We present a detailed experimental investigation which uncovers the nature of light bullets generated from self-focusing in a bulk dielectric medium with Kerr nonlinearity in the anomalous group velocity dispersion regime. By high dynamic range measurements of three-dimensional intensity profiles, we demonstrate that the light bullets consist of a sharply localized high-intensity core, which carries the self-compressed pulse and contains approximately 25% of the total energy, and a ring-shaped spatiotemporal periphery. Subdiffractive propagation along with dispersive broadening of the light bullets in free space after they exit the nonlinear medium indicate a strong space-time coupling within the bullet. This finding is confirmed by measurements of a spatiotemporal energy density flux that exhibits the same features as a stationary, polychromatic Bessel beam, thus highlighting the nature of the light bullets.
We report on the generation of ultrabroadband supercontinuum (SC) by filamentation of two optical-cycle, carrier-envelope phase-stable pulses at 2 μm in fused silica, sapphire, CaF₂ and YAG. The SC spectra extend from 450 nm to more than 2500 nm, and their particular shapes depend on dispersive properties of the materials. Prior to spectral super-broadening, we observe third-harmonic generation, which occurs in the condition of large phase and group velocity mismatch and consists of free and driven components. A double-peaked third-harmonic structure coexists with the SC pulse as demonstrated by the numerical simulations and verified experimentally. The SC pulses have stable carrier envelope phase with short-term rms fluctuations of ∼ 300 mrad, as simultaneously measured in YAG crystal by f-2f and f-3f interferometry, where the latter makes use of intrinsic third-harmonic generation.
We exploit inverse Raman scattering and solvated electron absorption to perform a quantitative characterization of the energy loss and ionization dynamics in water with tightly focused near-infrared femtosecond pulses. A comparison between experimental data and numerical simulations suggests that the ionization energy of water is 8 eV, rather than the commonly used value of 6.5 eV. We also introduce an equation for the Raman gain valid for ultra-short pulses that validates our experimental procedure.Keywords: Inverse Raman Scattering, light matter interaction, cold plasma Femtosecond laser pulses tightly focused in dielectric media have a wide range of applications in science and technology. Because of their capability to deposit high ionization doses in volumes of a few cubic microns, they can be used to induce permanent, microscopic refractive index modification in solid dielectrics, thus enabling three-dimensional integrated optics 1,2 . By focusing femtosecond pulses in liquids, it is possible to induce localized chemical reactions such as photo-polymerization on the micro-nano-scale 3 . In aqueous media, such as biological tissues, tightly focused femtosecond laser pulses have been successfully employed for eye surgery 4 and treatment of cancerous cells 5 . Recent studies show that by tuning the input pulse chirp an effective control on the energy deposition in water is reached 6 . Future developments of these applications will benefit from a more advanced control of the energy deposition by means of arbitrarily spatiotemporally tailored laser wavepackets 7 . In this context, suitable diagnostic tools for real time analysis of energy deposition dynamics as well as a better understanding of the initial stages of the energy absorption in the dielectric medium are of foremost importance.In previous experiments based on quantitative shadowgraphy, we characterized the propagation of a 120 fs pulse focused with low NA in water 8,9 . In this configuration, the laser pulse enters a filamentation regime 10 leaving behind a tenuous, few-mm-long plasma channel which gets solvated on a ps timescale. The pulse dynamics (featuring pulse splitting and superluminal pulse formation) was clearly seen in the probe as an absorption feature, which we attributed to the imaginary part of an unspecified cross-phase modulation process (XPM) between pump and probe. a) Electronic mail: stefano@stefanominardi.eu.; http://stefanominardi.eu.
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