Alessandro Averchi

Ultrashort laser pulse filamentation from spontaneous X Wave formation in air

The description of ultrashort laser pulse filamentation in condensed media as a spontaneous formation of X waves is shown to apply also to filaments generated in air. Within this framework, a simple explanation is brought for several features of the filament such as the subdiffractive propagation and the energy flux from the weakly localized tails of the X-waves to the intense core.

Spatio-temporal reshaping and X Wave dynamics in optical filaments.

Focus Serial: Frontiers of Nonlinear Optics We investigate ultrashort laser pulse filamentation within the framework of spontaneous X Wave formation. After a brief overview of the filamentation process we study the case of an intense filament co-propagating with a weaker seed pulse. The filament is shown to induce strong Cross-Phase-Modulation (XPM) effects on the weak seed pulse: driven by the pump, the seed pulse undergoes pulse splitting with the daughter pulses slaved to their pump counterparts. They undergo strong spatio-temporal reshaping and are transformed into XWaves traveling at the same group velocities as the pump split-off pulses. In the presence of a gain mechanism such as Four-Wave-Mixing or Stimulated Raman Scattering, energy is then transferred from the pump filament leading to amplification of the seed X Wave and formation of a temporally compressed intensity peak.

Terahertz pulse emission optimization from tailored femtosecond laser pulse filamentation in air

We study the generation of intense terahertz pulses produced by two-color laser pulse filamentation in air. We tailor the filamentation process and the produced plasma strings and study how the generated terahertz field is modified. An important terahertz pulse shortening is found for plasma strings with uniform electron density.

Stimulated Raman X waves in ultrashort optical pulse filamentation

We demonstrate that ultrashort pulse filamentation in liquids with strong Raman gain leads to the spontaneous formation of nonlinear X waves at a Raman-shifted wavelength. We measured as much as 75% energy conversion efficiency into a Raman X wave in ethanol starting from 1 ps pulses due to the group velocity matching between the pump and Raman X pulses. Large Raman gain of a weak seed signal was observed in water, associated with a strong spatiotemporal transformation of the seed into an X wave.

Kerr-induced spontaneous Bessel beam formation in the regime of strong two-photon absorption

We study the effect of Two-Photon Absorption (TPA) nonlinear losses on Gaussian pulses, with power that exceeds the critical power for self-focusing, propagating in bulk kerr media. Experiments performed in fused silica and silicon highlight a spontaneous reshaping of the input pulse into a pulsed Bessel beam. A filament is formed in which sub-diffractive propagation is sustained by the Bessel-nature of the pulse.

Characterization of conical emission of light filaments in media with anomalous dispersion

Conical emission featuring a diminishing emission angle with increasing wavelength detuning is observed in ultrashort light filaments in fused silica at a carrier wavelength with anomalous dispersion. In the measured angularly resolved spectrum of the filament, this conical emission appears as an elliptical, or O-like, structure that fits quantitatively to the elliptical spectrum of gain in the Kerr-driven spatiotemporal instability of nonlinear plane waves.

Colors of transparent submicron suspensions on approaching the Rayleigh regime.

The features of scattered and transmitted light by dilute suspensions of transparent submicron particles are investigated both in the spectral and in the perceived colorimetric domains, as a function of effective particle diameter D, particle-host refractive-index mismatch m, and scattering angle θ. Our results show that the wavelength λ-dependence of the scattering and extinction cross sections remains quite similar well beyond the Rayleigh regime up to particle sizes of a few hundreds nm, but only for specific scattering angles that depend on D and m, and tend to 90° on approaching the Rayleigh regime. Close to this limit (D/λ<<1), a simple criterion that relates the perceived scattering color at θ=90° and the ratio of the sample extinction coefficients at two properly selected wavelengths is demonstrated. A comparison between computed and measured data is presented.

Linear X-wave generation by means of cross-phase modulation in Kerr media.

We exploit cross-phase modulation by a strong driving pulse onto a weaker probe pulse at a different wavelength to induce the formation of an X wave possessing the typical nondispersive and nondiffractive propagation properties.

Tunable, octave-spannning supercontinuum driven by X-Waves formation in condensed Kerr media.

We generate an enhanced blue-shifted continuum in bulk Kerr media in ultrashort laser pulse filamentaiton at 1055 nm. At threshold, a spectrally isolated blue peak appears, while at higher energies the continuum expands from the blue peak and spans more than an octave in the spectrum. The central wavelength of the peak can be tuned in a 150 nm range. The effect is explained in terms of X-waves generation.

Controlled X wave formation in bulk quadratic and cubic nonlinear media

Here we report spontaneous conical wave formation in quadratic and cubic nonlinear media, demonstrating how the characteristics of the formed conical wave strictly depend on those of the pumping source.