Terahertz Generation and Domain Mapping in Periodically Poled Crystal

We measured multi-cycles of ps electric pulses (THz waves) generated by optical rectification in a periodically poled LiNbO₃ (PPLN) crystal using an electro-optic ZnTe detector of 1.55 mm-length. The input energy of fs pulse is about 160 uJ and its pulse duration is about 150 fs. The 2 mm-long PPLN crystal has the domain period of 30.8 mum. We found that the THz waves generated in the PPLN crystal always contained those generated in the bulk LiNbO₃ crystal. Multi-cycles of sub-ps electric pulses generated in the periodic domains allow us to map the distribution of the domain length directly. From the successful domain mapping, we confirmed that the sub-ps electric pulses were due to THz waves propagating reversely to the fs pump pulse, backward waves. No observation of the THz waves propagating parallel with the pump pulse, forward waves, was caused by the detection limit of our thick ZnTe crystal in high THz frequency range.