Generation of Terahertz Radiation using Nonlinear Cascade Frequency Conversion in KTP Crystal through the Phonon Absorption Region

One approach to constructing terahertz radiation sources for analyzing atmospheric gas composition is based on the use of nonlinear optical generators that convert the frequencies of intense near-infrared laser sources. This method’s downside is that its efficiency is proportional to the square of the generated frequency. This paper proposes a solution based on the use of cascade conversion, which involves generating intermediate frequencies in a local infrared transparency window of a nonlinear crystal and then downconverting these frequencies to the subterahertz region. For the first stage, an enhanced conversion due to a significant increase in quadratic susceptibility near optical phonons or stimulated polariton scattering is allowed. A second-stage model is considered for the process of subterahertz difference frequency generation under pumping in the local infrared transparency window in a KTP crystal. The fulfillment of collinear phase matching is demonstrated, and the figure of merit is estimated.