A Terahertz Vortex Beam Emitter With Tunable Topological Charge and Harmonic Excitation

Abstract

Terahertz (THz) vortex beams carrying orbital angular momentum (OAM) with high purity and tunable topological charge (TC) will undoubtedly bring extraordinary capacities for advanced imaging or communication systems. We propose a convenient and efficient method to generate a tunable and broadband vortex beam that is excited by superradiant Smith-Purcell radiation (SSPR) on a helical grating. This scheme fully explores the advantages of natural broadband evanescent wave carried by the electron beam and the Bloch's theorem regulated helical periodic systems. An explicit relation is established between the index of the spatial harmonic wave on the grating and the topological charge of the vortex beam. The electron energy in the SSPR can be customized to ignite the specific spatial harmonics and manipulate the OAM beam, accordingly. The separated radiation region also promises the high purity of the OAM spectrum. The harmonic excitation reduces the communality between the wavelength and the device size and alleviates the difficulty of device fabrication in the THz band. The proposed vortex scheme can not only be handily scaled to microwave and mid-infrared regions, but also bring possibilities to applications based on compact tunable vortex beam sources.

Bimodal Vortex Smith-Purcell Radiation

“…The triple-slots helical grating is employed as the interaction circuit to support the vortex SSP modes carrying orbital angular momentum. If these vortex SSP modes can be excited by the e beam and be diffracted into the free space, the VSPR with a single operating point can be achieved [5]. To achieve the multi-operation scheme, the Brillouin-zone folding phenomenon is introduced by modulating the depth of the groove.…”

“…Smith-Purcell radiation is emitted when the e beam flies over the periodic structure, which is a special kind of Cherenkov radiation and is firstly observed by Smith and Purcell [3]. By employing the helical tape, hologram grating, and helical grating, the vortex Smith-Purcell radiation (VSPR) can be readily generated with tunability in operation frequency and TC over broadband, which may remove the barrier of the generation of the vortex beam [4][5][6][7]. However, these methods can only generate a vortex beam with a single operation point at a time.…”

Chiral Plasmons Enable Coherent Vortex Smith–Purcell Radiation

“…Measured beam‐scanning characteristics and phase distributions verify the theoretical analysis and predictions on tunability and vorticity of V‐SPR with TC = −1 and −3, where the scheme can also be extended to the THz band. [ 24,25,27 ] The proposed methodology also indicates that the chiral plasmons can provide a versatile platform for investigating the manipulation of free‐electron radiation, thus boosting compact and flexible light sources with customized radiation features.…”

“…[ 17–22 ] Associating these artificial materials with swift electrons, numerous proposals have been reported to customize the phase, polarization, radiation direction, operating frequency, and coherence of the SPR, which contribute to the compact free‐electron radiation source and other applications. [ 12,14,15,23–31 ] Among them, as OAMs couple additional degrees of freedom to the EM wave, the generation of vortex Smith–Purcell radiation (V‐SPR) has drawn rapid attention in recent research. In 2019, Jing et al.…”

“…[ 26 ] To enhance the radiation intensity under DC e‐beam excitation, the harmonic‐operation scheme employing superradiant SPR is also explored. [ 27 ] The physical mechanism of the above studies is common, that is, the space harmonics of e‐beams are imposed with helical confinements to yield VBs. However, the increasing order of the space harmonics poses decreased intensity and mode overlap in the generation of VBs with high‐order TCs.…”