Terahertz-mid-infrared anisotropic vortex beams generation via few-cycle vortex-laser-induced air plasma

Abstract

We numerically demonstrated the generation of terahertz-mid-infrared (THz-MIR) anisotropic vortex beams from few-cycle vortex-laser-induced air plasma based on both the intra-pulse four-wave mixing and the photocurrent models. It is found that for the low-frequency THz components, their phase variance on the azimuthal angle follows a simple stepwise increase with a step of π, indicating the unsuccessful inheritance of the orbital angular momentum (OAM) from the few-cycle vortex driving laser. However, for the high-frequency MIR components, they are anisotropic vortex beams with a nonlinear variance in the phase profile showing angular acceleration, which indicates they carry OAM. Also, their intensity distributions vary from two-petal structure slowly to ring-shaped structure as the frequency increases. Physically, the behaviors of their phase and intensity distributions can be explained well by the interference theory of the two models. Interestingly, according to the fitting formula of the phase distributions, the calculated topological charges (TCs) of all components in the THz-MIR frequencies range are equal to that of the driving laser. Moreover, the phase nonlinearity and the intensity distributions strongly depend on the corresponding THz-MIR frequencies. The results from both schemes are the decoupling of the OAM, the TC, the geometry and the power distribution in anisotropic vortex beams.

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

“…Specifically, it has been demonstrated theoretically and experimentally that both Kerr nonlinearity and plasma currents contribute to THz generation during filamentation since filamentation is the result of dynamical balance between the Kerr self-focusing and plasma defocusing [43,44]. Basically, they dominate at different pump laser intensities and filament lengths, and contribute to different THz frequency ranges [32][33][34][35][36]. At the early stage of filament formation, self-focusing for Kerr effect always plays a leading role [43].…”

“…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] ∂…”

“…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].…”

“…As for frequency up-conversion process of high-order harmonic generation via vortex laser-gas interaction, the OAMs of the high-order harmonics for both integer and fractional schemes, always scales with the harmonic orders due to OAM conservation [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].…”

Recent advances in generation of terahertz vortex beams and their applications*

“…[108][109][110][111] Recent researchers found that these two fields with OAM, i.e., two-color vortex field (Gaussian 800 nm + vortex 400 nm) or a few-cycle vortex field (vortex 800 nm), can induce millimeter-scale gas-plasma to generate ultra-broadband intensity modulated THz vortex waves, which are also called THz angular accelerating beams. [112][113][114] Some experimental and simulation results for the generation of the intensity modulated THz vortex beams in Refs. [113,114] are shown in Fig.…”