Reconfigurable Terahertz Spatial Deflection Varifocal Metamirror.

A traditional optical lens usually has a fixed focus, and its focus controlling relies on a bulky lens component, which makes integration difficult. In this study, we propose a kind of terahertz spatial varifocal metamirror with a consistent metal-graphene unit structure whose focus can be flexibly adjusted. The focus deflection angle can be theoretically defined by superimposing certain encoded sequence on it according to Fourier convolution theorem. The configurable metamirror allows for the deflection of the focus position in space. The proposed configuration approach presents a design concept and offers potential advancements in the field of developing novel terahertz devices.

Transmission grating couples and enhances the second harmonic of the electron beam to generate tunable high-power terahertz radiation.

Vacuum electronic devices utilizing free-electron-based mechanisms are a crucial class of terahertz radiation sources that operate by modulating electron beams. In this study, we introduce what we believe is a novel approach to enhance the second harmonic of electron beams and substantially increase the output power at higher frequencies. Our method employs a planar grating for fundamental modulation and a transmission grating operating in the backward region to augment the harmonic coupling. The outcome is a high power output of the second harmonic signal. Contrasting with traditional linear electron beam harmonic devices, the proposed structure can achieve an output power increase of an order of magnitude. We have investigated this configuration computationally within the G-band. Our findings indicate that an electron beam density of 50 A/cm2 at 31.5 kV can produce a 0.202 THz center frequency signal with an output power of 4.59 W. As the electron beam voltage is adjusted from 23 kV to 38.5 kV, the output signal frequency shifts from 0.195 THz to 0.205 THz, generating several watts of power output. The starting oscillation current density at the center frequency point is 28 A/cm2, which is significantly lower in the G-band compared to conventional electron devices. This reduced current density has substantial implications for the advancement of terahertz vacuum devices.

Independently Tunable Multipurpose Absorber with Single Layer of Metal-Graphene Metamaterials.

This paper reports an independently tunable graphene-based metamaterial absorber (GMA) designed by etching two cascaded resonators with dissimilar sizes in the unit cell. Two perfect absorption peaks were obtained at 6.94 and 10.68 µm with simple single-layer metal-graphene metamaterials; the peaks show absorption values higher than 99%. The mechanism of absorption was analyzed theoretically. The independent tunability of the metamaterial absorber (MA) was realized by varying the Fermi level of graphene under a set of resonators. Furthermore, multi-band and wide-band absorption were observed by the proposed structure upon increasing the number of resonators and resizing them in the unit cell. The obtained results demonstrate the multipurpose performance of this type of absorber and indicate its potential application in diverse applications, such as solar energy harvesting and thermal absorbing.

Ultrawideband terahertz absorber with a graphene-loaded dielectric hemi-ellipsoid.

We herein present a high-performance ultrawideband terahertz absorber with a silicon hemi-ellipsoid (SHE) on a monolayer graphene that is separated by a dielectric spacer from a bottom metal reflector. The constitution of the absorber, which includes dielectric-mode structures and unstructured monolayer graphene, can minimize undesired optical losses in metals and avoid graphene processing. The absorber achieved an ultrawide absorption bandwidth from 2 THz to more than 10 THz with an average absorption of 95.72%, and the relative bandwidth is 133%. The excellent absorption properties are owing to the presence of graphene and the shape morphing of the SHE, in which multiple discrete graphene plasmon resonances (GPRs) and continuous multimode Fabry-Perot resonances (FPRs) can be excited. By coupling the GPRs and FPRs, the absorption spectrum is extended and smoothed to realize an ultrawideband absorber. The incident angular insensitivity within 50° of the absorber is discussed. The results will shed light on the better performance of terahertz trapping, imaging, communication and detection.

Independently tunable multi-band and ultra-wide-band absorbers based on multilayer metal-graphene metamaterials.

