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 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 S , and fading from the spectrum similarly to dark states when S is much larger than the HF splitting of the 3p 3/2 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 S and the involved quantum states, a selective excitation of otherwise unresolved hyperfine levels in excited electronic states.
Nonlinear effects in optical pumping of a cold and slow atomic beam By photoionizing hyperfine (HF) levels of the Cs state 6 2 P 3/2 in a slow and cold atom beam, we find how their population depends on the excitation laser power. The long time (around 180 μs) spent by the slow atoms inside the resonant laser beam is large enough to enable exploration of a unique atom-light interaction regime heavily affected by time-dependent optical pumping. We demonstrate that, under such conditions, the onset of nonlinear effects in the population dynamics and optical pumping occurs at excitation laser intensities much smaller than the conventional respective saturation values. The evolution of population within the HF structure is calculated by numerical integration of the multilevel optical Bloch equations. The agreement between numerical results and experiment outcomes is excellent. All main features in the experimental findings are explained by the occurrence of "dark" and "bright" resonances leading to power-dependent branching coefficients.
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Abstract:The time-dependent population dynamics of hyperfine (HF) sublevels of n 2 p 3/2 atomic states upon laser excitation in a cold medium of alkali atoms is examined. We demonstrate some peculiarities of the absorption HF multiplet formation in D2-line resulting from a long interaction time (∼200 µs) interaction between light and Na (n = 3) and Cs (n = 6) atoms in a cold and slow sub-thermal (T ∼ 1K) beam. We analytically describe a number of D2-line-shape effects that are of interest in spectroscopic studies of cold dusty white dwarfs: broadening by optical pumping, intensity redistribution within components of D2-line HF multiplet for partially closed transitions and asymmetry of absorption lines induced by AC Stark shifts for cyclic transitions.
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