Circularly polarized terahertz pulse generation in a plasma channel created by a uv high-intense laser pulse in the presence of a static magnetic field

Coulomb potential determining terahertz polarization in a two-color laser field

The orientation and ellipticity of terahertz (THz) polarization generated by a two-color strong field not only casts light on underlying mechanisms of laser-matter interaction, but also plays an important role for various applications. We develop the Coulomb-corrected classical trajectory Monte Carlo (CTMC) method to well reproduce the joint measurements, that the THz polarization generated by the linearly polarized 800 nm and circularly polarized 400 nm fields is independent on two-color phase delay. The trajectory analysis shows that the Coulomb potential twists the THz polarization by deflecting the orientation of asymptotic momentum of electron trajectories. Further, the CTMC calculations predict that, the two-color mid-infrared field can effectively accelerate the electron rapidly away from the parent core to relieve the disturbance of Coulomb potential, and simultaneously create large transverse acceleration of trajectories, leading to the circularly polarized THz radiation.

Three-dimensional modeling of intense unipolar THz pulses formation during their amplification in nonequilibrium extended Xe plasma channel

We develop a three-dimensional (3D) fully self-consistent model for analysis of an ultrashort THz pulse propagation and amplification in a nonequilibrium plasma channel formed in xenon by a femtosecond UV laser pulse. The model is based on the self-consistent solution of a second order wave equation in the cylindrical geometry and the kinetic Boltzmann equation for the electron velocity distribution function (EVDF) at different points of the spatially inhomogeneous nonequilibrium plasma channel. We analyze the wide range of plasma and seed pulse parameters and reveal the optimal regimes for producing high intensity outgoing THz fields as well as highly unipolar THz pulses within the proposed mechanism. It is demonstrated that the process of EVDF relaxation in plasma limits the amplification of THz pulses at the level of ∼10^{7}W/cm^{2}. Both focusing features of nonequilibrium plasma and the possibility of producing THz pulses with a high degree of unipolarity are confirmed for the case of 3D geometry.

Terahertz Pulse Generation by Strongly Magnetized, Laser-Created Plasmas

Relativistic interactions between ultraintense (>10^{18}  W cm^{-2}) laser pulses and magnetized underdense plasmas are known to produce few-cycle Cerenkov wake radiation in the terahertz (THz) domain. Using multidimensional particle-in-cell simulations, we demonstrate the possibility of generating high-field (>100  GV m^{-1}) THz bursts from helium gas plasmas embedded in strong (>100  T) magnetic fields perpendicular to the laser path. We show that two criteria must be satisfied for efficient THz generation. First, the plasma density should be adjusted to the laser pulse duration for a strong resonant excitation of the electromagnetic plasma wake. Second, in order to mitigate the damping of the transverse wake component across the density gradients at the plasma exit, the ratio of the relativistic electron cyclotron and plasma frequencies must be chosen slightly above unity, but not too large, lest the wake be degraded. Such conditions lead the outgoing THz wave to surpass in amplitude the electrostatic wakefield induced in a similar, yet unmagnetized plasma.

Unipolar terahertz pulse formation in a nonequilibrium plasma channel formed by an ultrashort uv laser pulse

We perform an alternative approach to produce highly unipolar terahertz pulses. The idea is based on the nonuniform amplification of seed ultrashort carrier-envelope phase (CEP) pulses in nonequilibrium fast relaxing plasma of air or nitrogen. If the gain coefficient drops significantly within the duration of a one-cycle CEP pulse, it undergoes significant distortion where the leading edge of the pulse is amplified and the trailing tail of opposite polarity is absorbed. The obtained results involve a self-consistent solution of the second-order wave equation and kinetic Boltzmann equation for the electron velocity distribution function relaxation.