本研究發現孔徑0.5x0.6mm2、長度25mm的週期性極化鈮酸鋰(periodically poled lithium niobate,PPLN)兆赫波方形波導晶體可以增強雷射差頻產生兆赫波(terahertz difference frequency generation,THzDFG)時的能量，其輻射出的正向(forward)與背向(backward)兆赫波波長分別是197與469m。這種晶體棒對於長波長的兆赫波而言是二維的兆赫波方形波導(rectangular waveguide)，然而對波長為1.54m的激發雷射光而言卻是塊狀晶體(bulk crystal)。當激發光強度皆為100MW/cm2的幫浦光(pump wave)與訊號光(signal wave)聚焦在二維的兆赫波方形波導晶體時，我們測量發現，如果與相同晶體長度與極化週期的鈮酸鋰兆赫波一維平面波導(slab waveguide)晶體做比較，二維波導所產生的正向兆赫波能量是一維波導的1.6倍，而背向兆赫波實驗結果則是1.8倍。

We report enhanced forward and backward THz-wave difference frequency generations at 197 and 469 m from a rectangular crystal rod made of periodically poled lithium niobate (PPLN) with an aperture of 0.5 x 0.6 mm2 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  m. 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/cm2 for the pump and signal waves in the two crystals.