Dual-resonance metagrating for enhancing or annihilating third-harmonic generation at terahertz frequencies

Terahertz nonlinear response has considerable potential for applications in fields such as high-speed communication and radar imaging. However, owing to the weakness of its nonlinear effects and the scarcity of high-power sources, its practical applications remain largely constrained. In this work, we present a way to manipulate third-harmonic generation (THG) at terahertz frequencies via the design and study of dual-resonance metagrating with a compound lattice. The compound lattice supports two photonic bands of the magnetic polariton mode and the spoof surface plasmon polariton (SSPP) mode, whose eigenfrequencies satisfy a threefold relationship. Due to the geometric symmetry of the structure, an accidental bound state in the continuum (BIC) emerges at the SSPP band. The combined contribution of the two distinct resonant modes can significantly enhance the THG process at a lower input power. Meanwhile, the quasi-BIC mode operating at the nonlinear frequency has the capacity to confine the generated nonlinear harmonics within the metagrating, thereby suppressing the radiation channel of the third-harmonic light. Specifically, based on the nonlinear efficiency and resonance mechanisms, four different THG regimes are identified, namely, region I (dual-resonance region), region II (annihilation region due to BIC), region III (transition region), and region IV (single-resonance region). Our findings offer an alternative paradigm to control nonlinear processes through metagrating to tailor the light-matter interactions.