We report enhanced forward and backward THz-wave difference frequency generations at 197 and 469 μm from a rectangular crystal rod made of periodically poled lithium niobate (PPLN) with an aperture of 0.5 × 0.6 mm~2 and length of 25 mm. The crystal rod appears as a waveguide for the THz waves but as a bulk material for the optical mixing waves at 1.54 μm. We measure enhancement factors of 1.6 and 1.8 for the forward and backward THz output powers, respectively, from the crystal rod when compared with those from a PPLN crystal slab waveguide of the same length, thickness, and domain period at an equal intensity of 100 MW/cm~2 for the pump and signal waves in the two crystals.
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