Terahertz emission from nonlinear interaction of laser beat wave with nanoparticles

Terahertz radiation is investigated using nonlinear interaction of a laser beat wave with a density-modulated medium of graphite nanoparticles. A beam-decentering parameter, b, is used to modify the polarizing field's profile to produce different-shaped laser pulse envelopes, e.g. Gaussian, top head, ring-shaped and cosh-Gaussian. The normal vectors corresponding to the basal planes of graphite nanoparticles are considered to be aligned parallel and perpendicular to the polarization of the propagating laser pulse. The electronic cloud of the graphite nanoparticles acquires a nonlinear oscillatory velocity under the influence of a nonlinear force that produces a strong nonlinear current at the beat-wave frequency (omega(T) = omega(1) - omega(2)). The strong nonlinear current allows the emission of radiation in the terahertz frequency regime. The terahertz radiation intensifies and attains a peak value when the laser beat-wave frequency (omega(T)) equals the plasmon frequency (omega(p)) of the nanoparticles. The terahertz radiation's amplitude is enhanced by up to the fourth order of magnitude in the case of a cosh-Gaussian laser pulse, when the beam-decentering parameter, b, equals 5. The present numerical results reveal that by changing the shape of a laser pulse and the properties of nanoparticles, one can control and tune the THz emission.