Control of third harmonic generation through the phase of an additional third harmonic field

Abstract

We present experimental results for the control of third harmonic generation in a xenon cell brought about by the presence of an additional phase-correlated third harmonic field, created by the fundamental beam in another harmonic generation stage. The dressing of the atoms by the extra field radically alters the polarization of the medium resulting in enhancement or suppression of the generated third harmonic depending on the relative phases between the dressing third harmonic and the fundamental fields. Equivalently, this can be viewed as the interference between the two dipole moments at 3ω (ω being the fundamental frequency) that are induced by the two driving fields, i.e. the fundamental and the phase-related third harmonic produced in the first stage. This phase relation is varied externally by a dispersive medium, allowing for the control of the total harmonic efficiency of the system. The effect under investigation may be employed in order to increase the yield of coherent VUV, XUV sources.

Introduction

In recent years there have been several demonstrations of control of excitation processes through quantum interference of different coherent excitation paths coupling the same initial and final state with a different number of photons [1]. The most familiar scheme is the interference of two excitation paths, one including absorption of laser photons and the other one absorption of photons of one of the laser harmonics, most commonly the third harmonic. Excitation probabilities can then be controlled by varying the relative phase of the two phase-related laser and harmonic fields. In this way phase control of ionization, dissociation, photofragment angular distributions and dissociation branching ratios has been achieved. Control of harmonic generation based on interference schemes of this type has been theoretically investigated by Charron et al. and Cormier and Lambropoulos in high field interactions [2]. In a recent publication [3]we have presented results of coherent control in four-photon excitation schemes where the possibility of coherent control of harmonic generation was briefly reported. In the present communication we present and discuss in detail aspects of coherent control of near resonant third harmonic generation when the harmonic production occurs in two distinct interaction regions separated by a third intermediate one in which the phases can be adjusted. It is worth noting that the present work as well as the work presented in Ref. [3]has been inspired by the earlier experimentally demonstrated [4]and theoretically interpreted [5]cancellation of four-photon resonances.