New type of metasurface allows unprecedented laser control

Many of today's technology relies on the ability to control the properties of laser light. This is true for everything from commercial VR headsets to microscopy for biomedical research. Today's laser systems are heavy and complicated because they rely on rotating parts to control the power, wavelength and shape of the laser beam. Researchers at Harvard John A. Paulson School of Engineering and Applied Sciences now have a single metasurface that is capable of tuning the various properties of laser light without the use of optical components. The metasurface can divide light into multiple beams, and can control their intensity and shape in an independent, precise, and efficient way. This research has opened the doors to lightweight, efficient optical systems that can be used for a variety of applications, including quantum sensing and VR/AR headsets. Federico Capasso is the Robert L. Wallace Professor in Applied Physics and Vinton Hayes Senior Fellow in Electrical Engineering at SEAS. He was also the senior author of this paper. Nature Communications published the research recently. Only two components make up the tunable laser: a laser diode as well as a reflective surface. This surface uses supercells instead of previous metasurfaces that relied on individual pillars to control the light. Instead, it uses a group of pillars working together to control different aspects. The diode emits light that hits the supercells of the metasurface. A portion of this light is reflected back to create a laser cavity. The second part of the light is reflected back into a beam that is separate from the first. Christina Spgele, a graduate student in SEAS, and the first author of this paper, stated that different colors are deflected when light hits the metasurface. "We were able to harness this effect, and designed it so that the laser can only operate at the wavelength we chose. Researchers can change the wavelength by simply moving the metasurface relative to the laser diode. Michele Tamagnone (ex-postdoctoral fellow at SEAS, co-author of this paper) stated that the design was more compact and simple than other wavelength-tunable lasers. Researchers also demonstrated that the shape and projection of holograms can be controlled by laser beams -- this time the Harvard shield, a century-old complex. The researchers also showed that they could split the incident light into three distinct beams with different properties. These beams included a conventional beam and an optical vortex. A beam known as the Bessel beam looks like a bullseye, and can be used in many applications, including optical tweezing. Capasso stated, "In addition to controlling all types of lasers, this ability generate multiple beams in parallel, directed at arbitrary angles and each implementing an a different function will enable many applications, from scientific instrumentation, augmented or virtual reality, and holography." ### Marco Piccardo, Marcus Ossiander, and Dmitry Kazakov co-authored the research. The Air Force Office of Scientific Research grant FA95550-19-1-0135, and the Office of Naval Research grant MURI no. N00014-20-2450

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