Spontaneous emission from two systems, namely,
N identical harmonic oscillators and
N identical two-level atoms, is studied without the use of the rotating-wave approximation. Certain new features of spontaneous emission, for instance, the dependence of the radiation rate on the initial dipole moment phase, are discussed.
We study the effects of the non-rotating wave approximation (non-RWA) on the spontaneous emission of a Λ-type three-level atom, where the metastable states are coupled to a common excited state by a weak laser field and the lower-level doublet is driven by a strong microwave field. When the non-RWA is applied to the interaction of the atom with the microwave field, the spontaneous emission spectrum is comprised of a series of ultrasharp lines. The physical interpretation of the spectral characteristics is given in light of the dressed states.
The collective spontaneous emission of
N multilevel atoms is studied in optical vector theory and without applying the rotating-wave approximation. The counter-rotating terms are included using a unitary transformation method. We analyze the decay dynamics starting from two initial conditions, the standard Dicke and timed Dicke states. In addition to the dependence on ensemble volume and density, we also study the effect of the ensemble geometry on cooperative emission for spheres, cubes, and quasi-two-dimensional shapes in different orientations. Finally, time-dependent cooperative spontaneous emission rates are introduced and investigated.