Ribbon Arrays Research Articles

Equivalent Boundary Conditions for Isotropic Arrays

Inductive and capacitive isotropic ribbon arrays with a period small compared with the wavelength, placed at the interface between two media, are considered. Boundary value problems for the scattering of plane waves of two polarizations, incident onto the arrays at arbitrary angles, are formulated. The boundary value problems are solved by the Galerkin method. A model of an array is proposed in the form of a semitransparent film on the surfaces of which equivalent boundary conditions relating the fields on both sides of the surfaces are imposed. The theoretical solution is compared with a numerical solutions obtained using a standard system of electromagnetic simulation. It is shown that, in the low-frequency region, the equivalent boundary conditions describe the behavior of the arrays with good accuracy.

Exploration of channel width scaling and edge states in transition metal dichalcogenides

We explore the impact of edge states in three types of transition metal dichalcogenides (TMDs), namely metallic Td-phase WTe2 and semiconducting 2H-phase MoTe2 and MoS2, by patterning thin flakes into ribbons with varying channel widths. No obvious charge depletion at the edges is observed for any of these three materials, in contrast to observations made for graphene nanoribbon devices. The semiconducting ribbons are characterized in a three-terminal field-effect transistor (FET) geometry. In addition, two ribbon array designs have been carefully investigated and found to exhibit current levels higher than those observed for conventional one-channel devices. Our results suggest that device structures incorporating a high number of edges can improve the performance of TMD FETs. This improvement is attributed to a higher local electric field, resulting from the edges, increasing the effective number of charge carriers, and the absence of any detrimental edge-related scattering.

Graphene-based nonvolatile terahertz switch with asymmetric electrodes

We propose a nonvolatile terahertz (THz) switch which is able to perform the switching with transient stimulus. The device utilizes graphene as its floating-gate layer, which changes the transmissivity of THz signal by trapping the tunneling charges. The conventional top-down electrode configuration is replaced by a left-right electrode configuration, so THz signals could transmit through this device with the transmissivity being controlled by voltage pulses. The two electrodes are made of metals with different work functions. The resultant asymmetrical energy band structure ensures that both electrical programming and erasing are viable. With the aid of localized surface plasmon resonances in graphene ribbon arrays, the modulation depth is 89% provided that the Femi level of graphene is tuned between 0 and 0.2 eV by proper voltage pulses.

Discrete plasmonic Talbot effect in single-mode graphene ribbon arrays

Controllable manipulation of surface plasmon polaritons in discrete waveguide arrays is critical in plasmonic applications. In this paper, exploiting the coupled-mode theory with the tight-binding approximation, the discrete plasmonic Talbot effect in the weak coupled single-mode graphene ribbon arrays is investigated. The Talbot distance can be flexible, modulated by the changing parameters (i.e., the carrier doping or array periods) of the graphene ribbon arrays, and the significant subwavelength Talbot distances (λ/13) are obtained. In the configuration, the coupled-mode theory with the tight-binding approximation is proved intensely efficient to describe the propagation of surface plasmon supermodes. Thus, our analysis can open a new avenue for developing graphene-based imaging devices and pave a way for their potential applications.

Tunable Goos-Hänchen shift from graphene ribbon array.

The Goos-Hänchen (GH) shift of light beam incident on graphene ribbon array is investigated by Green's function method. Due to the resonance effects of leaky surface plasmons on ribbons, the zeroth-order reflection field shows both giant positive and negative GH shifts. By tuning the graphene Fermi level, we can control the shift conveniently. This effect is important to graphene-based metasurface and electro-optical devices.

High-mobility air-stable n-type field-effect transistors based on large-area solution-processed organic single-crystal arrays

Solution-processed n-type organic semiconductor micro/nanocrystals (OSMCs) are fundamental elements for developing low-cost, large-area, and all organic logic/complementary circuits. However, the development of air-stable, highly aligned n-channel OSMC arrays for realizing high-performance devices lags far behind their p-channel counterparts. Herein, we present a simple one-step slope-coating method for the large-scale, solution-processed fabrication of highly aligned, air-stable, n-channel ribbon-shaped single-crystalline N,N′-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI) arrays. The slope and patterned photoresist (PR) stripes on the substrate are found to be crucial for the formation of large-area submicron ribbon arrays. The width and thickness of the BPE-PTCDI submicron ribbons can be finely tuned by controlling the solution concentration as well as the slope angle. The resulting BPE-PTCDI submicron ribbon arrays possess an optimum electron mobility up to 2.67 cm2·V–1·s–1 (with an average mobility of 1.13 cm2·V–1·s–1), which is remarkably higher than that of thin film counterparts and better than the performance reported previously for single-crystalline BPE-PTCDI-based devices. Moreover, the devices exhibit robust air stability and remain stable after exposing in air over 50 days. Our study facilitates the development of air-stable, n-channel organic field-effect transistors (OFETs) and paves the way towards the fabrication of high-performance, organic single crystal-based integrated circuits.

