Multiple and multifocal THz emission under external periodic electric field on laser irradiation of spherical and cylindrical nanoparticles

An important theoretical approach is proposed for the resonant excitation of multiple and multifocal THz radiation fields via matching of the wavenumbers when there is an interaction of two skew coshyperbolic (cosh) Gaussian laser beams with a medium having graphite nanoparticles of spherical and cylindrical shapes in presence of an external periodic electrostatic field. Through the analytical treatment, we discovered that two additional transverse components of the nonlinear ponderomotive force and hence, nonlinear currents are generated during the interaction of beams and electrostatic field with the medium containing nanoparticles, leading to the emission of multiple THz radiation field. Two mechanisms are responsible for the resonant excitation of the THz fields: (1) the beating mechanism of two similar skew cosh laser beams and, (2) the coupling mechanism of the incident laser beams' electric field with the external periodic electric field. The additional components of the generated THz fields are termed as induced THz field, naming (E) over right arrow (0ITHz2) and (E) over right arrow (0ITHz3). The external periodic electrostatic field and density ripples in the medium play an important role in matching of the wavenumbers for achieving multiple resonance conditions. These resonance conditions lead to frequency-tuneable radiation due to beating and coupling, occurring at similar to 8.2 THz and similar to 48 THz, for the longitudinal surface plasmon resonance and transverse plasmon resonance, respectively. The ratio of the two peaks increases with the magnitude of external electric field. A comparative theoretical study between the beating and induced fields and the effect of different beam parameters, shapes of nanoparticles, inter-particle distance, orientations of their basal planes with respect to incident lasers' field on the induced field has been investigated to uncover the significance of the external periodic field and the use of two kinds of nanoparticles.