Spectral structures induced by electron shelving.

We investigate the spectrum of resonance uorescence of laser driven three-level systems in the V or connguration where one transition is metastable. If the laser driving the weak transition has a suuciently small Rabi frequency we nd an extremely narrow structure in the resonance uorescence spectrum of the strong transition. Analytical expressions approximating the spectrum are given and the properties of the spectrum are discussed. With the help of a simpliied model of resonance uor-escence emitted on the strong transition we are able to show that the origin of the narrow peak can be traced back to electron shelving, i.e. to the existence of light and dark periods in the resonance uorescence emitted on the strong transition.