Abstract

We exploit coherent interaction properties to efficiently pump sodium from the 3s_1/2 ground state to the 4s_1/2 upper state via the 3p_3/2 state. Even though the excitation is nonlinear, the required intensities are reduced to levels required to resonantly pump single photon transitions in sodium, because of the particular choice of laser frequencies which are near resonance with the intermediate levels. Furthermore, the intermediate state is left unpopulated after the excitation sequence. A computer simulation solves the nonlinear Schrôdinger equation for this system, comprised of 32 hyperfine levels connected by 112 possible transitions. The levels are summed over the velocity distribution for mesospheric sodium to account for Doppler broadening which is the dominant dephasing process. Preliminary results indicate that the optimum excitation is a bichromatic π-pulse with a duration of 25 ps FWHM, detuning of 0.389 rad/ps at energy densities of 14.1 microjoules per square-centimeter at 589 nm and 1140 nm, respectively (see Fig. 1).

© 2000 IEEE