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非线性差频产生(Difference frequency generation,DFG)是实现太赫兹(Terahertz,THz)源的重要方式之一。利用微纳结构的DFG产生THz源可以不考虑相位匹配，同时是器件小型化、可集成化的重要研究方向。借助微纳结构的共振模式增强的局域电场在宽波段范围内实现高效的、可调谐的THz源是该领域的研究重点。本文研究了宽波段范围内具有高Q因子的光栅-波导结构中的DFG产生高效可调谐的THz辐射。理论上，通过调控相邻光栅中其中一个的位置扰动，从而实现光栅周期的加倍，进而使得布里渊区发生折叠，光线下方波导层中导模(Guide modes,GM)色散曲线折叠到光锥上方，形成超高Q因子的导模共振(Guide mode resonance,GMR)，可以实现在宽光谱范围内增强的THz产生。以硫化镉(Cadmium sulfide,CdS)光栅-波导为例，数值研究表明，在两束基频光光强均为100 kW/cm2时，THz的转换效率可达到10-8W-1的量级，为相同厚度CdS薄膜转换效率的109倍。通过改变两束基频光入射角，可实现不同共振基频组合，实现任意频率THz波产生，从而实现了在宽光谱范围内高效可调谐的THz源。Nonlinear difference frequency generation (DFG) is a key mechanism for realizing terahertz (THz) sources. Utilizing DFG within micro- and nano-structures circumvents phase-matching limitations while supporting device miniaturization and integrability, making it a significant area of research. Enhancing the local electric field through resonant modes in micro- and nano-structures has emerged as a promising approach to achieving efficient and tunable THz sources across a broad wavelength range. This study investigates the mechanism of DFG in high-Q-factor grating-waveguide structures for efficiently tunable THz radiation over a wide spectral range using numerical simulations based on the finite element method (COMSOL Multiphysics). Theoretical analysis reveals that modulating the positional perturbation of one of the adjacent gratings effectively doubles the grating period, causing Brillouin zone folding. This folding shifts the dispersion curve of the guided modes (GMs) within the waveguide layer above the light cone, forming a guided mode resonance (GMR) with an ultra-high Q-factor, thereby significantly enhancing THz generation across a broad spectral range. Using a cadmium sulfide (CdS) grating-waveguide structure as an example, numerical simulations demonstrate that the THz conversion efficiency reaches an order of 10⁻⁸ W⁻¹ when both fundamental frequency beams have an intensity of 100 kW/cm², which is 10⁹ times higher than the conversion efficiency of a CdS film of the same thickness. Moreover, the fundamental frequency resonance wavelength can be widely tuned by adjusting the incident angle. High-Q-factor resonance modes enable various fundamental frequency combinations by changing the incident angles of the two fundamental frequency beams, facilitating the generation of THz waves with arbitrary frequencies. This approach ultimately enables a highly efficient and tunable THz source over a wide spectral range, providing valuable insights for generating THz sources on micro- and nanophotonic platforms.
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