1. An electron beam generator comprising: a continuous wave (CW) laser; a modulator for varying the intensity of, or deflecting, the optical output beam of said laser at modulation rates of at least several megahertz; a photoemissive cathode positioned for illumination by the output beam of said laser, said cathode including a substrate optically transmissive to said laser light, said substrate having a back side facing in a direction to receive an optical output beam of said laser and a front side opposite said back side, an electrically conductive film deposited on a selected region of the front side of said substrate, said film optically semitransparent to said laser light, a film of photoemissive substance selected from the group consisting of cesium antimonide, sodium potassium antimonide, cesiated gallium phosphide, cesiated gallium arsenide phosphide, and cesiated and oxygenated gallium arsenide, said film being deposited on said electrically conductive, optically semitransparent film and operable to emit a high current density, rapidly modulable beam of electrons upon illumination by said laser; and a thick, electrically conductive, metallic coating adjacent to said selected region and in electrical and thermal contact with said electrically conductive, optically semitransparent film; and a light optical train positioned between said laser and said cathode and operable to produce with the output beam of said laser a pattern of laser light on said photoemissive cathode such that said film emits a high current density, rapidly modulable beam of electrons to form an electron image determined by said pattern and which is suitable for directwrite semiconductor lithography.