Theory of monochromatic terahertz generation via Cherenkov phase-matched difference frequency generation in LiNbO3 crystal

A theory of Cherenkov phase-matched monochromatic terahertz (THz)-wave generation via difference frequency generation in a nonlinear crystal is developed. An experimental situation (LiNbO3 pumped by dual-wavelength near-infrared nanosecond pulses) is considered. This theory accounts for the finite size of pump beam and allows us to explore the generation of transverse THz wave vector. The output characteristic of this THz source is analyzed based on the analytical expression, including radiation pattern, conversion efficiency, and tuning range. Calculated tuning curves are presented, which reasonably agree with previous experimental results. The influence of divergence of the focused pump beam on total radiated energy is studied in detail. Optimal pump beam size that maximizing generated THz energy is obtained.