We apply the optimal control technique to the topic of type I second-harmonic generation with ultrashort laser pulses. We design temporally tailored fundamental-frequency laser pulses that generate, in the regime of pump depletion, temporally symmetric second-harmonic optical pulses.
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.
We present experimental data on the coherent control of frequency conversion toward the vacuum-ultraviolet regime in xenon atoms, applying intense ultrashort picoseconds (ps) radiation pulses. We report on quantum interference between two resonantly enhanced frequency conversion processes (i.e., fifth harmonic generation and four-wave mixing). The conversion processes are driven by a fundamental (ps) laser pulse at 530 nm and the phase-locked second harmonic at 265 nm. Both fifth harmonic generation and four-wave mixing yield vacuum-ultraviolet radiation at 106 nm. The two frequency conversion pathways interfere with each other---either constructively or destructively, depending on the relative phase of the driving laser pulses. Thus, by variation of the relative phase, we substantially enhance or reduce the conversion efficiency (i.e., the yield of generated vacuum-ultraviolet radiation).