Harvard researchers have introduced a single metasurface that can effectively tune the different properties of laser light, including wavelength, without additional optical components. The metasurface can split light into multiple beams and control their shape and intensity in an independent, precise, and power-efficient way. The ability to precisely control the various properties of laser light is critical to technologies that span commercial (VR) headsets to microscopic imaging for biomedical research. Many current laser systems rely on separate, rotating components to control the wavelength, shape, and power of a laser beam, making these devices bulky and difficult to maintain. The new work from Harvard opens the door for lightweight and efficient optical systems for a range of applications, including quantum sensing and AR/VR headsets. The incident light can be split into three independent beams, each with different properties: a conventional beam (right), a beam known as a Bessel beam (center), and an optical vortex (left). Courtesy of Christina Spägele/Harvard SEAS. “Our approach paves the way to new methods to engineer the emission of optical sources and control multiple functions, such as focusing, holograms, polarization, and beam shaping, in parallel in a single metasurface,” said Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS, and senior author of the paper. The tunable laser is composed of just two parts: a laser diode and a reflective metasurface. Unlike previous metasurfaces, which relied on a network of individual pillars to control light, this surface uses so-called supercells — groups of pillars that work together to control different aspects of light. When light from the diode comes into contact with the supercells of the metasurface, part of the light is reflected back, creating a laser cavity between the diode and the metasurface. The other part of the light is reflected into a second beam that is independent from the first. “When light hits the metasurface, different colors are deflected in different directions,” said Christina Spägele, a graduate student at SEAS and first author of the paper. “We managed to harness this effect and design it so that only the wavelength that we selected has the correct direction to enter back in the diode, enabling the laser to operate only at that specific wavelength.” To change the wavelength, the researchers moved the metasurface, with respect to the laser diode. “The design is more compact and simpler than existing wavelength tunable lasers, since it does not require any rotating component,” said Michele Tamagnone, former postdoctoral fellow at SEAS and co-author of the paper. The team further demonstrated that the shape of the laser beam can be fully controlled to project a complex hologram, as well as the ability to split the incident light into three independent beams, each with different properties: a conventional beam, an optical vortex, and a beam known as a Bessel beam, which looks like a bull’s-eye and is used in applications such as optical tweezing. “In addition to controlling any type of laser, this ability to generate multiple beams in parallel and directed at arbitrary angles, each implementing a different function, will enable many applications from scientific instrumentation to augmented or virtual reality and holography,” Capasso said. The research was published in Nature Communications (www.doi.org/10.1038/s41467-021-24071-2).