AC Stark effect and fluorescence in modulated laser beams. II. Two-photon stepwise excitations
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For pt.I see ibid., vol.12, no.12, p.1939 (1979). The modulated fluorescence in an optical double resonance experiment, i.e. when an atom interacts with two intense monochromatic laser beams each of which is weakly modulated in amplitude at frequency Omega , is studied in detail. The modulation rate corresponding to the steady-state population of the highest atomic level exhibits resonant structures at the modulation frequencies Omega =0, +or- alpha0, +or-2 alpha0due to the dynamical Stark splitting of the various energy levels; alpha0being related to the Rabi frequencies alpha1and alpha2associated with the two lasers and the two atomic transitions, by alpha02= mod alpha1mod2+ mod alpha2mod2. The peak at Omega =+or- alpha0is the most intense and exhibits splitting when the applied fields are detuned from exact resonance with the respective atomic frequencies. Resonances are observed in the modulation rate of the intermediate-level population at the modulation frequencies Omega =0, +or-2 alpha0. In the presence of detunings of the applied fields, additional resonant structures occur at Omega =+or- alpha0which increase in magnitude with higher detunings and split. Explicit expressions for the positions, widths and weights of these resonant components are obtained analytically for intense, resonant fields. Numerical results for general values of the laser field strengths and the detunings are given.
