A research team has demonstrated the first nonlinear metasurface with engineered nonlocality, that simultaneously delivers high‑Q resonance enhancement and pixel‑level control of the wavefront of third‑harmonic light, enabling the realization of a compact chip that generates and steers visible light at will through the geometric phase imparted in the metasurface, and can control the steering angle by simply changing the pump polarization. Built in an ultrathin amorphous silicon film patterned with subwavelength meta‑atoms, the device converts infrared light near 1530 nm into visible light near ~510 nm and steers the generated light to desired angles—without suffering from traditional phase‑matching constraints.
“By merging long‑lived, lattice‑supported resonances with local geometric‑phase control, we combine strong nonlinear conversion driven by lattice resonances and precise beam shaping through the geometric phase, all within a single flat ultrathin device,” said Andrea Alù, Distinguished Professor at the City University of New York (CUNY). “The combination of engineered nonlocalities, diffractive optics and nonlinearities provides a new knob for nonlinear nanophotonics.”
The metasurface leverages a quasi–bound state in the continuum (qBIC) — a delocalized, high‑Q lattice mode — to strongly amplify the infrared pump inside the film. A spatially varying rotation of the meta‑atoms encodes a nonlinear geometric phase so that the third-harmonic wave inherits a controlled phase, producing a controllable far‑field steering angle. The metasurface demonstrates beam steering of THG into specific diffraction orders, controlled by the pump chirality: opposite circularly polarized pumps steers the third harmonic to opposite sides. Moreover, thanks to the high-Q factor of the nonlocal modes supported by this metasurface design, the THG efficiency is simultaneously boosted: the efficiency reported by the authors exceeds prior nonlinear gradient metasurfaces by ~ two orders of magnitude under comparable conditions.
Conventional nonlinear metasurfaces typically rely on localized resonances—excellent for pixel‑level control, but limited in Q‑factor and therefore in nonlinear efficiency. Conversely, periodic gratings can host lattice resonances with large Q, but they sacrifice wavefront programmability. This work bridges the gap: it is the first nonlinear platform to retain nonlocal, high‑Q enhancement while preserving subwavelength, per‑pixel phase control—a key step toward on‑chip visible/UV sources, LiDAR beam steering, quantum light generation, optical signal processing, and polarization‑encoded computing.
“Because the concept is geometry‑driven, it is largely material‑agnostic, and it can be readily extended to other nonlinear materials, such as wide‑bandgap semiconductors and quantum‑engineered heterostructures,” added Michele Cotrufo, lead author, former postdoctoral fellow at CUNY and currently Assistant Professor at the University of Rochester. “Additionally, while in this work we focused on a single metasurface layer, stacking or multiplexing multiple metasurfaces supporting slightly detuned qBICs can also spread the functionality across broader bandwidths.”
eLight
Nonlinear nonlocal metasurfaces