Recently, CAI Yangjian's team and Han Zhanghua's research group published a paper entitled "High-Q photonic flat-band resonances for enhancing third-harmonic" in Newton, the flagship journal of material science under Cell Press generation in all-dielectric metasurfaces "research article. Shandong Normal University is the first completed unit, Sun Kaili is the first author of the paper, and Professor Han Zhanghua and Professor CAI Yangjian are the corresponding authors. Co-corresponding authors also include Professor Huang Junjun of East China Normal University and Professor Andrea Alu of the City University of New York. Professor Wang Wei and Professor Wang Keren from Sichuan University also made important contributions to this work.

High Q resonances in conventional photonic structures, such as QBIC(quasi continuous domain bound states) or QGMs(quasi guiding modes), have been widely used to enhance light-matter interactions. However, their steep dispersion around the Γ point is associated with a resonance shift that increases with the incidence Angle, which means that their coupling to the associated momentum is rather weak when large intensity densities are achieved using tightly focused narrow linewidth beams. Flat-band resonances are therefore an ideal solution to this challenge, as a focused beam containing different Fourier components in such a system is still associated with a uniform high-Q resonance. In photonic systems, van Hove singularities make a large number of photon states gather at a specific frequency by supporting the flat band structure, thus significantly increasing the density of states at that frequency. However, they are usually located below the light cone, preventing them from coupling with free space radiation. In addition, photonic structures with Moire lattices can support high Q-flat band resonances radiating into free space, however, such structures usually have high machining accuracy requirements and are difficult to achieve flexible Q-factor regulation. Therefore, the development of a simple and general method to achieve high Q flat band resonance to improve the conversion efficiency of nonlinear harmonic generation will bring important significance for applications such as nonlinear imaging and data storage.

In this work, the authors propose a simple method for achieving high Q photonic flat bands that can be supported by both Hermitian and non-Hermitian systems. The key to achieving high Q photon flat band resonance is: By perturbation of the lattice, the dispersion GM band originally located at the edge of the first Brillouin region (FBZ) is folded near the Γ point, and a strong interaction occurs between the band and the leakage mode propagating in the opposite frequency direction, and the coupling strength U of the interaction can be flexibly controlled by changing the lattice perturbation, so as to obtain the desired flat band resonance in the momentum space. By further symmetry breaking, the BIC at the flat band Γ point can be transformed into a quasi BIC resonance with high Q factor, and its Q value can be flexibly regulated without affecting the dispersion relationship of the flat band. By utilizing these high-Q flat-band resonances and excited by a pumped laser with a narrow linewidth, the THG signal is enhanced by nearly 10,000 times compared to c-Si films of the same thickness.

The work was supported by the National Key Research and Development Program, the National Natural Science Foundation, the Shanghai Pujiang Program, and the U.S. Air Force Office of Scientific Research and the Simons Foundation.