ИСТИНА

Long-Wavelength Radiation from Femtosecond Filaments in Gases O.G. Kosareva1, V.A. Andreeva1, N.A. Panov1, D.E. Shipilo1, A.B. Savel’ev1, A.P. Shkurinov1, V.A. Makarov1, L. Bergé2, and S.L. Chin3 1. Department of Physics and International Laser Center, Lomonosov MSU, 119991, Moscow, Russia 2. CEA, DAM, DIF, 91297 Arpajon cedex, France 3. COPL and Departement de physique, Universite Laval, G1K7P4, Quebec, Canada An extended femtosecond filament is a nonlinear optical structure, which can emit a continuum of frequencies as well as quasi-isolated pulses in certain spectral ranges [1]. Generation of mid-infrared (MIR) ultrashort pulses can be enhanced by seeding the filament with the pulse at the central frequency close to its second harmonic [2]. With an 800 nm filament the coherent THz radiation can be delivered to the desired position far through the atmosphere, avoiding thereby strong water vapor absorption [3-5]. We have fully studied the long-wavelength part of 800 nm filament spectral continuum (0.8 <  < 3000 m) and identified new physical mechanisms supporting experimentally observed phenomena such as 3D Raman light bullet in the near-infrared range [1], single-cycle MIR pulse generation and enhancement [2], ring-type shape of the spatial distribution of terahertz radiation from the air-based plasma [6-8]. The search for the physical mechanisms has been performed numerically, based on the comprehensive unidirectional pulse propagation equation [9]. The lack of limitation on the range of frequencies and angles relative to the laser beam propagation axis allowed us to consider the frequencies down to 0.1 THz and angular divergence of terahertz radiation up to 15 degrees. We find that a 3D Raman bullet is created on the axis of the extended filament in a gas due to the Kerr nonlinearity. Its shift in central wavelength varies from ~840 to ~1000 nm with the propagation distance. The retarded Kerr effect leads to an increase in the energy converted into the bullet. In a single-color filament the radiation in the MIR range spreads out in a ring due to the medium dispersion. Here, we report an order of magnitude increase in the on-axis few-cycle MIR pulse energy if a visible seed pulse is added into a 800 nm filament. We show that the emission in the range ~0.1-15 THz in the (-2) filament follows mainly from the nonlinear nonstationary change of the photocurrent. With increasing frequency above 15 THz the Kerr nonlinearity due to neutrals contributes to THz generation through the 0=2process. THz radiation originates from the filament axis and diffracts on the plasma obstacle forming a ring in the far zone. By varying the relative contribution of the Kerr and the plasma nonlinearities, we have explicitly proved that this plasma–based THz generation scenario is of universal nature. The role of the Kerr nonlinearity in THz generation is limited to the intensity, hence favoring the action of the plasma. Our numerical simulation results explain and generalize recent experiments in this field [1,2,6-8]. We thank RFBR (15-02-99630, 14-02-31379), the Council of RF President for Support of Young Scientists (MK-4895.2014.2), RF President grant for Leading Scientific Schools (NSh-3796.2014.2), Dynasty Foundation, and CEA-France. References: [1] Y. Chen, F. Théberge, C. Marceau, H. Xu, N. Aközbek , О.G. Кosareva , S.L. Chin “Observation of filamentation-induced continuous self-frequency down shift in air”, Appl. Phys. B 91, 219 (2008). [2] T. Fuji and T. Suzuki, "Generation of sub-two-cycle mid-infrared pulses by four-wave mixing through filamentation in air," Opt. Lett. 32, 3330-3332 (2007). [3] X. Xie, J. Dai, X.C. Zhang, “Coherent control of THz wave generation in ambient air”, Phys. Rev. Lett., 96, 075005 (2006). [4] C. D’Amico, A. Houard, M. Franco, B. Prade, and A. Mysyrowicz, “Conical Forward THz Emission from Femtosecond-Laser-Beam Filamentation in Air”, Phys. Rev. Lett. 98, 235002 (2007). [5] L. Bergé, S. Skupin, C. Kohler, I. Babushkin, and J. Herrmann, “3D Numerical Simulations of THz Generation by Two-Color Laser Filaments”, Phys. Rev. Lett. 110, 073901 (2013) [6] Y. S. You, T. I. Oh, and K.Y. Kim, “Off-Axis Phase-Matched Terahertz Emission from Two-Color Laser-Induced Plasma Filaments”, Phys. Rev. Lett. 109, 183902 (2012) [7] P. Klarskov, A.C Strikwerda, K. Iwaszczuk, and P. Uhd Jepsen, “Experimental three-dimensional beam profiling and modeling of a terahertz beam generated from a two-color air plasma”, New Journal of Physics 15, 075012 (2013). [8] A. Gorodetsky, A.D. Koulouklidis, M. Massaouti, and S. Tzortzakis, “Physics of the conical broadband terahertz emission from two-color laser-induced plasma filaments”, Phys.Rev. A 89, 033838 (2014) [9] M. Kolesik, J.V. Moloney, “Nonlinear optical pulse propagation simulation: From Maxwell’s to unidirectional equations”, Phys. Rev. E, 70, 036604(2004).