ИСТИНА

One of the distinctive features of femtosecond laser filamentation is intensity and plasma concentration clamping. Arising due to the dynamical competition between Kerr focusing and plasma defocusing this phenomenon limits the peak intensity to the values 10^14 W/cm^2 and plasma concentration level to 10^16 – 10^17 1/cm^3. Further increase of the beam power leads to the beam splitting and multifilament formation rather than to an intense single filament with high plasma density. However it have been recently shown that fusing several filaments by introducing additional weak focusing one can obtain significant enhancement of energy deposition in the extended region around the focus resulting in higher plasma concentration and intensity [1,2,3]. This report aims to present the comprehensive picture of superfilament evolution including its formation from merging filaments and dissipation stages. We discuss energy deposition during the multifilament evolution and its dependency of the pulse energy as well as the influence of additional focusing conditions on superfilament formation. In the study we used a wide number of techniques: high-resolution broadband acoustics diagnostics [4], mode, luminescence and frequency-angular spectra measurements and interferometry. Combining this methods in different combinations in single-shot regime provided us with diverse and exhaustive data on superfilament structure, energy deposition and evolution. All the measurements was accomplished with pulse energy measurements that allowed to compare data from different experiments. A bunch of filaments was formed by radiation of a Ti:Sa laser system delivering pulses with 55 fs duration and 2–25 mJ energy at 10 Hz repetition rate with 8mm FWHM beam diameter. The bunch was generated in air under additional focusing by a lens. We use the lenses with 3.12, 1, 0.75, 0.5 and 0.3m focus lengths. For each focus length we made measurements in the extensive number of longitudinal coordinate points. The measurements were made in stochastic and amplitude regularized regimes. In the first one the separate filament formation is governed by an intrinsic amplitude and phase modulation of the laser beam, while in the second an amplitude mask was introduced into the beam to obtain the regularized filament array. In the work the mask was a thin opaque plate with four 4.2mm diameter holes placed on 5 mm center-to-center distance. For each focus length the reference signal for single filament regime has been measured. We conducted the most thorough investigation of the superfilament evolution for the 3.12m lens focusing as the focusing conditions in this regime are close to the reported in previous studies where the nice superfilamentation was observed [1-3,5]. In both stochastic and amplitude regularized modes huge nonlinear increase of the linear energy deposition has been observed. It increases from 5 μJ/cm for single filament to 300–400 μJ/cm in stochastic regime with pulse energy grows only 10 times and to more than 100 μJ/cm with amplitude regularization while the energy increases 5 times. However, the volumetric energy density in this regime is only slightly (~1.5 times) higher than for a single filament regime. When the pulse power P <= 20Pcr the nice single multifilament forms, while at the higher powers it tends to split into the several filaments. This complex superfilament structure should be taken into account in modern applications of superfilamentation. We revealed that superfilaments effectively forms only under loose focusing conditions when NA<0.01. When NA > 0.01 filament interactions in multifilament bundle don’t lead to nonlinear energy deposition enhancement and superfilament formation. References [1] D. V. Pushkarev, E. V. Mitina, D. S. Uryupina, R. V. Volkov, N. A. Panov, A. A. Karabutov, O. G. Kosareva, and A. B. Savel’ev. Nonlinear increase in the energy input into a medium at the fusion of regularized femtosecond filaments. JETP Letters, 106(9):561–564, 2017. [2] G Point, Y. Brelet, A. Houard, V. Jukna, C. Mili´an, G. Carbonnel, Y. Liu, A. Couairon, and A. Mysyrowicz. Superfilamentation in air. Phys. Rev. Lett., 112:223902, 2014. [3] D. Pushkarev, E. Mitina, D. Shipilo, N. Panov, D. Uryupina, A. Ushakov, R. Volkov, A. Karabutov, I. Babushkin, A. Demircan, U. Morgner, O. Kosareva, A. Savel’ev. Spatial dynamics and energy deposition in air superfilament created by a subTW femtosecond laser. New Journal of Physics, accepted for publication [4] D. S. Uryupina, A. S. Bychkov, D. V. Pushkarev, E. V. Mitina, A. B. Savel’ev, O. G. Kosareva, N. A. Panov, A. A. Karabutov, and E. B. Cherepetskaya. Laser optoacoustic diagnostics of femtosecond filaments in air using wideband piezoelectric transducers. Laser Physics Letters, 13(9):095401, September 2016. [5] D. Pushkarev, D. Shipilo, A. Lar’kin, E. Mitina, N. Panov, D. Uryupina, A. Ushakov, R. Volkov, S. Karpeev, S. Khonina, O. Kosareva, and A. Savel’ev. Effect of phase front modulation on the merging of multiple regularized femtosecond filaments. Laser Physics Letters, 15:045402–045402, 2018.