This invention relates to a wide band optical gate in the terahertz domain (wavelengths in the far infrared). It comprises a first optical source (2) emitting a first beam (FTHz) in said terahertz domain, a first plate made of a semiconducting material (1) illuminated by said terahertz beam and a second optical source (3) emitting a second beam (FIR) at a wavelength capable of saturating the first plate (1) made of a semi-conducting material and making it reflective at terahertz wavelengths. This invention also relates to a system for measuring terahertz signals and to a terahertz generator. It is particularly applicable to systems for measuring terahertz signals and to terahertz generators.