We show that the atomic dispersion and therefore the efficiency of the nonlinear generation at a given frequency can be controlled by the addition of an extra resonant laser field. Using this change in dispersion, we explain some of the recent observations on the vacuum ultraviolet generation in Xe and Hg.
Optical harmonic generation and mixing in the gas phase has been proposed as a technique for the generation of coherent radiation in the vacuum ultraviolet and soft x-ray spectral region. At the high field strengths required by these processes the interaction between atoms and the electromagnetic field shows intensity-dependent resonances. In this paper we modify harmonic generation theory to include the effect of these frequency shifts. Closed-form expressions for generated dipole moment, absorption probability, and coherence length are presented. The most important consequences of frequency shifts on resonantly enhanced processes are that the pump laser must be tuned away from the small-field resonance frequency, that the conversion efficiency may saturate, and that the dispersion of the medium may change sign. As an example, the generation of 198-\AA{} radiation by a five-photon mixing process in Li+ is considered.
New observations are reported of tunable vacuum ultraviolet light generated in the forbidden region to the red of the xenon $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{1}^{0}$ state due to the introduction of a second, resonant laser field. Striking observations of double-resonant ionization signals on the blue side of the xenon $6{s}^{\ensuremath{'}}$ level are also reported. Both sets of data are interpreted in terms of the changes in phase matching and the nonlinear susceptibility induced in the medium by the second laser. These changes are successfully analyzed in terms of the ac Stark effect in the three-photon level due to the second laser field.
We demonstrate efficient, pulsed, gas-phase, nonlinear frequency conversion in a quadruply resonant, double- Lambda system and, simultaneously, verify theoretical predictions of Rabi-frequency matching unique to absorbing nonlinear media. This system is used to up-convert ultraviolet light at 233 nm to the vacuum ultraviolet at 186 nm in atomic Pb vapor with small-signal conversion efficiencies exceeding 30% and with modest atomic density-length (NL) products (scale 10(14) cm(-2)) and optical power densities (10-100 kW/cm(2)).
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).