Abstract
We have investigated the terahertz radiation generated by femtosecond laser pulses in a periodically poled lithium niobate crystal. Two different types of terahertz radiation, forward and backward, could be generated in quasi-phase-matching structures. The generation of these two different types of radiation is due to difference frequency generation under the quasi-phase matching condition. A broadband femtosecond optical pulse contributes to wave mixing, which occurs between two components of an ultrashort laser pulse and a terahertz pulse, in a quasi-phase-matching crystal. We observed a backward terahertz radiation at approximately 1.3 THz and the frequencies of terahertz radiations are easily controlled by varying the domain structures.
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