Observation of filamentation-induced continuous self-frequency down shift in air

Spectral “soliton” transformation and four-wave mixing under femtosecond laser radiation filamentation in molecular gases

“…In particular we observed new spectral component -"spectral soliton" on the red side of the initial spectrum. Spectral shift of this "soliton" increases along the filament up to 100 nm from the fundamental wavelength of 805 nm at the distance of 3-4 m. The same spectral shift was observed recently in [3]. In the filament core (which was extracted by placing small 100-700 m in diameter diaphragm into the filament) the spectral intensity of this new component was much higher than the one at the fundamental frequency.…”

Post-filament self-trapping of ultrashort laser pulses

“…The same effect is achieved by artificially switching off optical nonlinearities at z 3 m, showing that postfilamentation beam self-trapping in our experiments is indeed due to the saturable self-focusing nonlinearity. Moreover, unlike modeling in [22], which, as the authors admit, fails to reproduce the long-wavelength part of the spectrum, motivating further in-depth studies, our simulations provide an accurate fit for the experimental spectra [ Fig. 1(a)].…”

“…2(a) 2(d) and 3(a), the (a∕rP∕P c scaling gives a∕γ ≈ 3 corresponding r FWHM ≈ 105 μm, which agrees very well with the FWHM radius of the self-trapped beam, r FWHM ≈ 100 μm at z 3 m, in our conditions. Alternative scenarios giving rise to beams with suppressed divergence behind the ionization region, including post-ionization long-range self-channeling of ultrashort laser pulses in air [19,20], involve a quasilinear dynamics of beams with intensities below ionization threshold, Bessel-like beams [21], and post-filamentation beam channeling sustained by the photon bath [22,23]. Numerical simulations have been performed to check whether any of these effects is involved in postfilamentation beam self-trapping in our experiments.…”

Mid-infrared laser filamentation in molecular gases

“…In particular, they most often generate similar output spectra because the blue shift due to ionization and pulse self-steepening [1,2,6,7] dominates the Raman-induced red shift [8]. However, the latter can be substantial in favorable conditions [9][10][11].…”