A team of researchers at the City University of New York (CUNY) has achieved a significant breakthrough in light manipulation, developing a metasurface capable of converting invisible infrared light into visible light and steering that light with remarkable precision. The innovation, detailed in a study published in eLight on , promises to unlock new possibilities for compact light sources and integrated optical systems.
The device, an ultra-thin chip patterned with nanoscale structures, operates by shifting the frequency of incoming infrared light to a higher frequency within the visible spectrum. Crucially, the direction of the emitted visible light beam can be controlled simply by altering the polarization of the original infrared light – eliminating the need for any mechanical components. In laboratory tests, the team successfully converted infrared light at approximately 1530 nanometers (commonly used in fiber optic communications) into green light around 510 nanometers, directing the beam with high accuracy.
“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,” explained Andrea Alù, 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.”
Addressing a Fundamental Tradeoff in Metasurface Design
Metasurfaces, artificial materials engineered with nanoscale structures, have become increasingly popular tools for controlling light. They offer the potential to bend, focus, and shape light in ways not possible with traditional optics. However, a longstanding challenge in the field has been balancing efficiency and control. Traditional designs often force engineers to choose between one or the other.
Some metasurface designs prioritize precise control over light at each point on the surface, but this typically comes at the cost of lower efficiency in strengthening the light signal. Conversely, designs that focus on maximizing efficiency through collective light-matter interactions often sacrifice the ability to finely tune the beam’s shape and direction. The CUNY team’s innovation represents a significant step toward overcoming this limitation, specifically in the realm of nonlinear light generation – the process of converting light from one color to another.
Nonlocalities and Quasi-Bound States in the Continuum
The CUNY metasurface achieves this balance through a novel design that leverages “nonlocalities” and a phenomenon known as a quasi-bound state in the continuum (QBSC). The chip utilizes an ultrathin amorphous silicon film patterned with subwavelength meta-atoms. The QBSC allows the incoming infrared light to be trapped and intensified across the entire surface, boosting efficiency. Simultaneously, each of these tiny structural elements is carefully rotated, imparting a position-dependent phase shift to the outgoing light – effectively acting as a built-in lens or prism.
This combination of collective resonance and precise phase control enables the generation of “third harmonic light,” where the frequency of the emitted light is three times that of the incoming light. The ability to steer this third harmonic light is then achieved by simply changing the polarization of the input beam, offering a simple and effective control mechanism.
Enhanced Efficiency and Potential Applications
The researchers report that the third harmonic signal produced by their chip is approximately 100 times more efficient than similar beam-shaping devices that lack these collective resonances. This significant improvement in efficiency opens up a range of potential applications.
“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 Michele Cotrufo, lead author of the study and 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.”
Potential applications extend beyond those mentioned by Cotrufo. The ability to efficiently upconvert infrared light to visible light could also improve infrared imaging systems, as highlighted by recent research demonstrating high-efficiency infrared upconversion imaging using nonlinear silicon metasurfaces . This could lead to clearer images in low-light conditions and improved detection of thermal signatures.
The CUNY team suggests that future iterations of the technology could involve stacking or combining multiple metasurfaces, each optimized for a slightly different wavelength, to broaden the device’s operational range. This could lead to even more versatile and powerful optical systems.
The research was funded by the U.S. Department of Defense, the Simons Foundation, and the European Research Council.
