An optical frequency mixer has been developed that uses a novel nanostructured metamaterial to concurrently generate 11 different colors of light. The metamaterial is made from an array of gallium arsenide (GaAs) nanocylinders. Each nanocylinder is only about 500 nm tall and about 400 nm in diameter. The nanostructures are laid out in a square pattern about 840 nm from each other. The tiny, repeating structures can interact with electromagnetic waves in ways that conventional materials cannot. Current methods for mixing light use phase matching, a technique that uses crystals to align the lightwaves perfectly. Because a single crystal can efficiently match the phases of one color of incoming light only, it can produce just one different color of light. Sandia National Laboratories postdoctoral appointee Polina Vabishchevich (left) and senior scientist Igal Brener made a metamaterial that mixes two lasers to produce 11 colors ranging from the near infrared to ultraviolet. Courtesy of Randy Montoya/Sandia National Laboratories. In a different approach, the team from Sandia National Laboratories used two NIR lasers with wavelengths tuned to the metamaterial’s resonant frequencies. Different mixing products were used to generate 11 different colors from the two laser pulses, without the need for phase matching. The novel metamixer exploits the combined attributes of resonantly enhanced electromagnetic fields at the metasurface resonant frequencies; the even-order and odd-order optical nonlinearities of GaAs; and the relaxed phase-matching conditions. Sandia National Laboratories’ new light mixing metamaterial, made up of an array of nanocylinders, produces 11 colors. The infrared light is actually 10 times stronger than the red light. Courtesy of Michael Vittitow. The even- and odd-order nonlinearities of the GaAs-based dielectric metasurface enabled a variety of simultaneous nonlinear optical processes across a broad spectral range. Specifically, seven different nonlinear processes (second-harmonic, third-harmonic, and fourth-harmonic generation; sum-frequency generation; two-photon absorption-induced photoluminescence; four-wave mixing; and six-wave mixing) were shown to simultaneously give rise to 11 new frequencies spanning the UV to NIR spectral range. The research team believes that the ultracompact optical mixer could enable a number of applications in biology, chemistry, sensing, communications, and quantum optics that require light at specific wavelengths. The team further believes that the novel metasurface could be optimized for other nonlinear mixing processes such as difference frequency generation. This would enable the production of femtosecond pulses covering the mid-IR spectral range. Though the conversion efficiency for the optical metamixer is very low, researcher Igal Brener believes the efficiency can be greatly improved with further work, perhaps by stacking multiple layers of metamaterial. The research was published in Nature Communications (doi:10.1038/s41467-018-04944-9).
