Enhanced terahertz high-harmonic generation from high-Q quasi-bound states in the continuum empowered by permittivity-broken metasurface

The extraordinary emergence of all-dielectric resonant meta-photonics underpinned by high refractive index and low optical loss materials promises a standout platform for unprecedented manipulation and subwavelength control of light. Recent significant breakthroughs in meta-photonics have to do with the exploration of the non-radiative eigenmodes that lie inside the light cone, called the bound states in the continuum (BICs), which have demonstrated considerable potential to achieve high quality (Q) factors but require ultra-small structure asymmetry, complicating fabrication and hindering precise control of Q factors. Here, we propose a scheme to excite quasi-BICs by breaking symmetry in the permittivity of the comprising meta-atoms where small permittivity perturbation renders the same effect in the conventional geometrically asymmetric design. Empowered by the insignificant permittivity perturbation introduced from chemical doping and carrier injection, arbitrarily small permittivity asymmetry, that is, infinitely high-Q factor, can be precisely and dynamically tailored. As an example of application, we numerically show the THz range, exceptionally high conversion efficiency for both third-harmonic and fifth-harmonic generation from the permittivity-broken metasurface (MS) benefiting from the extreme field confinement at high-Q quasi-BICs resonance. Our results present a feasible and efficient strategy to mitigate the shortcomings of previous photonic platform and shall shed light on the advances of nonlinear all-dielectric THz-MS.