We obtain subnatural linewidth (i.e. < Γ) for probe absorption in room-temperature Rb vapor using electromagnetically induced transparency (EIT) in a Λ system. For stationary atoms, the EIT dip for a resonant control laser is as wide as the control Rabi frequency Ωc. But in thermal vapor, the moving atoms fill the transparency band so that the final EIT dip remains subnatural even when Ωc > Γ. We observe linewidths as small as Γ/7 in the D2 line of Rb.PACS numbers: 42.50. Gy,32.80.Qk,42.50.Md,32.80.Wr Laser spectroscopy in a room-temperature gas is often limited by Doppler broadening due to the thermal velocity of gas particles. While techniques such as saturatedabsorption spectroscopy can be used to eliminate the first-order Doppler effect and get linewidths close to the natural linewidth, the natural linewidth itself appears as a fundamental limit to the resolution that can be achieved in precision spectroscopy. In addition, when lasers are locked to atomic transitions (for use as frequency standards), the natural linewidth determines the tightness of the lock. It is therefore desirable to develop techniques for getting below the natural linewidth.In this work, we propose and demonstrate a technique to obtain subnatural linewidth in a Doppler broadened medium. The technique has been adapted from recent developments in the use of control lasers in three level systems as a means of modifying the absorption properties of a probe beam [1], in what is generally called coherent-control spectroscopy. More specifically, we use the phenomenon of electromagnetically induced transparency (EIT) in a Λ-type system, in which an initially absorbing medium is rendered transparent to a weak probe when a strong control laser is applied to a second transition [2]. It is well known that the EIT dip on resonance for stationary atoms can be subnatural if the Rabi frequency of the control laser is sufficiently small [3]. However, in thermal vapor, the effect of the large Doppler width was thought to have a detrimental effect on observing any subnatural features. Indeed, theoretical work in such Doppler broadened media predicted that one can at best achieve sub-Doppler resolution by detuning the control laser [4]. However, in earlier work [5], we have shown that we can obtain sub-natural linewidth either by detuning the control by an amount that is larger than the Doppler width, or by using a slightly-detuned control along with a counterpropagating pump beam that allows the probe to address only zero-velocity atoms. Here, we show that one can observe subnatural linewidth for the EIT dip even when the control is on resonance. * Electronic address: vasant@physics.iisc.ernet.in; URL: www.physics.iisc.ernet.in/~vasant There has been a previous report of subnatural linewidth for the EIT dip using a Λ system in the D 1 line of Rb [3]. However, the dip was subnatural only when the control Rabi frequency was less than the natural linewidth, Γ, and reached a smallest value of Γ/4 partly limited by the 3-MHz linewidth of the laser. By contr...

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