Abstract <p>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.</p>