Original Article

High-efficiency terahertz third harmonic generation from quasi-bound states in the continuum in permittivity-asymmetric hybrid metasurface

¹ Shaanxi Key Laboratory of Liquid Crystal Polymer Intelligent Display, Technological Institute of Materials & Energy Science (TIMES), School of Electronic Information, Xijing University, Xi’an 710123, PR China
² Key Laboratory of Ultrafast Photoelectric Technology and Terahertz Science in Shaanxi, Xi’an University of Technology, Xi’an 710054, PR China
³ National Key Laboratory of Microwave Photonics (Tianyuan Laboratory), College of Electronic and Information Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, PR China
⁴ Terahertz Research Center, School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, PR China

^* e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 10 December 2025
Accepted: 13 April 2026
Published online: 19 May 2026

Abstract

Optical resonators with high quality (Q) factors are highly desirable in meta-optic and nanophotonic fields. Recently, bound states in the continuum (BIC) have emerged as a pivotal solution to enhance the Q factors of optical resonators to theoretically infinite values, leveraging non-dissipative energy confinement, specific resonance conditions, and destructive interference. However, in all-dielectric structures, the field distribution of BIC is predominantly confined within the metasurface (MS), posing significant challenges to the precise control of mode volume. Here, we propose a metal-dielectric hybrid MS that supports two symmetry-protected BIC, both of which are topologically protected by polarization vortices. Notably, the BIC mode volume is drastically reduced via coupling with surface plasmon polaritons, forming a hybrid mode with enhanced field localization. Specifically, we achieve high-efficiency terahertz third harmonic generation (THG) by breaking the dielectric constant symmetry of the structure to excite quasi-BIC. When the incident power of the fundamental frequency wave reaches 1 kW/cm², the THG conversion efficiency peaks at an impressive 1.5%. Our findings not only provide a pathway for designing optical components with both high Q factors and low mode volumes but also offer a feasible strategy to boost nonlinear optical effect, opening new avenues for advanced nanophotonic applications.