Terahertz (THz) waves occupy a segment of the electromagnetic spectrum between the infrared and microwave bands. As such, they can be used for imaging and sensing in ways not possible with conventional technologies such as X-ray and microwave. Because THz radiation transmits through almost anything that is not metal or liquid, the waves can see through most materials that might be used to conceal explosives or other materials, such as packaging, corrugated cardboard, clothing, shoes, backpacks, and book bags. Multiple broadband coherent generation and detection techniques have been demonstrated which use inorganic crystals. However, the crystal GaSe has not been explored in this context. This invention is directed to a broad bandwidth detector to measure the intensity information of THz frequency pulses. The detector includes coupling optics coupled to a coherent optical source, a GaSe substrate, a polarization detector, and means to measure the information. This system is particularly well suited to detect time-domain waveforms andorder frequency-domain spectra of short duration (1ps) THz frequency pulses.