Quasi‐BIC Enhanced Broadband Terahertz Generation in All‐Dielectric Metasurface

Abstract Terahertz waves have widespread applications in various fields ranging from wireless communication to fundamental physics. Achieving bright and broadband terahertz sources still remains a challenge. Recent research has revealed that all‐dielectric metasurfaces with strong electromagnetic resonances and negligible Ohmic loss can enhance nonlinear optical processes and weaken the influences of phase‐matching conditions, providing new opportunities for developing terahertz sources. Here, an all‐dielectric metasurface supporting quasi‐BIC (bound states in the continuum), which enhances terahertz emission in a nonlinear optical film under the excitation of femtosecond laser pulses, is proposed. It is observed that the terahertz emission in lithium niobate (LiNbO₃) films is enhanced up to 17 times by a silicon dioxide metasurface and that the terahertz emission enhancement spans the frequency range in 0.1–4.5 THz. This mechanism for the terahertz emission enhancement is attributed to the tight confinement of the pump field in the LNibO₃ which is caused by quasi‐BIC resonances. The all‐dielectric metasurface is patterned on the LiNbO₃ film instead of etching it, providing a simple way for engineering nonlinear optical processes in LiNbO₃. This work paves the way for broadband terahertz emitters.