Filamentation describes the ability of a very intense ultrashort laser pulse to remain focused over an extended distance (from several Rayleigh lengths to several kilometers).
Filaments arise from the nonlinear propagation of ultrashort, high-power laser pulses in transparent media. They result from a dynamic balance between Kerr-self-focusing and defocusing by negative higher-order Kerr effect (HOKE) and/or self-induced plasma, depending on the pulse duration and wavelength. The relative contributions of these defocusing processes is one of the most active research topic in the field of filamentation (See. e.g. P. Béjot et al. Phys. Rev. Lett. 104, 103903 (2010) and 106, 243902 (2011)) Our research now aims at identifying the physical processes behind the HOKE.
In parallel with the experimental work, we develop and maintain a propagation code based on the Unidirectional pulse propagation equation (UPPE) describing the filamentation of ultrashort pulses in radial symmetry.
In view of our biophotonics applications, we have developed a 400 nm broadband source, based on filamentation in Argon, to selectively excite biological samples with shaped laser.
Indeed, increasing interest has been recently devoted to selective excitation of biological samples by means of “pump-pump” spectroscopy or more sophisticated coherent control schemes. These experiments require to shape a broadband low noise coherent source, which until recently was only feasible in the near infrared, whereas the absorption bands of most of the relevant optically active biological molecules (tryptophan, flavin, heme molecules...) are located in the UV-Visible region. Therefore, previous coherent control experiments in this field usually involved two-photon excited fluorescence induced by shaped intense ultrashort 800 nm laser pulses. This was used for instance to selectively excite the fluorescence in living organisms. However, due to their inherently weak cross-section and nonlinear nature, two-photon based experiments need a tightly focused laser, which could turn out to be critical to implement in long-distance applications. Our novel approach is then to use a broadband 400 nm pulse in one photon excited fluorescence experiments.
To better control the shape of the broadband pulse, we developed a theoretical model which describes the propagation of a 400 nm filament pulse (P.Béjot et al, Optics Express, 15 (20), 13295-13309 (2007)). These theoretical calculations let us to confirm our prediction that the generated broad spectrum has specific statistical properties which can be used for significantly reduce the shot-to-shot noise of the laser (P. Béjot et al, Appl. Phys. B. 87 (1), 1-4 (2007)).