Generation of terahertz radiation with fractional or integer OAMs from a fractional-order vortex two-color field

Abstract

This paper investigates the generation of the ultra-broadwidth (0.1-30 THz) terahertz (THz) radiation carrying fractional/integer orbital angular momentums (OAMs) via the interaction of the two-color (ω0 and 2ω0) laser field carrying initial fractional topological charges (TCs) with air in a moderate pump intensity regime (20 TW/cm2<Ipump<50 TW/cm2). The two four-wave mixing (FWM) processes (i.e., ω0+ω′0-2ω0→ωTHz and 2ω0-ω0-ω′0→ωTHz) are responsible for THz generation. The two processes can produce two THz pulses. They interfere with each other and THz interference vortex beams are obtained. More importantly, the generation probability from the first FWM process grows while that of the second process declines in the positive frequency region over distance. This is largely due to the combined action of phase mismatch and the blue shift of the THz center frequency. For a longer distance, THz fractional/integer vortex beams (FVBs/IVBs) are produced by the dominant FWM process (ω0+ω′0-2ω0→ωTHz). Therefore, via employing different combinations of the initial TCs of the ω0 and 2ω0 pulses, one can manipulate the generation of the THz vortex beams with arbitrary fractional-order or integer-order TCs at some specific propagation distances. What is even more interesting is that, when employing half-integer TCs, THz FVBs with varying TC over distance can be produced, companied with birth and annihilation of the alternative vortex pair. This is principally due to diffraction-related effects and the unstable nature of the fractional vortex structures. This simple manipulation for THz waves carrying arbitrary fractional or integer TCs in this scheme encourages the applications for optically rotation, manipulation of molecular or cell assays and image edge enhancement in the field of biomedicine.

Terahertz quasi-perfect vortex beam with integer-order and fractional-order generated by spiral spherical harmonic axicon

“…[23] In 2022, Wang et al used a fractional-order vortex two-color field to generate THz fractional vortex beam in simulation. [24] Furthermore, it is demonstrated that THz vortex beams can be used in communication [20,25] and magnetic excitations. [26] Nevertheless, THz vortex beam has the same disadvantages as optical vortex beam in which ring radius increases with the topological charge number increasing.…”