Design of an efficient terahertz wave source from a gap waveguide embedded in a silicon slot waveguide

Here, we propose an efficient scheme for terahertz (THz) wave generation on the basis of difference frequency mixing (DFM) using a GaP ridge waveguide embedded in a silicon slot waveguide. Phase matching in the DFM process, between the nonlinear polarisation wave induced by two near-infrared pumps and the generated THz wave in the low-refractive-index slot waveguide, was achieved by utilising the modal birefringence of the fundamental transverse electric- and transverse magnetic-like modes at telecom wavelengths in the GaP ridge waveguide. The effective cross-sectional area of the THz wave in the waveguide was small, 220 µm² at 2.26 THz, resulting in a photon conversion efficiency of 5.7×10^-2%. The THz output power approached the multi-µW level using the proposed waveguide structure.