The metasurface consists of a microscopic chip patterned with structures smaller than the wavelength of light. When an infrared laser illuminates the surface, the design converts the incoming light to a higher color, or frequency, and emits it as a tightly confined beam whose direction is set by the polarization state of the input.
In experiments, the team converted infrared light at about 1530 nanometers, similar to wavelengths used in fiber optic networks, into green light near 510 nanometers. By rotating the polarization of the incoming beam, they steered the generated visible beam to specific output angles on demand.
"Think of it as a flat, microscopic spotlight that not only changes the color of light but also points the beam wherever you want, all on a single chip," said Andrea Alu, founding director of the CUNY ASRC Photonics Initiative and Distinguished Professor at the CUNY Graduate Center. "By making different parts of the surface work together, we get both very efficient conversion of light and precise control over where that light goes."
Engineers have used metasurfaces for years to bend and shape light using arrays of nanostructures on flat substrates. Traditional designs typically face a tradeoff between flexible, pixel level control of the wavefront, which often yields low nonlinear conversion efficiency, and collective resonances that boost nonlinear signals but limit fine control over the outgoing beam shape.
The new chip overcomes this tradeoff for nonlinear light generation, where one color of light is converted into another. The device supports a special kind of collective resonance known as a quasi bound state in the continuum, which traps and amplifies the incoming infrared field across the entire surface to strongly enhance nonlinear interactions.
At the same time, each nanoscale building block on the metasurface is rotated in a carefully engineered pattern, so the outgoing light acquires a position dependent phase profile. This geometric phase control makes the surface act like a built in lens or prism, enabling the emitted beam to be shaped and steered while benefiting from the collective resonance.
Through this combination of nonlocal resonance and local geometric phase control, the chip generates third harmonic light whose frequency is three times that of the incident infrared beam and directs it into chosen directions in space. Switching the polarization of the input flips the steering direction, providing a simple way to route the visible output without mechanical motion.
Measurements show that the third harmonic signal from the metasurface is about 100 times stronger than in comparable nonlinear beam shaping devices that do not exploit such collective resonances. This large enhancement points to a practical path toward efficient, chip scale frequency converters that also function as agile beam steering elements.
"This platform opens a path to ultra compact light sources and beam steering elements for technologies like LiDAR, quantum light generation, and optical signal processing, all integrated directly on a chip," said lead author Michele Cotrufo, a former CUNY postdoctoral fellow who is now an assistant professor at the University of Rochester. "Because the concept is driven by geometry, not by one specific material, it can be applied to many other nonlinear materials and across different colors of light, including the ultraviolet."
The researchers note that future architectures could stack or laterally combine several metasurfaces, each tuned to slightly different resonances, to maintain high efficiency over a broader range of wavelengths. Such multi band or broadband nonlinear metasurfaces could be engineered for advanced imaging systems, spectroscopy tools or integrated photonic processors that require multiple colors of light.
Beyond immediate device applications, the work illustrates how nonlocal effects in metasurfaces can be harnessed in nonlinear regimes without sacrificing spatial control. The approach could be adapted to generate and manipulate other nonlinear processes, such as second harmonic generation or frequency mixing, in compact photonic platforms.
The study, published in the journal eLight under the title "Nonlinear nonlocal metasurfaces," describes the design principles, fabrication steps and optical measurements that confirm the metasurface operation. The authors also outline routes to further boost performance by optimizing material choices and resonance quality, and by integrating the structures with on chip light sources.
This research was supported by the U.S. Department of Defense, the Simons Foundation, and the European Research Council, reflecting broad interest in metasurface based solutions for next generation photonics. The results suggest that flat, resonant structures that both convert and steer light could become key building blocks for LiDAR units, quantum light sources and optical computing elements that fit directly onto semiconductor chips.
Research Report:Nonlinear nonlocal metasurfaces
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