The near-zero threshold and the very high gain of the active optical microlaser are investigated both theoretically and experimentally. These properties are found to be determined by the synergy of several quantum-statistical processes taking 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 merging of the spontaneous emission with stimulated emission (StE) and of the consequent anomalous onset of the collective atomic behavior at very low excitation levels. An excitation threshold of about 50 pJ has been determined experimentally with a molecular oxazine microlaser excited by a femtosecond-pulse source. The relevance within the overall StE dynamics of the processes of mode-competition fluorescence loss, interatom transverse Bose correlations, and periodic excitation are 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 microcavity dynamics to quantum-dynamical systems such as the ‘‘microscopic’’ parametric oscillator and to Raman and Compton scattering is also considered.

• Received 3 March 1992

DOI:https://doi.org/10.1103/PhysRevA.46.4220