The invention relates to an optically-controlled field-emission cathode, comprising a substrate (10, 20, 30, 40, 50, 60, 70, 80, 90, 100) having at least one conducting surface (11, 21, 31, 41, 51, 61, 71, 81, 91, 101) and at least one conducting emitter element (16, 26, 36, 46, 56, 66, 76, 86, 96, 106) in the vicinity of a conducting surface, characterized in that it also comprises at least one photoconducting element (13, 23, 33, 43, 53, 63, 73, 83, 93, 103) electrically connected in series between at least one emitter element and a conducting surface of the substrate. Another subject of the invention is an amplifier tube comprising such a cathode. The application is for Vacuum tubes, in particular for microwave amplification, with a view for example to applications in telecommunications.