We have performed electromagnetically induced transparency (EIT) and two-photon absorption experiments in ladder-type three-level systems in a hot sodium atomic vapor, using the 3S 1/2 -3P 1/2 -4D 3/2 , 3S 1/2 -3P 3/2 -4D 3/2,5/2 , 3S 1/2 -3P 1/2 -5S 1/2 , and 3S 1/2 -3P 3/2 -5S 1/2 transitions. In particular, in the most pronounced 3S 1/2 -3P 1/2 -4D 3/2 system, we have observed quite unique spectral line shapes that are superpositions of sharp dips and peaks, in contrast to ordinary EIT spectra. The peaks and dips have apparently different physical origins and the line shape can be interpreted as the interference between EIT and two-step excitation.

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We have observed ultraslow propagation of matched pulses in a parametric amplifying hot sodium vapor. A fractional delay of more than 1 and a gain of 28 have been simultaneously achieved for a probe pulse of width 100 ns, accompanying a conjugate Stokes pulse with similar gain and delay under a strong coupling beam. The results are compared with the preceding work by Boyer et al. [Phys. Rev. Lett. 99, 143601 (2007)].