Two-dimensional (2D) materials including graphene, transition metal dichalcogenides, black phosphorus, MXenes, and semimetals have attracted extensive and widespread interest over the past years for their many intriguing properties and phenomena, underlying physics, and great potential for applications. The vast library of 2D materials and their heterostructures provides a diverse range of electrical, photonic, mechanical, and chemical properties with boundless opportunities for photonics and plasmonic devices. The infrared (IR) regime, with wavelengths across 0.78 μm to 1000 μm, has particular technological significance in industrial, military, commercial, and medical settings while facing challenges especially in the limit of materials. Here, we present a comprehensive review of the varied approaches taken to leverage the properties of the 2D materials for IR applications in photodetection and sensing, light emission and modulation, surface plasmon and phonon polaritons, non-linear optics, and Smith− Purcell radiation, among others. The strategies examined include the growth and processing of 2D materials, the use of various 2D materials like semiconductors, semimetals, Weyl-semimetals and 2D heterostructures or mixed-dimensional hybrid structures, and the engineering of light−matter interactions through nanophotonics, metasurfaces, and 2D polaritons. Finally, we give an outlook on the challenges in realizing high-performance and ambient-stable devices and the prospects for future research and large-scale commercial applications.

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.

Regenerated amplification induced by a Fabry-Perot (F-P) cavity is introduced to enhance the interaction efficiency of the free-electron-driven spoof surface plasmon. A direct-current electron beam flies above the meta-grating surface and seeds noise-level plasmonic waves. This weak signal experiences multiple back-and-forth regenerated amplifications in the F-P cavity loaded grating system, and the system outputs a pulsed radiation when the signal leaves the cavity. When compared with the condition without the F-P cavity, the equivalent beam-wave interaction length is effectively extended, and the interaction efficiency is improved by orders of magnitude. The proof-of-principle scheme is verified in both backward-wave and forward-wave modes using the particle-in-cell simulation. This scheme is promising for developing high-efficient on-chip terahertz free-electron radiation sources.

A free-electron-driven multi-frequency terahertz (THz) radiation based on a super-grating structure is elucidated in this paper. The super-grating, i.e., periodically depth-modulated metallic grating, has a peculiar dispersion characteristic, similar to the energy bands in a crystal due to the Brillouin zone folding effect. The multi-frequency radiation is stimulated in several directions with the excitation of a free electron as the synchronization points are in the radiative region. The radiation frequency can be independently tuned by the groove depths of the super-grating. The number of frequencies is tailored by the modulated period. Additionally, the multi-frequency THz radiation exhibits a frequency-locked effect during the energy variation of the free electron. Moreover, the radiation field intensity shows a significant enhancement compared with that of a conventional Smith-Purcell radiation. The work is promising for developing efficient on-chip THz radiation sources and boosts advanced THz applications such as communications, multi-frequency imaging, and beam diagnostics, etc.

A broadband vortex beam generator provides a promising solution for various applications. Since the space-charge wave of the free-electron bunch inherently covers a wide frequency range, the free-electron-driven devices can be utilized to generate broadband radiation. This work presents a wideband tunable multi-mode vortex beam generator based on the Smith–Purcell radiation (SPR) from a helical grating. The generated broadband vortex radiation presents prominent flexibility in the manipulation of frequency, topological charge, and radiation direction. The frequency range and the topological charge depend on the operating harmonic order, operating voltage, and structural parameters. The radiation direction varies with the operating frequency and covers a wide range in each band. Moreover, the proposed principle can be scaled to the terahertz frequency band. This work advances the application of the vortex beam in the millimeter wave-terahertz communication system.