The temporal evolution of a plasma channel at the trail of a self-guided femtosecond laser pulse was studied experimentally and theoretically in air, nitrogen (with an admixture of ∼3% O_{2}), and argon in a wide range of gas pressures (from 2 to 760 Torr). Measurements by means of transverse optical interferometry and pulsed terahertz scattering techniques showed that plasma density in air and nitrogen at atmospheric pressure reduces by an order of magnitude within 3-4 ns and that the decay rate decreases with decreasing pressure. The argon plasma did not decay within several nanoseconds for pressures of 50-760 Torr. We extended our theoretical model previously applied for atmospheric pressure air plasma to explain the plasma decay in the gases under study and to show that allowance for plasma channel expansion affects plasma decay at low pressures.

We report on the first experimental observation of graphene optical emission induced by the intense THz pulse. P-doped CVD graphene with the initial Fermi energy of about 200 meV was used, optical photons was detected in the wavelength range of 340-600 nm. Emission started when THz field amplitude exceeded 100 kV/cm. For THz fields from 200 to 300 kV/cm the temperature of optical radiation was constant, while the number of emitted photons increased several dozen times. This fact clearly indicates multiplication of electron-hole pairs induced by an external field itself and not due to electron heating. The experimental data are in a good agreement with the theory of Landau-Zener interband transitions. It is shown theoretically that Landau-Zener transitions are possible even in the case of heavily doped graphene because the strong THz field removes quasiparticles from the region of interband transitions during several femtoseconds, which cancels the Pauli blocking effect. *oladyshkin@gmail.comThe nonlinear optical properties of graphene are currently actively investigated in view of their prospective use in plasmonics, optoelectronics, and photonics [1]. The specific features of gapless dispersion of Dirac fermions, including the so-called Dirac cones, make graphene a unique material. In the neighborhood of the Dirac point, fermions have a massless dispersion law up to energies of order 1.5 eV and high Fermi velocity 8 F 10 cm/s  , thus providing ultrahigh nonlinear susceptibility (both quadratic and cubic) of graphene in the infrared and terahertz ranges [2][3][4][5][6][7][8][9][10]. The interaction of graphene with terahertz radiation is presently arousing particular interest from the viewpoint of various applications [11]. Terahertz video pulses are also used for studying relaxation processes in graphene [12,13].An interesting effectthe THz-pulse-initiated carrier multiplication (CM) was studied experimentally in [12]. This effect was attributed in [12] to impact ionization (II), i.e., to interband reverse Auger recombination. At the same time, according to the theoretical results obtained in [14] within the framework of the 2D Fermi-liquid model, the Auger processes have low efficiency in the region of the linear dispersion law 1 ; consequently, in the simulation made in [12] the Auger resonances were broadened "artificially" within the framework of the numerical scheme 2 . The CM effect was also observed in the case of optical pumping [13,[16][17][18][19][20][21][22], when the contribution of the reverse Auger 1 The situation is different for graphene in magnetic field (see, e.g., [15]). 2 The authors of [12] assumed that dislocations could be a possible physical mechanism of such broadening.relaxation to the formation of electron population in the conducting band did not exceed 5-10% [17].It is worth noting that the ballistic (directly initiated by the field) interband transitions were neglected in [12] in view of the small photon energy as compared to the typical kinetic energy of fermions in doped graphe...

The influence of a short intense (with an electric field strength up to 250 kV cm−1) terahertz (THz) pulse on the generation of second harmonic (SH) of Ti : sapphire laser radiation in crystals of zinc blende type (InAs and GaAs), characterised by nonzero bulk quadratic susceptibility, is investigated. It is experimentally shown for InAs(100) that, in the case of s-polarised first and second harmonics, an application of s-polarised THz field changes significantly the SH signal. The THz field-induced azimuthal dependence of the SH signal is in good agreement with the results of theoretical calculation within a phenomenological approach. The dependence of the SH signal on the delay time between the optical and THz pulses is investigated. This dependence for the GaAs crystal repeats the envelope of the THz pulse intensity, whereas in the case of InAs crystal there is a significant discrepancy, caused by the nonlinear dynamics of strong THz field in InAs.

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Second harmonic (SH) generation of femtosecond laser radiation induced by a short terahertz pulse in a layer of isotropic medium with cubic nonlinearity was investigated. A comprehensive 1D theory of SH field dynamics with allowance for different propagation velocities of interacting terahertz, fundamental, and SH pulses was developed. In particular, it was shown that for media with a coherent length of the fundamental and second optical harmonics smaller than the walk-off length of the fundamental optical and THz pulses, a SH radiation signal is generated only when the fundamental optical and terahertz pulses overlap near the entrance and exit surfaces of the nonlinear layer rather than when the pulses are superimposed in the bulk. The prediction was verified experimentally by using a 3-mm-thick fused quartz. Good agreement between the theory and the experiment was demonstrated.