E.O. Smetanina

Light bullets from near-IR filament in fused silica

The formation of light bullets during femtosecond laser pulse filamentation in the presence of anomalous group velocity dispersion has been recorded for the first time. The minimum experimentally detected width of the light bullet autocorrelation function is 27 fs, which corresponds to a duration of about 13.5 fs. The duration of the light bullet at a wavelength of 1800 nm is about two periods of the light field oscillation. The numerically calculated width of the autocorrelation function for such a light bullet is 23 fs, which is in good agreement with the experimental value.

Anti-Stokes wing of femtosecond laser filament supercontinuum in fused silica.

We have demonstrated that in the IR pulse filament the anomalous dispersion of fused silica leads to the formation of an isolated anti-Stokes wing (ASW), which is located in the visible region of the supercontinuum (SC). It is shown that the isolated ASW is formed by the interference of the light field of a SC undergoing anomalous group velocity dispersion.

Light bullets from a femtosecond filament

The scenario of the formation of light bullets in the presence of anomalous group velocity dispersion is presented within the same general scenario for condensed matter and humid air. The temporal and spectral parameters of light bullets during filamentation in fused silica and humid air are obtained. A light bullet (LB) is a short-lived formation in a femtosecond filament with a high spatiotemporal light field localization. The sequence formation of the quasi-periodical LB is obtained numerically and is confirmed experimentally by autocorrelation measurements of the LBs duration. The estimation of the LB duration reaches few-cycle value. It is established that the generation of each LB is accompanied by the ejection of a supercontinuum (SC) in the visible spectrum and an isolated anti-Stokes wing is formed in the visible area of the SC as a result of destructive interference of broadband spectral components. It was found that the energy of a visible SC increases discretely according to the number of LBs in the filament. We demonstrated that the model of ionization in solid dielectric which is used in numerical simulation fundamentally affects the obtained scenario of LB formation. The possibility of the formation of LBs under the filamentation of middle-IR pulses in the atmosphere was shown with numerical simulation.

Spatio-temporal evolution scenarios of femtosecond laser pulse filamentation in fused silica

We investigated the evolution of femtosecond laser pulses at different wavelengths corresponding to normal, zero, and anomalous regimes of group velocity dispersion (GVD) in fused silica. The laser pulse filamentation in different GVD regimes under the same similarity parameters was first considered. It was established numerically that the scenario of the pulse filamentation depends both on temporal factors, which are determined by pulse GVD and self-phase modulation, and spatial factors associated with Kerr self-focusing and plasma defocusing. In presence of strong normal GVD the dispersive stretching causes, a pulse power decrease followed by lowering of the intensity in filament, electron density reduction in plasma channel, and suppressing of the refocusing. For zero GVD the multipeak regime of radiation propagation is realized in the filament as a result of recurring self-focusings of powerful pulse tail, which was defocused in laser plasma. When GVD is anomalous a sequence of “light bullets” with duration about 10 fs forms in the filament. And the peak intensity in “light bullet” stays the same ≈ 5 × 1013 W/cm2. In the regime of anomalous GVD power is transferred from the pulse edges to its center, where the repeated self-focusings occur and form a “light bullet” sequence.

Formation of conical emission of supercontinuum during filamentation of femtosecond laser radiation in fused silica

The formation of conical emission of supercontinuum during filamentation of femtosecond laser pulses with central wavelengths in a wide range is studied experimentally, numerically, and analytically. The frequency-angular intensity distribution of the spectral components of conical emission is determined by the interference of supercontinuum emission in a filament of a femtosecond laser pulse. The interference of supercontinuum emission has a general character, exists at different regimes of group velocity dispersion, gives rise to the fine spectral structure after the pulse splitting into subpulses and the formation of a distributed supercontinuum source in an extended filament, and causes the decomposition of the continuous spectrum of conical emission into many high-contrast maxima after pulse refocusing in the filament. In spectroscopic studies with a tunable femtosecond radiation source based on a TOPAS parametric amplifier, we used an original scheme with a wedge fused silica sample. Numerical simulations have been performed using a system of equations of nonlinear-optical interaction of laser radiation under conditions of diffraction, wave nonstationarity, and material dispersion in fused silica. The analytic study is based on the interference model of formation of conical emission by supercontinuum sources moving in a filament.

Light bullets from Mid-IR femtosecond filament in air

We numerically investigated the formation of light bullets under filamentation of 3.8- μm femtosecond pulse in presence of anomalous GVD in humid air. The dispersion of humid air in infrared spectral region was characterized by model fits of the real part of refractive index from HITRAN database. The fit for the 2.8-4.2 μm region describes the areas of anomalous GVD near 3.1 μm and 4 μm in humid air. During the nonlinear propagation of femtosecond pulse in humid air, the compressed in space and time wave packet – light bullet – appears in the central time layers of the pulse and moves to the pulse tail. The duration of light bullet reaches the few-cycle value and the energy fluence in the in the light bullet cross-section is above 1 J/cm2. Together with the light bullet formation the spectrum of the pulse broadens strongly and covers the spectral region from 3 μm to 5 μm. The interference model was used for investigation of the frequency-angular distribution of the supercontinuum spectrum components.

Femtosecond filamentation of Bessel–Gaussian beams under conditions of anomalous group-velocity dispersion

This paper presents the results of an experimental and numerical study of filamentation of axicon-focused femtosecond laser radiation in fused quartz. It is shown that the wave-front curvature of the laser beam incident on the axicon affects the number of plasma channels in the filament and the distance between them. The parameters of the solitonlike wave packets (light bullets), such as the peak intensity, duration, and transverse size are independent of the wave-front curvature of the radiation.

Supercontinuum conical emission accompanying filamentation of a femtosecond laser pulse in fused quartz

The frequency-angular spectrum of the supercontinuum accompanying the filamentation of femtosecond laser pulses in KU-1 fused quartz at various wavelengths has been experimentally and numerically investigated. The splitting effect of divergent conical-emission radiation of the supercontinuum into discrete rings when the radiation is refocused is compared for various wavelengths.

Supercontinuum spectrum in IR Bessel-Gauss and Gauss pulsed beam filament under anomalous group velocity dispersion in fused silica

Paper concerns experimental and numerical investigation on supercontinuum emission in the process of femtosecond beam filamentation under different geometrical focusing in presence of anomalous group velocity dispersion in fused silica. It was shown that energy of supercontinuum visible part increases discretely with increasing of input laser pulse energy. It is connected with light bullets formation. Also it was found that energy of supercontinuum visible part connected with each bullet doesn’t depend on focusing geometry.