Abstract
This paper is devoted to clarifying the implications of hyperfine (HF) interaction in the formation of adiabatic (i.e., "laser-dressed") states and their expression in the Autler-Townes (AT) spectra. We first use the Morris-Shore model [J. R. Morris and B. W. Shore, Phys. Rev. A 27, 906 (1983)] to illustrate how bright and dark states are formed in a simple reference system where closely spaced energy levels are coupled to a single state with a strong laser field with the respective Rabi frequency Omega(S). We then expand the simulations to realistic hyperfine level systems in Na atoms for a more general case when non-negligible HF interaction can be treated as a perturbation in the total system Hamiltonian. A numerical analysis of the adiabatic states that are formed by coupling of the 3p(3/2) and 4d(5/2) states by the strong laser field and probed by a weak laser field on the 3s(1/2) - 3p(3/2) transition yielded two important conclusions. Firstly, the perturbation introduced by the HF interaction leads to the observation of what we term "chameleon" states-states that change their appearance in the AT spectrum, behaving as bright states at small to moderate Omega(S), and fading from the spectrum similarly to dark states when Omega(S) is much larger than the HF splitting of the 3p3/2 state. Secondly, excitation by the probe field from two different HF levels of the ground state allows one to address orthogonal sets of adiabatic states; this enables, with appropriate choice of Omega(S) and the involved quantum states, a selective excitation of otherwise unresolved hyperfine levels in excited electronic states.