Narrow absorption lines induced by electron shelving

We investigate the absorption spectrum of a laser driven V system in which one of the upper levels is metastable. If the Rabi frequency of the laser driving the metastable transition is suuciently small we nd an extremely narrow peak in the stationary absorption spectrum of a weak probe on the strong transition. We give analytical expressions approximating the stationary absorption spectrum. The absorption spectrum of a two-level system is investigated for the case where an upper limit for the time separation of successive emissions is given. The absorption spectrum then exhibits a delta function like peak and using this we then show that the origin of the narrow peak in the absorption spectrum 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.