Generation and evolution of different terahertz singular beams from long gas-plasma filaments

Abstract

We theoretically and numerically investigate the generation and evolution of different pulsed terahertz (THz) singular beams with an ultrabroad bandwidth (0.1–40 THz) in long gas-plasma filaments induced by a shaped two-color laser field, i.e., a vortex fundamental pulse (ω0) and a Gaussian second harmonic pulse (2ω0). Based on the unidirectional propagation model under group-velocity moving reference frame, the simulating results demonstrate that three different THz singular beams, including the THz necklace beams with a π-stepwise phase profile, the THz angular accelerating vortex beams (AAVBs) with nonlinear phase profile, and the THz vortex beams with linear phase profile, are generated. The THz necklace beams are generated first at millimeter-scale length. Then, with the increase of the filament length, THz AAVBs and THz vortex beams appear in turn almost periodically. Our calculations confirm that all these different THz singular beams result from the coherent superposition of the two collinear THz vortex beams with variable relative amplitudes and conjugated topological charges (TCs), i.e., +2 and −2. These two THz vortex beams could come from the two four-wave mixing (FWM) processes, respectively, i.e., ω0+ω0−2ω0→ωTHz and –(ω0+ω0) + 2ω0→ωTHz. The evolution of the different THz singular beams depends on the combined effect of the pump ω0−2ω0 time delay and the separate, periodical, and helical plasma channels. And the TC sign of the generated THz singular beams can be easily controlled by changing the sign of the ω0−2ω0 time delay. We believe that these results will deepen the understanding of the THz singular beam generation mechanism and orbital angular momentum (OAM) conversion in laser induced gas-filamentation.

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

“…While the clamping intensity for a LG mode is much bigger than that of a Gaussian one [23]. It is verified that when the pump intensity is at the range of 20-50 TW cm −2 , Kerr nonlinearity dominates over plasma currents [34,35]. The fractional-order vortex two-color field with an identical initial TCs of l 1 = l 2 = 1.8 at t = 0.5T (T is the time cycle) is taken as an example to check the characteristics of the transverse spatial and OAM spectrum distributions.…”

“…In this paper, a numerical simulation of the laser-gas interaction based on the unidirectional pulse propagation equation (UPPE) [32][33][34][35] is carried out to simulate the laser nonlinear propagation. The spectral-domain UPPE in the group-velocity moving reference frame is performed, which can be expressed as [34,38,39] ∂…”

“…We use 64 × 64 × 4096 grids for the spatial parameters x, y and time t with the steps of Δx = Δy ≈ 15.6 μm and Δt ≈ 0.08 fs, respectively. We employed the intensity-dependent propagation step size Δz n = Δz 0 I 0 /I n , where I 0 and I n are the peak intensity of the initial and the nth step, respectively [34,38]. It has been widely known that plasma-based THz sources induced by femtosecond laser with tens of TW cm −2 are characterized by an ultrabroad bandwidth [40,41].…”

“…The development of ultrafast vortex beams has opened new possibilities for the generation of filamentation [22,23], supercontinuum [24], high-order harmonics [25][26][27][28][29][30][31], and even terahertz (THz) pulses [32][33][34][35][36] with complex structures. It has been demonstrated that the nonlinear phenomena are crucial during the interaction of laser with bulk media and atmospheric air [23,[25][26][27][28][29][30][31].…”

“…In contrast, when it turns to the frequency down-conversion process of THz generation via vortex laser-induced gas plasma, the THz transverse spatial distributions and also physical mechanics are far more different, which depend on the pump intensity, the pump laser modes, the pump chirp parameter, the THz frequency, the filament length, and so on [32][33][34][35][36]. Particularly, for long filaments induced by integer-order vortex two-color field, three types of singular beams (i.e., THz necklace beams, THz azimuthal accelerating vortex beams (AAVBs), THz IVBs) can be produced at different filament lengths, which result from the coherent superposition of the two collinear THz vortex pulses with variable relative amplitudes and conjugated TCs [34]. What if two-color vortex pulses with initial fractional TCs are employed to interact with air?…”