Efficient Excitation of Atomic Rydberg States

by

John D. Corless
(1997)

We have investigated the optical excitation of atomic Rydberg states with emphasis on efficiency and selectivity. We consider both direct excitation from the ground to the Rydberg state as well as excitation via an intermediate resonance.

In the direct excitation case, we find that a major limitation to the selectivity of the process is the optical mixing of the nearly degenerate Rydberg states with the same principal quantum number, but differing angular momentum quantum numbers. The interaction between these states is characterized by Rabi frequencies that exceed the optical frequency even for very modest optical field strengths. This interaction gives rise to angular distributions peaked orthogonal to the laser polarization direction, emission of high harmonics of the laser field, as well as laser induced stabilization. We derive analytic results for this interaction as well as develop a model based on Landau-Zener level-crossing theory. Experimentally, we observe this strong interaction between Rydberg states by examining three-photon ionization in atomic potassium when a picosecond dye laser is tuned through two-photon resonance with the Rydberg series. The ionization signal becomes suppressed when the optical mixing of the Rydberg states becomes large. The details of this suppression depend on the peak intensity of the laser field.

We then consider doubly-resonant excitation of Rydberg states and investigate the dependence of the transfer efficiency on the time delay between the two resonant laser pulses. We find that even in the presence of Doppler broadening, transverse spatial variation of the laser beam, and laser amplitude fluctuations that the transfer effciency from the ground state to the Rydberg state is maximized when the laser pulses are applied in the counterintuitive order. We investigate these predictions experimentally in a three-level cascade system in atomic sodium vapor and verify that the population transfer efficiency is maximized in the counterintuitive regime. We further find that in the presence of laser amplitude fluctuations, the population transfer signal fluctuates less when the pulses are applied in the counterintuitive order.

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