Self-compression of femtosecond pulses in argon with a power close to the self-focusing threshold

Self-compression of femtosecond pulses in noble gases with an inputpower close to the self-focusing threshold has been investigatedexperimentally and theoretically. It is demonstrated that eithermultiphoton ionization (MPI) or space--time focusing andself-steepening effects can induce pulse shortening, but theypredominate at different beam intensities during the propagation.The latter effects play a key role in the final pulseself-compression. By choosing an appropriate focusing parameter,action distance of the space--time focusing and self-steepeningeffects can be lengthened, which can promote a shock pulse structurewith a duration as short as two optical cycles. It is also foundthat, for our calculation cases in which an input pulse power isclose to the self-focusing threshold, either group velocitydispersion (GVD) or multiphoton absorption (MPA) has a negligibleinfluence on pulse characteristics in the propagation process.