Axion Quasibound States in the Continuum and Their Enhanced Axion Nonlinearity

Overview & Synopsis

Dynamical axion quasiparticles (DAQs) in condensed-matter systmes have emerged as a fascinating platform for exploring axion physics and nonlinear optical phenomena. However, the amplitude of the coherent oscillating $θ$ field is even smaller than that of the static counterpart, making the experimental observation of DAQs particularly challenging. Here, we introduce symmetry-sensitive bound states in the continuum (BICs) physics to enable the all-optical detection of DAQs and enhance DAQ-mediated second-harmonic generation (SHG). Specifically, we uncover that DAQs induce optical shear that breaks the $C_{2z}$ symmetry of metagratings without geometric engineering, thereby converting symmetry-protected BICs into axion quasi-BICs (qBICs). In this context, the evolution of the quality factor and topological charge in momentum space, together with the appearance of optical resonances in far field associated with axion qBICs, offers achiral optical signatures for DAQ detection beyond the widely used Kerr and Faraday effects. Furthermore, by exploiting axion qBICs in the critical coupling regime to enhance fundamental optical field, we show that the SHG efficiency can be enhanced more than $10^5$-fold compared to bulk axion materials, achieving a conversion efficiency exceeding $10^{-6}~W^{-1}$. The SHG is governed by the magnetoelectric coupling, which differs from that in conventional SHG materials. Our findings establish a new approach for probing DAQs via achiral optical effects, while providing a versatile route for reconfigurable photonics and terahertz nonlinear optics.