Observation of difference-frequency generation by mixing of terahertz and near-infrared laser beams in a GaSe crystal

We have observed difference-frequency generation by mixing terahertz and near-infrared laser beams in a GaSe crystal (i.e., terahertz frequency upconversion). This process has a potential for detecting terahertz waves. Using an InGaAs photodiode operating at room temperature the minimum detectable energy per pulse was 9.75nJ. When a fast photomultiplier tube was used instead, the lowest-energy per pulse was measured to be 245pJ. In addition, the temporal profile of the terahertz pulses was measured with a ns resolution.