Narrow line spectroscopy with femtosecond laser pulses using a background-free frequency-resolved correlator system

Atomic spectroscopy using broadband ultrashort laser pulses with durations in the 100-fs range seems nearly impossible because the large bandwidth of the pulses exceeds the line-width of atomic transitions by several orders of magnitude. Therefore the energy loss of ultrashort pulses crossing a narrow-linewidth medium is very low, even if the center frequency of the pulses is tuned to the transition frequency. For instance, the total absorption of a 100-fs pulse centered at the D/sub 2/-absorption line of Rb near 780 nm is well below 10/sup -3/ if the pulse is sent through a 75-mm-long Rb vapor cell at room temperature. Much more information can be obtained by measurement of the change of the pulse phase rather than the pulse energy after the pulse has crossed the medium. We found that a measurement technique similar to previously published second harmonic generation (SHG) frequency-resolved optical gating (FROG) is very convenient for that purpose. In our first experiments we send light pulses with a duration of 100 fs, a Fourier-limited bandwidth of 10 nm, and a pulse energy of 1 nJ through a 75-mm long Rb vapor cell. The pulses were analyzed with use of a SHG-FROG system composed on a single shot correlator and dispersive optics that depicted temporal and spectral information as two-dimensional images on a CCD sensor. The SHG-FROG signals depended strongly on the Rb vapor pressure inside the cell, which was controlled by the cell temperature. While these experiments analyzed steady states, the new technique principally allows the monitoring of transient states as well. We present first results on the investigation of optical pumped material.