A device for generating RF energy from electromagnetic radiation of another form, such as light, includes an emitter responsive to the electromagnetic radiation for producing a beam of charged particles, an electrode spaced from the emitter to define a path for the charged particles, and a resonant structure for supporting RF oscillations and disposed with respect to the path to enable energy transfer between the charged particles and an RF field associated with the RF oscillations. When biased, the devices operate in a multi-pass mode, wherein the charged particles undergo multiple oscillations while remaining in phase with the RF field. When unbiased, the devices operate in a half-cycle mode to produce RF oscillations with no externally applied input power other than the electromagnetic radiation.