Control of third harmonic generation through the phase of an additional third harmonic field
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.
Section snippets
Experimental set-up
The experimental set-up is shown in Fig. 1. It is a conventional phase control arrangement consisting of two static cells both filled with Xe gas and separated by a chamber which is used as a variable phase-shifter [3]. The second static cell is equipped with an electrode that allows for the measurement of ionization. The Xe gas pressure is approximately 1 mbar in the first cell and is varied from 0.1 to 10 mbar in the second static cell. The connection of the two cells with the phase-shifter
Results and discussion
The third harmonic generation scheme in the Xe cells is three-photon near-resonant with the first excited state of Xe, namely the 6s[3/2]2 state, as shown in Fig. 2 where the index l in the photon energy ℏωl indicates the laser (l=1) or the third harmonic (l=3) wave and the prime indicates photons produced in the second cell. Since a focused beam geometry is employed in both cells, negative dispersion is required for optimum phase matching [6]. Above the 6s[3/2]2 state, Xe becomes negatively
Conclusions
Coherent control of VUV generation has been demonstrated in third harmonic generation in Xe, involving two successive generation stages. The total intensity of the VUV signal can be varied by a factor of 3 by varying the relative phase between the fundamental laser field and the third harmonic field produced in the first conversion stage. The modulation of the VUV signal is due to the interference between different linear and non-linear scattering processes in the second conversion stage. The
Acknowledgements
This work has been carried out in the Ultraviolet Laser Facility operating at FO.R.T.H.-I.E.S.L. with support from the HCM Programme (Contract No ERB-CHGE-CT920007).
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