FEMTOSECOND-PULSE 2-PHOTON RESONANT DIFFERENCE-FREQUENCY MIXING IN GASES - A TECHNIQUE FOR TUNABLE VACUUM-ULTRAVIOLET FEMTOSECOND-PULSE GENERATION AND A SPECTROSCOPIC TOOL FOR STUDYING ATOMS IN STRONG LASER FIELDS

Two-photon resonant and near-resonant four-wave difference-frequency m ixing in gases in the interaction regime when laser pulse durations ar e comparable tear shorter than the medium polarization relaxation time T-2' is investigated. The results of experimental studies of the proc ess in Ar and Kr using pump pulses from the ArF-excimer laser are pres ented demonstrating a generation of tunable short pulse radiation in t he range 102-124 nm. The results are discussed in terms of a theoretic al model based on a self-consistent solution of the Bloch equations fo r the atomic transitions and the Maxwell equations for the fields. Thi s enables one to interpret specific nonstationary resonant and quasire sonant phenomena involved in the frequency conversion process. It is s hown that the femtosecond-pulse four-wave frequency-mixing technique w ith probe pulses significantly shorter than the pump pulses makes it p ossible to study the coherent dynamics of an atomic transition exposed to an intense field. Using atomic Kr as the nonlinear medium, coheren t Rabi oscillations and the subsequent phase relaxation of excitation were observed under the condition of two-photon interaction of Kr with femtosecond 193-nm laser pulses. The obtained information is importan t for controlling and optimizing processes of two-photon resonant freq uency conversion and for time-resolved studies of Rydberg states in at oms and molecules.