Coupled-Mode Theory for Cherenkov-Type Guided-Wave Terahertz Generation Via Cascaded Difference Frequency Generation

A scheme for monochromatic terahertz (THz) generation via cascading enhanced Cherenkov-type difference frequency generation (DFG) in a sandwich-like waveguide is proposed. The novel scheme has the potential to overcome the quantum-defect limit and to provide an efficient output coupling. This process is elucidated by developing a coupled-mode theory and taking into account the pump depletion, waveguide mode properties, and THz output coupling. The effect of cascading enhancement is analyzed by comparing with non-cascaded DFG situation. It is predicted that THz power can be boosted by nearly 8-fold with a 400 MW/cm $^{2}$ pump in a 40-mm-long Si-LiNbO $_3$ -Si waveguide.