Macroscopic dark periods without a metastable state

The well-known three-level system of Dehmelt Bull. Am. Phys. Soc. 20, 60 (1975) exhibits macroscopic dark periods because one of the upper levels is metastable. In this paper we show that one can also have the same phenomenon for a three-level-atom system with both upper states rapidly decaying to the ground state, provided (a) their level separation is very small and (b) their dipole moments for the ground-state transition are parallel. For suitably chosen laser parameters we also exhibit an exact nonradiating solution of the complete time development for atom plus radiation field. This leads to the surprising effect that the atom, after emission of a number of photons, will stop to fluoresce although still irradiated by the laser. The mean number of photons emitted before this switching off of the atom is calculated, as well as the mean duration of the dark periods for the general case.