Tunable plasmon-enhanced birefringence in ribbon array of anisotropic two-dimensional materials

We explore the far-field scattering properties of anisotropic 2D materials in ribbon array configuration. Our study reveals the plasmon-enhanced linear birefringence in these ultrathin metasurfaces, where linearly polarized incident light can be scattered into its orthogonal polarization or be converted into circular polarized light. We found wide modulation in both amplitude and phase of the scattered light via tuning the operating frequency or material's anisotropy and develop models to explain the observed scattering behavior.

Frequency comb generation using plasmonic resonances in a time-dependent graphene ribbon array

Graphene has emerged as a promising material for subwavelength photonics, among other reasons because of its large tunability, extreme confinement of plasmonic modes, and limited losses. Here we use the interplay of time modulation and plasmonic resonances in graphene gratings to efficiently generate frequency combs. By using rigorous simulations and a coupled-mode theory model, we show that the combs can be strongly tailored via both the grating and time modulation properties, and can range from the mid-infrared to far-infrared. The grating properties, via the resonance lifetime, strongly influence the conversion efficiency, while the temporal modulation defines the comb's frequency spacing and shape. We examine in detail the dynamics of this interplay between cavity and modulation. With the grating mechanism, a similar frequency comb generation is obtained with a modulation amplitude that is three orders of magnitude smaller than in the planar case.

Excitation of surface plasmon polaritons by electron beam with graphene ribbon arrays

Graphene has emerged as an alternative material to support surface plasmon polaritons (SPPs) with its excellent properties such as the tight electromagnetic field localization, low dissipative loss, and versatile tunability. Thus, graphene surface plasmon polaritons (GSPs) provide an exciting platform to develop a series of novel devices and systems from the optical band to the terahertz (THz) band. In this paper, theoretical and simulated studies about the excitation of SPPs by an injected electron beam with periodic graphene ribbon arrays deposited on a dielectric medium are presented. The analytical dispersion expression of the GSP mode on the graphene ribbon arrays is obtained by using a modal expansion method along with periodic boundary conditions in the structure. With this result, the dispersion relation, propagation loss, and field pattern of the propagating GSPs for both periodic graphene microribbon arrays and the complete graphene sheet are investigated and analyzed in the THz band. It is shown that the electromagnetic field with a better concentration on the interface can be realized with graphene ribbon arrays compared with the graphene sheet for a given frequency. Besides, the excitation of GSPs by an injected electron beam with graphene ribbon arrays is modeled and implemented by the particle-in-cell simulation based on the finite difference time domain algorithm. GSPs can be excited effectively when the dispersion line of the electron beam and SPPs on the graphene ribbon arrays is matched with each other well. Besides, the dependences of output power on electron beam parameters such as the distance of the electron beam above the graphene ribbon surface and beam voltage are studied and analyzed. Finally, the tunability of graphene conductivity via biased voltage with a ground metal is considered and the tunable excitation of GSPs on the structure with biased drive voltage by the injected electron beam is also realized. The present work can find a lot of potential applications such as low loss plasmonic waveguides, graphene metamaterials, and tunable THz electronic sources.

Electronic Skin with Multifunction Sensors Based on Thermosensation.

A multifunctional electronic skin (e-skin) with multimodal sensing capabilities of perceiving mechanical and thermal stimuli, discriminating matter type, and sensing wind is developed using the thermosensation of a platinum ribbon array, whose temperature varies with conductive or convective heat transfer toward the surroundings. Pressure is perceived by a porous elastomer covering on the heated platinum ribbon, which bears mechanical-thermal conversion to allow high integration with other sensors.

Selective formation of C2 products from the electrochemical conversion of CO2 on CuO-derived copper electrodes comprised of nanoporous ribbon arrays

Promotion of CC bond coupling in the electrochemical conversion of CO2 to fuels is of great scientific and practical interest. Selective formation of C2 over the C1 products, however, is a formidable challenge on all electrocatalysts known in literature. Here, we report the selectivity of CuO-derived porous copper nanoribbon arrays as the electrode to convert CO2 to C2 products. The CC bond coupling occurred at electrode potentials <−0.701V vs. RHE. The total Faradaic efficiency towards the formation of these C2 compounds (C2H4, C2H6 and C2H5OH) reached to ∼40% at −0.816V vs. RHE under ambient pressure and temperature. More importantly, at the same condition, the total Faradaic efficiency for C1 products (CO and HCOO−) was <3%, which were major products when a Cu2O-derived Cu electrode was used. Methane was not observed, a key product on a Cu foil electrode. This increased selectivity towards the formation of C2 chemicals, meanwhile suppressed C1 chemicals, are attributed to the presence of surface defects and a large number of grain boundaries on the CuO-derived porous Cu nanoribbon arrays electrode. Moreover, the activation of CO2 was found to likely occur at the copper surface; while the presence of copper oxide layer reported in literature may result from the interaction between copper and water during the post analysis process.

