SPONTANEOUS AND STIMULATED-EMISSION IN THE THRESHOLDLESS MICROLASER

High gain and virtual zero threshold have been recognized to be distinctive properties of the microlaser since its original proposal and realization by our laboratory in 1988. These properties are investigated both theoretically and experimentally. They are found to be determined by the synergy of several quantum-statistical processes that take place in the condition of extreme field confinement provided by the peculiar Casimir-type topology of the optical microcavity. The determination of the microcavity mode structure leads to a detailed study of the process of spontaneous emission (SpE), its merging with stimulated emission, and the consequent anomalous onset of the collective atomic behavior at low excitation levels. A microlaser excitation threshold of approximately 50 pJ has been determined experimentally with a molecular Oxazine microlaser excited by a femtosecond source. The relevance in atomic dynamics of the processes of SpE inhibition-enhancement, mode competition, fluorescence loss, interatomic transverse Bose correlations, and periodic excitation-is investigated both theoretically and experimentally. A discussion of the overall process in terms of a second-order phase transition in a nonequilibrium statistical problem is given. The extension of the microlaser dynamics to other quantum systems, such as the microscopic parametric oscillator, and to Raman and Compton scattering is considered.