Molecular amplification and generation of microwaves

This paper is a general introduction to the field of amplification and generation of microwaves using molecular rather than electronic processes. The basic physical properties of molecular systems as related to amplification are reviewed. The properties of molecular amplifiers, such as gain, bandwidth, saturation power, and noise figure are discussed and several specific types of amplifiers are described. These include the molecular-beam maser, the "hot-grid cell", amplifiers excited by radio-frequency pulses and by "adiabatic fast passage", and amplifiers based on multilevel molecular internal energy systems, including "optically pumped" amplifiers. Molecular amplifiers may add very little noise to the signal to be amplified: noise figures under 1 dB can be obtained. With suitable feedback, such amplifiers become oscillators of extremely high spectral purity. High gains can be achieved using regeneration, but bandwidths are relatively small. These range from the order of tens of kilocycles for amplifiers using a gaseous molecular system, to megacycles, using solids. Molecular amplifiers saturate at low input powers, of the order of microwatts. Variations of the devices discussed may provide a means of generating millimeter and submillimeter waves.