Theory of transient spontaneous emission by an atom in a planar microcavity

We study the transient regime of spontaneous emission by an atom excited in a planar high-Q microcavity. The variation of the spatial distribution of the radiation outside the cavity is determined as a function of the increas ing number of internal reflections for transition dipole moments parallel a nd perpendicular to the mirrors and for mirror separations equal to lambda/ 2 and lambda/4. Particular attention is given to the parallel dipole in the lambda/2 cavity, where the number of reflections is closely proportional t o the elapsed time. It is shown that the transient regime extends over time s of the order of the cavity decay time, in contrast to the effectively one -dimensional cavities previously studied, where the steady state is achieve d after the cavity round-trip time. The field distribution inside the cavit y tends to a steady-state form whose dimension parallel to the mirrors-is o f the order of the transverse coherence length. Despite this transverse loc alization of the field excitation, it is shown that the conditions for the achievement of strong atom-field coupling and the observation of Rabi oscil lations cannot be met in the planar microcavity. [S1050-2947(99)06911-5].