A device for coherent amplification or generation of electromagnetic waves by use of excitation energy in resonant atomic or molecular systems. “Maser” is an acronym for microwave amplification by stimulated emission of radiation. The device uses an unstable ensemble of atoms or molecules that may be stimulated by an electromagnetic wave to radiate energy at the same frequency and phase as the stimulating wave, thus providing coherent amplification. Amplifiers and oscillators operating on the same principle as the maser exist in many regions of the electromagnetic spectrum. Those operating in the optical region were once called optical masers, but they are now universally called lasers (the “l” stands for “light”). Amplification by maser action is also observed arising naturally from interstellar gases.