Proposal for Simultaneous Two-Color Smith–Purcell Terahertz Radiation Through Effective Surface Plasmon Excitation

Proximity interaction of a travelling electron beam near the surface of a structured metallic slab excites effective surface Plasmon at lower frequencies. Due to the existence of a velocity phase match, the Terahertz (THz) frequencies-known as Smith-Purcell radiation (SPR)-are produced. Here, we investigate two-color SPR characteristics of a ladder-type slab by means of the femtosecond pre-bunched perturbation (in parallel and counter propagating configurations). Our single-bunch test showed a successful two-color generation in the THz domain with variable frequency separations. At the allowed frequencies of SPR, an umklapp process in electron-Plasmon momentum redshifts the ESP's momentum to a strong THz generation regime-beyond the light zone. Calculating the three-dimensional (3D) dispersion relation, we find out that the slab width has a great effect on required beam energy and also velocity matching between phase of ESP and beam. Furthermore, by filling the slits with higher dielectric materials, a stronger and narrower SPR was achieved. The two-color device showed a strong coherent SPR when a long sequence of prebunched beams with variable bunch-to-bunch distances was launched to the system. Numerical methods-finite integral and particle-in-cell-are used to study the dispersion relation and the interaction of electrons with slab, respectively. These simulations showed complete agreement with the reported 2D and 3D results and fully supported the wavelengths predicted by the analytic equations.