Enhanced broadband terahertz radiation generation near the reststrahlen band in sub-wavelength leaky-mode LiNbO3 waveguides

The generation of terahertz (THz) radiation in a compact geometry is crucial for the implementation of on-chip, coherent THz radiation sources. Here, via numerical time-domain simulations, we show that LiNbO3 (LN) waveguides having sub-wavelength core widths can provide highly efficient optical-to-THz radiation frequency conversion over short lengths. By exploiting the nonlinear susceptibility, χ33(2), enhancement of LN near its phonon reststrahlen band and utilizing THz leaky-mode guidance to minimize reststrahlen band absorption and improve phase matching, we show that broadband (0.2-11.6 THz) electric field pulses of 3.4 kV/cm can be generated at an optical-to-THz conversion efficiency of 2.5×10-4. These sub-wavelength leaky-mode waveguides provide a compact platform for wideband, coherent THz radiation sources.