No abstract

Ultrashort terahertz (THz) beam is often affected by spatio-temporal distortion due to its long wavelength, and this spatio-temporal coupling will degrade the focusing effect. Here, we develop a numerical model for calculating the vector focusing of ultrashort pulses with arbitrary spatial and temporal distribution, which adopt the vector diffraction integral and coherent spectrum superposition methods. Leveraging it, the focusing of a broadband pulse can be characterized together with its vector characteristics and we explore the influences of multifarious spatio-temporal couplings on the focusing of ultrashort THz pulses. In addition, the possibility of focusing the ultrashort THz radiation from an undulator to 1 GV/m is also discussed. This will provide further guidance for the non-equilibrium state of matter induced by ultrashort strong-field THz pulses.

Terahertz vortex beams need to be developed for supporting new applications in fundamental sciences. In this paper, a novel and feasible scheme is presented for generation of helically microbunched electron beam for the emission of terahertz (THz) vortex pulses. In the scheme, a seed laser pulse with longitudinal and azimuthal modulation is firstly produced by optical beating of dual-vortex pulses. Through the laser-electron interaction, such a complex profile of laser can be converted to a periodically azimuth-dependence energy modulation in the electron beam, which finally forms a helical microbunching structure in the beam after a dispersion section. Then, this electron beam is applied to emit THz vortex beams and its quantum number can be continuously tuned by changing the topological charges of the optical laser pulses. This approach is suited to conventional accelerator system consisted of a modulator and a chicane. We expect this presentation can access to new researches in THz science.