A system for generating radio frequency, RF, electromagnetic radiation, the system comprising: a maser comprising: a photonic crystal structure formed of dielectric material and comprising: (1a1) an array of cavities having a defect region disposed therein, and (1a2) an elongated slot disposed in the defect region, wherein the array of cavities and the elongated slot define a waveguide having a waveguide mode, and a vapor disposed in the elongated slot and comprising: (1b1) one or more input electronic transitions, and (1b2) an output electronic transition coupled to the one or more input electronic transitions and operable to emit a target RF electromagnetic radiation, the output electronic transition resonant with the waveguide mode of the waveguide; a laser system configured to provide input optical signals to the elongated slot of the photonic crystal structure, the input optical signals capable of exciting the one or more input electronic transitions of the vapor; and signal processing electronics in communication with the laser system and configured to control one or more properties of the input optical signals, the one or more properties comprising at least one of an intensity, a phase, or a frequency. a maser comprising: a photonic crystal structure formed of dielectric material and comprising: (1a1) an array of cavities having a defect region disposed therein, and (1a2) an elongated slot disposed in the defect region, wherein the array of cavities and the elongated slot define a waveguide having a waveguide mode, and a vapor disposed in the elongated slot and comprising: (1b1) one or more input electronic transitions, and (1b2) an output electronic transition coupled to the one or more input electronic transitions and operable to emit a target RF electromagnetic radiation, the output electronic transition resonant with the waveguide mode of the waveguide; a photonic crystal structure formed of dielectric material and comprising: (1a1) an array of cavities having a defect region disposed therein, and (1a2) an elongated slot disposed in the defect region, wherein the array of cavities and the elongated slot define a waveguide having a waveguide mode, and (1a1) an array of cavities having a defect region disposed therein, and (1a2) an elongated slot disposed in the defect region, wherein the array of cavities and the elongated slot define a waveguide having a waveguide mode, and a vapor disposed in the elongated slot and comprising: (1b1) one or more input electronic transitions, and (1b2) an output electronic transition coupled to the one or more input electronic transitions and operable to emit a target RF electromagnetic radiation, the output electronic transition resonant with the waveguide mode of the waveguide; (1b1) one or more input electronic transitions, and (1b2) an output electronic transition coupled to the one or more input electronic transitions and operable to emit a target RF electromagnetic radiation, the output electronic transition resonant with the waveguide mode of the waveguide; a laser system configured to provide input optical signals to the elongated slot of the photonic crystal structure, the input optical signals capable of exciting the one or more input electronic transitions of the vapor; and signal processing electronics in communication with the laser system and configured to control one or more properties of the input optical signals, the one or more properties comprising at least one of an intensity, a phase, or a frequency.