Coupling effects of the sum-frequency process and difference-frequency process on upconversion terahertz-wave detection via a DAST crystal.

Frequency upconversion technology with good performance including high sensitivity, fast response, and room-temperature operation is a promising method for terahertz-wave detection. The sum-frequency conversion and difference-frequency conversion jointly affect the detection ability for upconversion detection using organic crystals as nonlinear media. The concurrence of both processes has been ignored in past studies, which results in discrepancies between theoretical simulations and experimental results. In this paper, four-wave interaction equations involving two nonlinear conversion processes are proposed, and the effect of the sum-frequency process is analyzed in upconversion terahertz-wave detection via a 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal. The ratio of the sum-frequency signal to the difference-frequency signal varies for different terahertz frequencies and crystal thicknesses. Experiments suggest that theoretical simulations are good at predicting physical processes. Under certain conditions, the detection efficiency can be improved by simultaneously utilizing the two signals. The total signal photon number is not sensitive to the crystal thickness. Furthermore, the theoretical exploration of terahertz single-photon detection provides a noteworthy reference for future experiments.