Analysis of THz-wave surface-emitted difference-frequency generation in periodically poled lithium niobate waveguide

It was shown that the periodically poled LiNbO3-waveguide with period of poling Lambda approximate to lambda /n(g) (lambda is the wavelength of emitted THz-wave, n(g) is a refractive index corresponding to optical group velocity) emits THz-wave difference-frequency generation (DFG) in the direction normal to the surface of the planar waveguide. The 5% distinction between the manufactured and required periods of gratings results only in a small deflection (similar to6 degrees) of the output THz-beam from the normal direction. The dependence of DFG efficiency on mode size is analyzed. The output THz power at lambda = 150 mum is estimated as 2 mW, taking into account imperfections in coupling incident beams with guided modes. It was shown that the efficiency of THz-wave DFG in surface-emitting geometry is more than for collinear geometry in bulk crystal, especially in the high-absorption wavelength region.