Quantum-well metasurface boosts free-space second-harmonic generation to giant levels
Researchers combine a GaAs/AlGaAs multi-quantum-well stack with a high-Q TiO2 metasurface to access and enhance second-order nonlinearities from near-infrared to visible. Engineered interband transitions yield an intrinsic χ(2) ≈ 1.6 nm/V at 1.57 μm, while coupling to a guided-mode resonance in the metasurface elevates the effective χ(2) to ~14 nm/V (≈17 nm/V after correcting for SH absorption), enabling free-space-accessible SHG with normal incidence. The device uses a 16-period MQW film (~595 nm) and a metasurface that provides polarization conversion and strong field overlap; the strongest SHG occurs at small incidence angles with a high-Q resonance (Q ~1125). This co-design demonstrates a scalable route to compact, efficient nonlinear photonic devices and could extend to other nonlinear processes.
