Breakdown of Power Series Expansion

For many materials the polarization induced by an external field is found to be proportional to the field itself. In weak fields the electric susceptibility as a measure of this proportionality can be expanded in a power series. We have shown that the perturbative power series expansion breaks down, as indicated by the ratio of successive terms in the expansion (see Figure), at about 1x1013 W/cm2. In the same intensity regime there occurs the transition from perturbative to non-perturbative interaction in low-order harmonic generation. At higher intensities ab-initio theory that accounts for the nonperturbative interaction between the atom and the external field can be used to resolve the change of the susceptibility as a function of time over the duration of the pulse.

A. Spott et al., Phys. Rev. A 90, 013426 (2014); 91, 023402 (2015); 96, 053404 (2017).

HHG Control via Angular Spatial Chirp

We have studied the use of a pulse with angular chirp at the focal plane in high-order harmonic generation. Results of our numerical simulations show that each harmonic is emitted with an angular chirp which scales inversely with the harmonic order. This leads to a control of the separation of the harmonics in two dimensions and the temporal periodicity of the harmonic pulse trains - in collaboration with Chip Durfee (Colorado School of Mines).

C. Hernandez-Garcia et al., Phys. Rev. A 93, 023825 (2016)

From Intensity Clamping to Tunable Pulses

The balance of the defocusing effect of the self-generated plasma and the Kerr self-focusing effect during laser pulse filamentation leads to a limited peak intensity in the filament. Most recently, we demonstrated that the clamped intensity can be measured by comparing the fluorescence yields for filaments at two different wavelengths and two experimental conditions. The Figure shows S-matrix results representing the two conditions for filamentation in air at 400 nm and 800 nm. The crossing point of the two lines determines the clamped intensities. The intensities in the filament are sufficiently high to induce third-harmonic generation and other parametric processes. Results of experiments and numerical simulations provide evidence for a nonlinear phase-locking mechanism which couples the two pulses together inside the filament. This leads to a new scheme to efficiently generate tunable and stable ultrashort laser pulses in the visible spectrum by four-wave mixing during filamentation - in collaboration with N. Aközbek (U.S. Army Aviation and Missile Command, Huntsville, USA) and S.L. Chin (Laval University, Quebec, Canada).

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F. Theberge et al., Phys. Rev. Lett. 97, 023904 (2006)
J.F. Daigle et al., Phys. Rev. A 82, 023405 (2010)