Dynamically and independently tunable absorbers based on multilayer metal-graphene metamaterials are proposed to achieve multi-band and ultra-wide-band absorbing properties at mid-infrared frequencies. Dual-band, triple-band and even more bands absorption can be arbitrarily customized by etching the appropriate number of tandem gold strips in each meta-molecule, as well as stacking multiple metal-graphene layers. Through tuning the Fermi energy level of the graphene in each metal-graphene layer separately, the multiple absorption resonances can be dynamically and independently adjusted. With side-by-side arrangement of the gold strips in each supercell, the proposed structure is rendered to be a promising candidate for ultra-wide-band absorber. The extreme bandwidth exceeding 80% absorption up to 7.5THz can be achieved with a dual-layered structure, and the average peak absorption is 88.5% in the wide-band range for lossless insulating interlayer. For a triple-layered structure, the average peak absorption is 84.7% from 27.5 THz to 38.4 THz with a minimum of 60%. The absorption windows can be even further broadened with more metal-graphene layers. All these results will benefit the integrated microstructure research with simple structure and flexible tunability, and the multilayer structure has potential applications in information processing fields such as filtering, sensing, cloaking objects and other multispectral devices.

Terahertz radiation in graphene hyperbolic medium excited by an electric dipole.

In this Letter, the enhanced and directional radiation in a wide terahertz (THz) frequency range in a graphene hyperbolic medium excited by an electric dipole is presented. The numerical simulations and theoretical analyses indicate that the enhanced radiation comes from the strong surface plasmon couplings in the graphene hyperbolic medium, consisting of alternative graphene and dielectric substrate layers. The simulation results also show that the peak power flow of the enhanced THz radiation in the graphene hyperbolic medium is dramatically enhanced by more than 1 order of magnitude over that in a general medium within a certain distance from the dipole, and the electromagnetic fields are strongly concentrated in a narrow angle. Also, the radiation fields can be manipulated, and the fields' angular distributions can be tuned by adjusting the dielectric permittivity and thickness of the substrates, and the chemical potential of graphene. Accordingly, it provides a good opportunity for developing miniature, integratable, high-power-density, and tunable radiation sources in the THz band at room temperature.

Dynamically tunable band stop filter enabled by the metal-graphene metamaterials.

Dynamically tunable band stop filter based on metal-graphene metamaterials is proposed and numerically investigated at mid-infrared frequencies. The proposed filter is constructed by unit cells with simple gold strips on the stack of monolayer graphene and the substrate of BaF2. A stable modulation depth up to -23.26 dB can be achieved. Due to the cooperative effect of the "bright-bright" elements, the amount of the gold strips in each unit cell determines the number of the stop-bands, providing a simple and flexible approach to develop multispectral devices. Further investigations illustrate that the location of the stop bands not only can be adjusted by varying the length of gold strips, but also can be dynamically controlled by tuning the Fermi energy level of graphene, and deep modulation is acquired through designing the carrier mobility. With the sensitivity as high as 2393 nm/RIU of the resonances to the varieties of surrounding medium, the structure is also enabled to be an index based sensor. The results will benefit the on plane or integrated micro-structure research with simple structure and flexible tunability, and can be applied in multi-band stop filters, sensors and other graphene-based multispectral devices.

Mediated coupling of surface plasmon polaritons by a moving electron beam.

The mediated coupling of surface plasmon polaritons (SPPs) by a parallel moving electron beam is demonstrated in this paper. The theoretical analysis shows that the electron beam excited spoof surface plasmon polaritons (SSPs) on the grating placed above the metal films play the role as the excitation source in the mediated coupling. The numerical calculations and particle-in-cell simulations demonstrate the significant advantages of the SSPs mediately coupled SPPs in contrast with that coupled by the parallel moving electron beam directly. The photo density of the mediately coupled SPPs reaches up to 106 per cm2 for the electron beam with the charge density 100 nC/cm, which is two orders of magnitude larger than that of the directly coupled SPPs. The tuning band of the mediately coupled SPPs reaches up to 9% for the beam energy ranging from 10 keV to 30 keV, while it almost cannot be tuned for the direct coupling. The lifetime of the mediately coupled SPPs, which reaches up to hundreds of femtoseconds, is also much longer. Accordingly, the mediated coupling may bring great significances for the applications of SPPs.

Plasmon modes of circular cylindrical double-layer graphene.

