New research from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) paves the way to smaller and more efficient components for telecommunications, quantum communication, and other photonic technologies.
The work, from the lab of Professor Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering, combines layered engineered semiconductor materials with special nanostructured devices called metasurfaces, creating a unique semiconductor device that exhibits an extraordinarily strong ability to mix and transform different colors of light, including at near-infrared wavelengths that exist in the technologically valuable range of fiber-optic networks.
The research, a collaboration between Harvard SEAS, University of Texas Austin and the University of California, Irvine, was published in Nature Nanotechnology.
Whether in a classroom laser pointer, a biomedical sensor, or a quantum computer, many of today’s optical devices rely on a basic process known as nonlinear frequency conversion. This process generates new frequencies, or colors, of light when high-intensity laser beams are passed through a special material. Typically, these materials are crystals such as lithium niobate or gallium arsenide whose specific atomic structures naturally enable these color-changing processes.
Today’s optical devices are thus limited by the inherent properties of conventional crystals, making it difficult to build compact, scalable photonic devices, and hindering many advances – from chip-scale atomic clocks for GPS tracking, to ultra-fast fiber optic communications.
The new work closes a gap between material design and device design. Instead of just accepting the nonlinear properties of a conventional crystal, the research team designed a layered semiconductor and a metasurface together so that every part of the device, including electronic states, optical fields, and geometry, worked in tandem toward efficient frequency conversion at a given wavelength.
“This work combines, in a creative way, quantum engineering of an underlying material and enhancing its nonlinearity, together with optimized metasurface design,” Capasso said. “The overall nonlinear response is greatly enhanced and made usable for free-space optics.”
Multi-quantum wells and metasurfaces
The team started with crystalline materials of gallium arsenide and aluminum gallium arsenide, grown as a stack of ultra-thin semiconductor layers called multi-quantum wells. By carefully choosing the thickness and arrangement of these layers, they engineered electronic energy levels that produce strong interaction between light and matter. Importantly, they did this using a different type of electronic transition than what has traditionally been used, in order to achieve strong nonlinear effects in quantum wells, and enabling the effects to operate at much shorter wavelengths.
To fully unlock the material’s potential, the researchers then patterned its surface with an array of tiny light-shaping nanopillars – or metasurfaces, which trap and shape light at subwavelength scales. The Capasso group is a leader in metasurface technology.
The precisely tailored nanopillars on the metasurface orient the electromagnetic field in the correct directions within the layered semiconductor while concentrating the field inside the material, increasing the intensity of light within the structure. The researchers also controlled the symmetry of the fields, allowing interactions that would otherwise cancel out.
Together, these effects boost the effective nonlinear conversion of the light to three orders of magnitude higher than what would be observed from the unpatterned wafer, and higher than previously reported values for comparable devices at near‑infrared wavelengths — demonstrating the device’s potential utility in fiber-optic networks.
Collaborators led by Professor Seth Bank at the University of Texas at Austin designed the multi-quantum well material, while the Capasso group designed how the electromagnetic field behaves within that material, “so that it can make the most of the material property,” said Pernille Undrum Fathi, a Ph.D. student in Capasso’s lab and first author of the paper.
Compatibility with existing semiconductor manufacturing
The new platform is entirely based on standard compound semiconductor materials and planar nanofabrication, making it compatible with existing semiconductor manufacturing processes. Because the nonlinear response is so strong, similar devices could be made extremely compact while still converting light efficiently.
Potential uses include chip‑scale frequency converters to generate colors of light that are difficult to produce directly with lasers, and sources of entangled photon pairs for photonic quantum communication and computation.
“I’m very excited about the fact that this initial demonstration opens the door for so much more to be done at the intersection of nanophotonics and materials science,” Fathi said.
The research received federal support from an Air Force Office of Scientific Research Multidisciplinary University Research Initiative grant under award No. FA9550-22-1-0307). The work was performed in part at the Harvard University Center for Nanoscale Systems (CNS); a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which is supported by the National Science Foundation under NSF award No. ECCS-2025158.