An apparatus for investigating a sample, the apparatus comprising an emitter (1) for irradiating the sample (27) with a beam of emitted electromagnetic radiation; and a detector (49) for detecting the radiation reflected from the sample, wherein there is an optically non-linear member (15) which functions as both an active part of the emitter and an active part of the detector, said emitter and detector using the same part of the optically non-linear member (15). The electromagnetic radiation is primarily intended to be in the terahertz (THz) frequency range.