Nonlinear optics changes the frequency of light and enables functions such as signal mixing and entangled-photon generation. Conventional crystals often require a long optical path. An international team instead combined a 595-nanometre-thick GaAs/AlGaAs multi-quantum-well structure with a precisely patterned array of titanium-dioxide nanopillars.

The asymmetric quantum wells produced a strong nonlinear response near 1.57 micrometres, but the useful component was difficult to access from free space. The metasurface locally redirected and enhanced the field, while tilting the sample by just 0.3 degrees broke the symmetry needed for second-harmonic generation.

The material alone reached a second-order nonlinear susceptibility of 1.6 nm/V, and the combined structure an effective value of about 14 nm/V. That is a measured property of a specific resonant sample, not the efficiency of a complete future device. The response also used a narrow resonance and deviated slightly from ideal behaviour at higher power, probably because of heating.

Possible uses include compact frequency converters, photonic signal processing and sources for quantum communication. Wider bandwidth, lower losses, improved fabrication and integration with sources and waveguides are still required. Pilot chip demonstrations could emerge within 3–6 years, while dependable mass deployment is likely to take longer.