10-kw laser focused down into a 750-µm spot on a 3- × 3-mm diamond, which is a lot of energy focused down on a very small spot, and we didn’t burn it,” researcher Haig Atikian said. “This is important because, as laser systems become more and more power hungry, you need to come up with creative ways to make the optical components more robust.” An illustration of a high-powered continuous-wave laser hitting nanostructures on a diamond mirror. Harvard SEAS researchers opened up opportunities in fields spanning from manufacturing and defense to communications and sensing by designing and building highly reflective mirrors that can direct the beams from high-powered CW lasers without incurring damage. Courtesy of the Loncar Lab/Harvard SEAS. High-power CW lasers are used in construction and manufacturing, defense, medical surgery, communications and sensing, optical spectroscopy, and other areas. These applications require optical components that can tolerate high CW optical powers and successfully direct light from the laser to the target. The diamond mirrors could broaden the scope and use of high-power CW laser applications and lead to a new category of optics that can operate under extreme conditions. “This approach has potential to improve or create new applications of high-power lasers,” Loncar said. The researchers envision the mirrors being used for defense applications, semiconductor manufacturing, industrial manufacturing, and deep-space communications. The mirror technology is also not limited to diamond alone, they said. Reflectors could be fabricated from a range of materials. For example, monolithic mirrors leveraging the extremely large bandgap of fused silica could be used to benefit ultrafast laser applications. The Harvard Office of Technology Development protected the intellectual property associated with the research and is exploring commercialization opportunities. The research was published in Nature Communications (www.doi.org/10.1038/s41467-022-30335-2).