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 using two experimentally observed 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.

References:

A. Becker et al., Appl. Phys. B 73, 287 (2001)
N. Aközbek et al., Phys. Rev. Lett. 89, 143901 (2002)
F. Theberge et al., Phys. Rev. Lett. 97, 023904 (2006)
J.F. Daigle et al., Phys. Rev. A 82, 023405 (2010)

Collaborations:

N. Aközbek (U.S. Army Aviation and Missile Command, Huntsville, USA)
S.L. Chin (Laval University, Quebec, Canada)