Abstract

Our paper addresses the problem of nonlinear conversion of high intensity femtosecond laser pulse spectra, during the process of rapid multiphoton gas ionization. Previously, various modifications of laser spectra resulting from the nonlinear response of media have been observed in investigations of intense laser beam interactions with rarefied gases. The observed "elemental” modifications include blue shifting of the pulse frequency produced by temporal variations in the refractive index due to gas ionization [1-3], superbroadening of the focused laser pulse spectrum caused by avalanche gas ionization [4,5], and high-order optical field harmonic emission [6-9]. In investigations of the last process, the atom ionization has usually been treated as a negative effect that breaks the phase matching conditions between the fundamental and excited waves. However, in the recent paper of Corkum [10] and in our paper [11] it has been demonstrated that ionization itself is precisely the mechanism which is responsible for high-order harmonic coherent pulse generation. Hence, atoms ionized in the strong laser field constitute a. nonlinear medium which has attractive potential for producing supershort (spectrum broadening), tunable (blue-shifting), ultrashort wave (higher harmonics) radiation. The question arises: can these possibilities be exploited simultaneously to create a unique source of coherent radiation? Our paper proves theoretically this possibility and defines the relevant conditions for experimental demonstration.

© 1994 Optical Society of America