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Institute of Optics


Resonant Interactions of Atoms with Modulated Optical Fields


Stephanos Papademetriou

The interaction of an amplitude-modulated field with an ensemble of radiatively broadened two-level atoms yields a complicated series of parametric resonances, when both the Rabi frequency and modulation frequency are large compared with the linewidth of the atomic transition. A special case of amplitude modulation is that of 100% depth of modulation, where the carrier frequency is completely removed (AM field). We experimentally and theoretically investigate this problem in the particular case of a two-level atom. Lock-in detection techniques resolve up to a sixth order process in the nonlinear dynamics of the interaction. We find excellent agreement between theory and experiment.

With the aid of matrix-continued fractions, we develop a semiclassical theory to describe a pump-probe interaction in a three-level cascade atomic system: A 100% AM field interacting with the lower transition, while a weak monochromatic field is tuned in frequency in the upper transition. The absorption spectrum of the monochromatic probe in the upper transition reveals the complicated structure of the interaction of the AM field in the lower transition. A dressed-state calculation is helpful in interpreting the different peaks of the absorption spectrum in terms of transitions among the infinite number of dressed states within every manifold of states. It also provides analytic results for the amplitudes and linewidths of the different peaks of the absorption spectrum, which are in complete agreement with the matrix-continued fractions calculation.

We have also devised an experiment in atomic sodium to test these theoretical predictions, using the 32S1/2 (F = 2, mF = 2), 32P3/2 (F = 3, mF = 3), and 42D5/2 (F = 4, mF = 4) sublevels for the three-level cascade atom. We create the optical fields, with the aid of acousto-optic modulators from two different frequency-stabilized dye lasers. For a given value of detuning between the center frequency of the AM field and the lower transition, we measure the absorption spectrum of the probe in the upper transition, for a range of values of the Rabi frequency of the AM field. We find excellent agreement between theory and experiment for all the experimental parameters.

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Last modified 13 September 2006