Researchers from the University of Pennsylvania and Duke University designed and built 2D arrays of closely packed microlasers that demonstrated the stability of a single microlaser, and that collectively achieved power density that were orders of magnitude higher. While single-mode lasers eliminate variations caused by noise in their beams and improve their coherence, they are dimmer and less powerful than lasers containing multiple simultaneous modes. The work, which was funded by the U.S. Army, combines higher laser power with beam stability and coherence. Robots and autonomous vehicles that use lidar to perform optical sensing and ranging and manufacturing materials processing techniques that use lasers are among the applications the research supports. One method to achieve a high-power, single-mode laser is to couple multiple identical single-mode lasers to form a laser array. “Intuitively, this laser array would have an enhanced emission power, but because of the nature of complexity associated with a coupled system, it will also have multiple supermodes,” said Liang Feng, associate professor in the departments of Materials Science and Engineering and Electrical and Systems Engineering at the University of Pennsylvania. “Unfortunately, the competition between modes makes the laser array less coherent.” Army-funded researchers designed and built two-dimensional arrays of closely packed microlasers that have the stability of a single microlaser but can collectively achieve power density orders of magnitude higher, paving the way for improved lasers, high-speed computing, and optical communications for the Army. Courtesy of the University of Pennsylvania. When two lasers are coupled, two supermodes are produced. That number increases quadratically as lasers are arrayed in the 2D grids used for sensing and lidar applications, however. “Single-mode operation is critical because the radiance and brightness of the laser array increase with number of lasers only if they are all phase-locked into a single supermode,” said Xingdu Qiao, a doctoral candidate at University of Pennsylvania. Qiao and his fellow team members added what they described as a “dissipative superpartner” to achieve phase-locked, single-mode lasing in a laser array. The team, he said, was inspired by the concept of supersymmetry from physics; in particle physics, supersymmetry is the theory that all elementary particles of the two main classes (bosons and fermions) have a yet-undiscovered superpartner in the other class. The mathematical tools that predict the properties of each particle’s hypothetical superpartner can also be applied to the properties of lasers. A challenge involves the adaptation of supersymmetry’s mathematical transformations to produce a complete array of superpartners that has the energy levels needed to cancel out all but the desired mode of the original. The researchers solved the mathematical relationships that governed the directions in which the individual elements in an array aligned in a row, to produce an array with five rows and five columns of microlasers. Before the team’s research, superpartner arrays could only have been one-dimensional, with each of the laser elements aligned in a row. “When the lossy supersymmetric partner array and the original laser array are coupled together, all the supermodes except for the fundamental mode are dissipated, resulting in single-mode lasing with 25 times the power and more than 100 times the power density of the original array,” said Zihe Gao, a postdoctoral fellow at the University of Pennsylvania. “We envision a much more dramatic power scaling by applying our generic scheme for a much larger array, even in three dimensions. The engineering behind it is the same.” The study also shows the compatibility of the technique with earlier research (performed by the same team) on vortex lasers that can precisely control orbital angular momentum. The ability to manipulate that property of light could enable photonic systems encoded at higher densities than previously imagined. The National Science Foundation and a Sloan Research Fellowship also supported the research, which was published in Science (www.doi.org/10.1126/science.abg3904).