A photoacoustic detector for detecting a gas which includes a resonant cavity having an inner wall capable of absorbing electromagnetic radiation and a passageway for circulating the gas through the resonant cavity. The detector also includes an electromagnetic source for generating an electromagnetic energy and an optical window associated with the electromagnetic source for 5 introducing the electromagnetic energy into the resonant cavity towards the absorbing inner wall surface, thereby preventing the electromagnetic energy from radiating throughout the whole length of the resonant cavity. A pressure sensor is provided for detecting a pressure signal inside the resonant cavity which is representative of the gas being detected. A method is also provided for detecting a gas using the photoacoustic detector.