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It is shown, both through simulations and experiments, that the in-line holographic microscopy technique can be used to retrieve very small refractive-index perturbations caused during the filamentation of ultrashort laser pulses. This technique provides the possibility of having spatially and temporally (four dimensions) resolved measurements of refractive-index changes, down to 10-4, from objects with diameters as small as 10 µm. Moreover, we demonstrate the power of the technique in discriminating multiple filaments in a precise quantitative way. [ABSTRACT FROM AUTHOR]

We propose a comprehensive physical model explaining the conical character of broadband terahertz generation from femtosecond two-color laser-induced air plasma filaments. We show, that, in contrast to other models, emission is always conical, resulting from phase matching of the radiation produced inside the filament combined with a partial back reflection of the generated terahertz field from the filament itself due to the frequency-dependent critical plasma density. The obtained conical angle varies from 2° to 10°, depending on the plasma density distribution and filament length. Unlike previously proposed models, our model shows good agreement with our experiments as well as a wide range of experimental findings from the literature. [ABSTRACT FROM AUTHOR]

We numerically demonstrate that femtosecond ring-Airy wave packets are able to overcome the reference intensity clamping of 4?1013 W/cm² for filaments generated with Gaussian beams at low numerical apertures and form an intense sharp intensity peak on axis. Numerical simulations, with unidirectional propagation models for the pulse envelope and the carrier resolved electric field, reveal that the driving mechanism for this unexpected intensity increase is due to the self-generated plasma. The plasma formation, in conjunction with the circular geometry of the beam, force the wave packet into a multistage collapse process which takes place faster than the saturating mechanisms can compensate. We report here a nonstandard mechanism that increases the intensity of a collapsing wave packet, due to the joint contributions of the cubic phase of the Airy beam and the formation of a partially reflecting plasma. [ABSTRACT FROM AUTHOR]