Abstract

The standard schemes of laser Doppler-cooling are not effective in the case of atoms with narrow widths of transitions. The main reason is, that the average light pressure force is weak in this case, and the cooling time may be too large compared with the time of flight of the atom through the region of interaction with the laser beam. The Doppler-cooling method may be improved to be useful for cooling and manipulating of atoms with narrow widths of transitions by using some physical mechanism, which reduces the spontaneous decay time of excited atoms. We propose in this paper to apply the effect of Dicke superradiance for reduction of the decay time and hence for increasing the average acting light force on the atoms The necessary condition for Dicke superradiance to take place is the simultaneous excitation of the atoms of the ensemble More precisely, of the group of atoms having velocities near some velocity v with a width Δ v of the velocity distribution such, that k_L Δv < 1 / τ_D, where k_L and τ_D are the wave number of the laser field and the duration of the Dicke superradiance. As it is well known, τ_D ≅ T/ N ≪ T, for N ≫ 1, where T is the spontaneous decay time, and N is the number of the excited atoms. It is clear, that the Doppler-cooling by cw laser beams could not be used in this case, because of failing to excite simultaneously the ensemble of the atoms to provide afterwards the decay by the Dicke superradiance. Laser pulses with chirped frequency acting in the regime of adiabatic passage seem to be the best tool for realization of this cooling scheme.

© 1996 IEEE