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

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 (omega (0) and 2 omega (0)) laser field carrying initial fractional topological charges (TCs) with air in a moderate pump intensity regime (20 TW cm(-2) < I (pump) < 50 TW cm(-2)). The two four-wave mixing (FWM) processes (i.e., omega (0) + omega '(0) - 2 omega (0) -> omega (THz) and 2 omega (0) - omega (0) - omega '(0) -> omega (THz)) 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 vortex beams (FVBs)/integer vortex beams (IVBs) are produced by the dominant FWM process (omega (0) + omega '(0) - 2 omega (0) -> omega (THz)). Therefore, via employing different combinations of the initial TCs of the omega (0) and 2 omega (0) 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.