Continuous wave (cw) amplification and laser oscillation without population inversion have been observed for the first time in a L scheme within the sodium D 1 line. This is also the first demonstration in which the lasing medium was an atomic beam; this is an approach which, in addition to elucidating the physics, lays a foundation for extensions into the ultraviolet. Calculations using realistic atomic structure were critical to the choice of experimental approach. Observations agree with full densitymatrix calculations and clearly show there was no population inversion. Lasing without inversion (LWI) has attracted attention in recent years [1], as it opens new perspectives for laser physics via the effects of atomic coherence and interference [2]. The L and V schemes are frequently discussed techniques for LWI in atomic systems. In a L scheme [3] the fields have a common upper level, and LWI is insured by the coherence between two lower levels; whereas, in a V scheme [4] the fields share a lower level and LWI is made possible by coherence between upper levels. All previous LWI experiments employing a L scheme observed only transient gain (less than a few nsec) or gain on a probe laser pulse [5]. Electromagnetically induced transparency (EIT) based on these schemes has also been observed [6]; and related experiments involving a V scheme have been reported [7].The first cw laser oscillator without inversion was recently reported by Zibrov et al. in a V configuration [8]. Here we describe a very different experimental demonstration of a cw laser oscillator without population inversion. It is based on the L scheme and is closest to the concept proposed by Imamoǧlu, Field, and Harris [9]. The active medium is a sodium atomic beam and the transitions are within the D 1 line. By using a weak probe laser, we first demonstrate complete transparency and then inversionless gain. Next, a laser cavity is installed and aligned. With the probe blocked, we find that the laser starts spontaneously from vacuum fluctuations.A generic L scheme for our LWI experiments is shown in Fig. 1. The atomic system consists of an upper level a connected by a dipole-allowed transition to two lower levels: to c via a strong "driving" field with Rabi frequency V D , and to b via the "lasing" transition with Rabi frequency V L . In preliminary studies a very weak "probe" field with Rabi frequency V P was tuned through the region of the a $ b transition frequency. To observe gain and lasing, some population is transferred into state a using an incoherent pump field ͑b $ a͒ with a transition rate r I . Level a decays radiatively to levels c and b at rates g c and g b , respectively; its radiative lifetime is 1͑͞g c 1 g b ͒.Since the active medium is an atomic beam, fresh atoms enter the probe interaction region in state b, and we model the exit via a decay of all states at a relatively slow rate g 0 .Significant insight into this generic L configuration is obtained by considering analytic forms for the steady state solutions of the density matri...

Atomic coherence effects within the sodium D\ line are shown to lead to the suppression of optical pumping, to the switching of light on and off when the coherence effects are turned on and off, and especially to lasing without inversion.PACS numbers: 42.50.-p, 42.55.-f Atomic coherence effects [1][2][3][4] are the basis of the most commonly discussed schemes for lasing without inversion (LWI) [5][6][7][8][9][10][11]. These schemes typically envision an intense beam preparing the medium so as to allow LWI on a weak probe. In the present paper, we present an experimental and theoretical demonstration of LWI and related coherence effects using a different type of system. In this system two strong beams prepare the atomic coherence as in Figs. 1 (a) and 1 (b) and LWI is observed as an increased intensity on these same strong beams.To understand the basic idea behind these new experiments, consider the case of optical pumping within the states associated with the Na D\ line as in Fig. 1 (b). The field and intensity driving transitions 6'-* a are labeled FIG. 1. (a) Two strong beams prepare a coherent superposition of b and b'. This leads to LWI on the same two transitions when a is populated via a weak pump at rate R from level c (four-level system), (b) The structure of the 3 2 5i/2 and 7> 2 P\/i levels of sodium. Shown above each sublevel is our designation for it; the subscript on a level designation is the mp value for the level. Shown below the sublevels are their fractional steady state populations under the action of two strong RCP fields (heavy lines) of equal intensity; initially isotropic populations are assumed. The weak LCP field that provides an upper level population is shown as dashed lines.E\ and I\ (frequency vi), respectively; similarly, for b-+a they are E^ and 12 (frequency V2); see Fig. 1(b). From a naive rate equation perspective, one would expect that in the presence of both E\ and E 2 all the atoms would be optically pumped into the 62 state, i.e., at steady state p/> 2 6 2 = l in the absence of collisions. However, due to the presence of coherent fields at v\ and V2, atomic coherences pbb' are generated between the pairs of states (6 0 ,66), (6 + 1,6 + 1), and (6-1,6-1); thus population is trapped in these pairs of states and their population percentages are shown in Fig. 1 (b).The atomic coherence between and trapping within these paired states modifies the optical pumping of the ground states. It is manifested in the phenomenon of "coherence switching" which is reported here. Having demonstrated (via coherence switching) the presence of ground-state coherences and population trapping, we are in a position to observe LWI by exciting a small fraction of the atoms to the a state [ Fig. 1(a)] or the ao and a\ states [ Fig. Kb)].In order to most simply understand the physics of the present coherence switching and LWI, we consider the Na atomic structure in Fig. 1(b). The essence of this structure, as it applies to the present problem, is explained by two four-level groupings: # 0 , (6-1,6-1 )...