Electromagnetically induced transparency in Doppler-broadened three-level systems with resonant standing-wave drive

We study electromagnetically induced transparency for a probe travelling-wave (TW) laser field in closed Doppler-broadened three-level systems driven by a resonant standing-wave (SW) laser field of moderate intensity (its Rabi frequencies are smaller than the Doppler width of the driven transition). We show that probe windows of transparency occur only for values of the probe-to-drive field frequency ratio R close to half-integer values. For optical transitions and typical values of Doppler broadening for atoms in a vapor cell, we show that for R > 1 a SW drive field is appreciably more efficient than a TW driving in inducing probe transparency. As an example, we consider parameters for a real closed cascade scheme in barium atoms with R ≃ 1.5 showing that probe transmission values of 50% are possible for cases in which the transmission is almost negligible either without driving field or with only one of the TW components of the drive.