Radiative properties of a two-level system in the presence of mirrors

Using a first-order approximation in time-dependent perturbation theory, we evaluate the probability per unit time of spontaneous emission by a two-level system coupled to a Hermitian massless scalar field in the presence of either one or two infinite perfectly reflecting plates (mirrors). We consider the effects of a finite interaction time between the system and the quantized field. Furthermore, using the image method in imaginary time, we study radiative processes at finite temperature. We show that vacuum and thermal fluctuations give independent contributions to the total transition rate and that both are modified by the presence of the mirrors. In the two-parallel-plate geometry, we find that the position average of the spontaneous emission rate is equal to the free-space rate for resonant cavities, but is suppressed in nonresonant cavities.