The authors have studied the behavior of two-level-like Ba atoms driven by a strong near-resonant excitation field and coupled to a cavity-modified frequency-dependent vacuum reservoir. By tuning the cavity, it is found that the steady-state level of atomic excitation can be increased or decreased relative to the free-space norm. This change arises because of a cavity-mediated change in the atom's state after undergoing spontaneous decay. It is also found that the spectrum of resonance fluorescence observed out the side of the cavity is perturbed by the cavity in a manner that is again controlled by the cavity tuning and indicative of interesting changes in the atomic decay dynamics. A discussion of the atom-field dressed states provides some insight into the physics involved in the experiment. It is shown that spectral engineering of the vacuum leads to qualitatively new types of atomic behavior. Cavities using corrected elements or utilizing different sizes or geometries can be expected to increase significantly the magnitude of the effects.
