Abstract: Optically pumped masers are described. The devices are useful for microwave amplification, microwave phase shifting and microwave limiting function. The devices are based on electron spin polarization of a stable free radical species induced by an intermolecular energy transfer process acting via a photo induced radical-triplet pair mechanism. A paramagnetic triplet precursor chromophore in a matrix with the stable free radical species is irradiated, preferably using a pulsed laser to initiate the process with concomitant increase in matrix magnetic susceptibility. Microwave radiation incident on the photo activated matrix is reflected as amplified, phase shifted or limited microwave fields dependent on relevant device parameters and on the power of the incident microwave radiation.