Cover story: Wang JT, Tonkaev P, Koshelev K et al. Resonantly enhanced second- and third-harmonic generation in dielectric nonlinear metasurfaces. Opto-Electron Adv 7, 230186 (2024).

Nonlinear dielectric metasurfaces provide a promising approach to control and manipulate optical frequency conversion processes at the nanoscale, thus facilitating both advances in fundamental research and the development of new applications in photonics, optical microscopy, and sensing. The group led by Prof.  Nicolae C. Panoiu from University College London proposes symmetry-broken metasurfaces made of centrosymmetric amorphous silicon to resonantly enhance second- and third-harmonic generation. Exploiting optical quasi-bound states in the continuum and guided mode resonances, the relative contribution of surface (local) and bulk (nonlocal) effects to second-harmonic generation and the bulk contribution to third-harmonic generation from the amorphous silicon meta-atoms are comprehensively studied through rigorous numerical calculations. In addition, optical resonances with high quality factors, which greatly boost light-matter interaction, are used to experimentally demonstrate giant enhancement of about 550 times of second-harmonic generation and nearly 5000-fold increase of third-harmonic generation. A good agreement between theoretical predictions and experimental measurements was observed. To gain deeper insights into the physics of the investigated nonlinear optical processes, the relation between nonlinear light emission and the structural asymmetry of the metasurface was numerically investigated, revealing that the generated harmonic signals arising from sharp resonances are highly dependent on the asymmetry of the meta-atoms. This work suggests a powerful strategy to enhance nonlinear optical interactions at the nanoscale and to effectively manipulate higher-harmonic generation in dielectric structures made of centrosymmetric materials, thus enabling novel photonic applications and devices with new or improved functionalities.