Subnatural linewidth for probe absorption in an electromagnetically-induced-transparency medium due to Doppler averaging

We obtain subnatural linewidth (i.e.,<Gamma) for probe absorption in room-temperature Rb vapor using electromagnetically induced transparency (EIT) in a Lambda system. For stationary atoms, the EIT dip for a resonant control laser is roughly one-half as wide as the control Rabi frequency Omega(c). But in thermal vapor, the moving atoms fill the transparency band so that the final EIT dip remains subnatural even when Omega(c)>>Gamma. We observe a linewidth of Gamma/4 for Omega(c) = 8 Gamma in the D-2 line of Rb.