Laser oscillation without population inversion is demonstrated experimentally in a V-type atomic configuration within the D 1 and D 2 lines of Rb vapor. It is shown that the effect is due to the atomic interference. The experimental results, as first predicted by careful theoretical analysis, are in a good agreement with detailed calculations.

Spectroscopic properties of coherently prepared, optically dense atomic media are studied experimentally and analyzed theoretically. It is shown that in such media the power broadening of the resonances can be substantially reduced. A density-dependent spectral narrowing of the electromagnetically induced transparency (EIT) window and novel, even narrower, resonances superimposed on the EIT line are observed in dense Rb vapor. A nonlinear two-photon spectroscopic technique based on coherent atomic media and combining high resolution with a large signal-to-noise ratio seems feasible.

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We present a proof-of-principle experiment demonstrating a resonant enhancement of the index of refraction accompanied by vanishing absorption in a cell containing a coherently prepared Rb vapor. The results are in good agreement with detailed theoretical predictions. [S0031-9007(96)00142-1] PACS numbers: 42.50. Ar, In the present Letter we report the first demonstration of a resonant enhancement of the index of refraction without absorption [1]. Surprising and counterintuitive effects involving quantum coherence and interference have recently become laboratory realities. For example, phenomena such as electromagnetically induced transparency (EIT) [2] and lasing without population inversion (LWI) [3], predicted theoretically at the end of the 1980s have now been demonstrated experimentally [4][5][6][7]. In both of the above effects quantum coherence dramatically modifies the absorptive properties of the medium. The unusual behavior of the dispersive part of the susceptibility of coherently prepared medium is no less intriguing. For example, it was shown theoretically that it is possible to have a completely transparent medium with large dispersion (i.e., rapid variation of index of refraction with frequency) or with large index of refraction [1]. Both of these effects can be achieved using atomic phase coherence in a resonant medium, which is normally optically thick. Coherence effects, however, allow us to prepare an optical medium such that the medium has vanishing absorption, while the dispersive part of the susceptibility is enhanced.The interest in these phenomena is due, on one hand, to the fundamental nature of coherence effects and, on the other hand, to possible applications. For example, the dispersive properties of coherently prepared media have been studied with an eye toward generation of pulses with very slow group velocity [8], effective control of nonlinearities [9], high precision magnetometry [10], and laser acceleration of particles [1].Several recent experiments [11] have demonstrated the large dispersion of the index of refraction accompanying EIT. Index enhancement, however, allows not only for large dispersion, but also for a large refractive index itself, while maintaining a transparent medium.The conceptual foundation of the present experiment can be understood by considering the simple L atomic configuration of Fig. 1. The coherent strong driving field with Rabi frequency ͑V͒ and weak coherent probe ͑V p ͒ allow us to prepare the atom in a coherent superposition of states b and c [12]. When the detunings of these two fields from their respective atomic resonances are equal, EIT is obtained. The incoherent pumping (represented by the rate r) alters this coherent superposition by pumping some of the population into other states. Depending upon the actual parameters of the system this may result in gain, loss, or complete transparency for the probe field.The linear gain (absorption) coefficient (G) is proportional to the imaginary part of complex susceptibility ͑x 00 ͒. In the case when th...