High-Q photonic flat-band resonances for enhancing third-harmonic generation in all-dielectric metasurfaces

Nonlinear optical processes are crucial to many applications, such as telecommunications and medical imaging. Traditional materials, however, exhibit only weak optical nonlinearity. High-quality (Q) factor photonic resonances are a promising means of enhancing nonlinear optical processes. Nevertheless, despite significant progress in boosting high-harmonic generations by harnessing high-Q resonances, further improvement of the nonlinear conversion efficiency is sought. Here, we propose a means of achieving flat-band high-Q resonances in all-dielectric metasurfaces featuring distorted photonic lattices. We demonstrate this experimentally in silicon metasurfaces, showing stable high-Q (∼1,600) flat-band resonances that persist over incident angles between −12° and 12° while highlighting optimizations that would further extend the angular range. We then show that these resonances lead to a large enhancement of third-harmonic generation by a factor of nearly 10⁴ times compared to silicon films of the same thickness. The introduced flat-band design unlocks new potential for a broad range of applications, including nonlinear imaging, photodetection, optical signal processing, and data storage, thereby laying the groundwork for the development of next-generation photonic devices.