In this paper, a theoretical investigation on plasmon modes in a circular cylindrical double-layer graphene structure is presented. Due to the interlayer electromagnetic interaction, there exist two branches of plasmon modes, the optical plasmon mode and the acoustic plasmon mode. The characteristics of these two modes, such as mode pattern, effective mode index and propagation loss, are analyzed. The modal behaviors can be effectively tuned by changing the distance between two graphene layers, the chemical potential of graphene and the permittivity of interlayer dielectric. Importantly, the breakup of tradeoff between mode confinement and propagation loss is discovered in the distance-dependent modal behavior, which originates from the unique dispersion properties of a double-layer graphene system. As a consequence, both strong mode confinement and longer propagation length can be achieved. Our results may provide good opportunities for developing applications based on graphene plasmonics in circular cylindrical structure.

Coherent and Tunable Terahertz Radiation from Graphene Surface Plasmon Polarirons Excited by Cyclotron Electron Beam.

Terahertz (THz) radiation can revolutionize modern science and technology. To this date, it remains big challenges to develop intense, coherent and tunable THz radiation sources that can cover the whole THz frequency region either by means of only electronics (both vacuum electronics and semiconductor electronics) or of only photonics (lasers, for example, quantum cascade laser). Here we present a mechanism which can overcome these difficulties in THz radiation generation. Due to the natural periodicity of 2π of both the circular cylindrical graphene structure and cyclotron electron beam (CEB), the surface plasmon polaritions (SPPs) dispersion can cross the light line of dielectric, making transformation of SPPs into radiation immediately possible. The dual natural periodicity also brings significant excellences to the excitation and the transformation. The fundamental and hybrid SPPs modes can be excited and transformed into radiation. The excited SPPs propagate along the cyclotron trajectory together with the beam and gain energy from the beam continuously. The radiation density is enhanced over 300 times, up to 10(5) W/cm(2). The radiation frequency can be widely tuned by adjusting the beam energy or chemical potential. This mechanism opens a way for developing desired THz radiation sources to cover the whole THz frequency regime.

Electron beam excitation of surface plasmon polaritons.

In this paper, the excitations of surface plasmon polaritons (SPPs) by both perpendicular and parallel electron beam are investigated. The results of analytical theory and numerical calculation show that the mechanisms of these two excitations are essentially different, and the behavior and properties of SPPs in metal structures strongly depend on the methods of excitation. For the perpendicular excitation, SPPs contain plenty of frequency components, propagate with attenuation and are always accompanied with the transition radiation. Whereas for parallel excitation, SPPs waves are coherent, tunable, propagating without attenuation and the transition radiation does not occur. We also show that there are two modes for the parallel excited SPPs on the metal films and they all can be excited efficiently by the parallel moving electron beam. And the operating frequency of SPPs can be tuned in a large frequency range by adjusting the beam energy.

Surface polariton Cherenkov light radiation source.

A physical phenomenon has been found: in a structure of nanometal film with dielectric-medium loading, the surface polaritons excited by a uniformly moving electron bunch can be transformed into Cherenkov radiation with intensity enhancement in the medium. Based on this mechanism, the surface polariton Cherenkov light radiation source is presented and explored in the Letter. The results show that surface polariton Cherenkov light radiation source can generate radiation, from visible light to the ultraviolet frequency regime and the radiation power density can reach or even exceed 10(8) W/cm(2) depending on the beam energy and current density. It is a tunable and miniature light radiation source promising to be integrated on a chip and built into a light radiation source array.

Electromagnetic diffraction radiation of a subwavelength-hole array excited by an electron beam.

This paper explores the physics of the electromagnetic diffraction radiation of a subwavelength holes array excited by a set of evanescent waves generated by a line charge of electron beam moving parallel to the array. Activated by a uniformly moving line charge, numerous physical phenomena occur such as the diffraction radiation on both sides of the array as well as the electromagnetic penetration or transmission below or above the cut-off through the holes. As a result the subwavelength holes array becomes a radiation array. Making use of the integral equation with relevant Green's functions, an analytical theory for such a radiation system is built up. The results of the numerical calculations based on the theory agree well with that obtained by the computer simulation. The relation among the effective surface plasmon wave, the electromagnetic penetration or transmission of the holes and the diffraction radiation is revealed. The energy dependence of and the influence of the hole thickness on the diffraction radiation and the electromagnetic penetration or transmission are investigated in detail. Therefore, a distinct diffraction radiation phenomenon is discovered.