The effect of frequency chirp on the generated terahertz (THz) wave by the interaction between intense laser pulses and under dense plasma in the presence of transverse magnetic field is investigated theoretically in the present model. An expression for the electron Lorentz factor coupling cyclotron motion nonlinearity is derived for static magnetic field perpendicular to the laser propagation axis. The beating lasers produce a nonlinear ponderomotive force due to their oscillatory motion. This force drives a nonlinear current, at beat frequency, which produces the THz radiation. The influence of the external magnetic field on the optimization process of THz field amplitude is investigated numerically. A linear frequency chirp increases the duration of nonlinear interaction of laser pulse with plasma electrons and hence, enforces the interaction for longer duration. The presence of magnetic field further provides the additional momentum to the THz photon to obtain a significant gain in output yield. Our numerical simulations reveal that there is a significant enhancement in the THz field strength for optimized value of the chirp parameter and magnetic field.

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The present paper reports on the combined influence of the chirp characteristics of pump pulses and spatially periodic density (density ripple) plasma, on terahertz (THz) radiation generation, by beating of two chirped pulses. The beating lasers exert a nonlinear ponderomotive force along the z-direction. A self-consistent field is generated due to the nonlinear oscillations of plasma electrons; as a result of these linear and nonlinear forces the plasma electrons attain an oscillatory velocity that couples with the density ripple to generate a stronger transient transverse current, driving THz radiation. The importance of chirp parameter, amplitude and periodicity of density structure are discussed for emitted THz radiation. Our numerical simulations disclose that the variation of the chirp frequency parameter and ripple amplitude have considerable roles in improving the nonlinear oscillating current. By optimizing the chirp parameter and amplitude of the density ripple, a notable change in the magnitude of the terahertz field amplitude is found. The present paper maybe useful for broadband THz pulses, for use in plasma diagnostics and time-domain spectroscopy.

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The present communication deals with a scheme to generate terahertz (THz) radiation by electromagnetic Gaussian beams beating in a hot collisionless plasma having a density ripple on its surface, parallel to the z-axis. These p-polarised laser beams propagate in the x-z plane, incident obliquely to the density ripple on the plasma surface, and exert a ponderomotive force on electrons. The plasma electrons start oscillating because the plasma neutrality disturbed by the nonlinearity arises due to the ponderomotive force. This oscillatory velocity beats with the density ripple; as a result, an irrotational current density J NL arises at the beating frequency ω 1 − ω 2 (with ∇ × J NL = 0). This nonlinear current density urges a wave whose frequency is in the THz range. Our results show that, for a set of laser and plasma parameters, the power of emitted THz radiation scales as the square of the density ripple amplitude, as well as the amplitude of the emitted THz wave, decreases with the THz frequency and increases with the incidence angle up to an optimum value. In our case, the maximum normalised amplitude of emitted THz radiation is reached up to 0.038 at laser intensity ~7 × 10 14 W cm −2 , θ = 30 • and electron temperature ~5 keV with 30% density ripple.