Tunable graphene-based mid-infrared plasmonic wide-angle narrowband perfect absorber.

In this paper, the periodic double-layer graphene ribbon arrays placed near a metallic ground plate coated by a dielectric layer are proposed and analyzed by the coupled-mode theory (CMT) to predict the perfect absorption response in the mid-infrared region. Numerical simulations of the finite-difference time-domain (FDTD) method confirm this effect and give the underlying physical origin. The anti-symmetric dipole-dipole coupling mode is supported by the double-layer graphene ribbons and acts as the electrical resonance to suppress the reflection, because of the impedance matching. The transmission from this system is restricted by the ultra-thick metallic ground plate. All incident electromagnetic energy is efficiently confined in the interlayer between graphene ribbons and the metallic plate, and the dramatic narrowband perfect absorption peak with the FWHM (full width at half maximums) of 300 nm hence is achieved. The spectral position of the absorption peak can be dynamically tuned by a small change in the chemical potential of graphene, in addition to varying geometrical parameters of the absorber. Meanwhile, this device exhibits good absorption stability over a wide angle range of incidence around ± 60° at least. Such absorber will benefit the fabrication of mid-infrared nano-photonic devices for optical filtering and storage.

Broadband light absorption in graphene ribbons by canceling strong coupling at subwavelength scale.

We theoretically investigate the broadband light absorption in the THz range by canceling the strong coupling in an array of graphene ribbons at subwavelength scale. A series of resonators with different absorption frequencies can achieve a broadband absorber, however, the suppression of absorption always accompanies since the mutual coupling between resonators cause the mode splitting. By adjusting the near- and far-field coupling between the plasmon resonances of the graphene ribbon array to the critical point, the absorption linewidth is broadened for almost one magnitude larger than that of individual graphene ribbon, to be ~1 THz. Our study provides not only insight understanding but also new approaches towards the broadband graphene absorber.

Plasmonic Tuning at Mid-Infrared Wavelengths by Composite Arrays of Graphene Ribbons

This work reports on a numerical simulation study regarding the systematic tuning of plasmonic resonance wavelength in the mid-infrared regime, by using composite arrays of graphene ribbons. On top of a glass substrate, a variety of composite arrays that consist of graphene ribbons are carefully designed. The layer numbers of the multi-layer graphene ribbons and the Fermi energy levels of the graphene are respectively varied, and the corresponding light transmittance is calculated in the mid-infrared wavelength range from 4 to 20 μm, via the finite difference time domain (FDTD) method. The results reveal that the plasmonic resonance wavelength associated with the graphene arrays can be tuned in the mid-infrared wavelength range by adjusting the parameters of the arrays. Based on the study, we suggest that the structure of the graphene composite arrays proposed in this work be implemented in the design for plasmonic tuning devices at mid-infrared wavelengths.

Graphene-ribbon-coupled tunable enhanced transmission through metallic grating

We report the tunable enhanced transmission of light through a hybrid metal–graphene structure, in which a graphene ribbon array is situated over a metallic grating. The graphene ribbon is employed to make the graphene–insulator–metal waveguide of finite length as a Fabry–Perot (F–P) cavity. When the slit of metallic grating is opened at the position with a maximal magnetic field in F–P resonant cavity, the transmission of light through metallic grating is greatly enhanced since the strongly localized magnetic field is effectively coupled to the slit. The transmission spectrum and the enhancement factor can be adjusted by changing geometrical parameters including the width and the length of slit, the width of graphene ribbon and the period of metallic grating. The transmission peaks exhibit a broad tuning range with a small change in the Fermi energy level of graphene. Moreover, the enhancement factor of transmission peak can be manipulated by the Fermi energy level and the carrier mobility of graphene, and an enhancement factor of 154 is obtained. The findings expand our understanding of hybrid metal–graphene plasmons and have potential applications in building active plasmonic devices.

Nonlinear terahertz frequency conversion via graphene microribbon array

By exploiting the interesting trait of graphene to have electrically tunable first- and third-order conductivities besides its capability to support plasmonic resonances at terahertz frequencies, here, through the nonlinear finite-difference time-domain numerical technique we developed, we demonstrate a noticeable improvement in the conversion efficiency of third-harmonic generation (THG) from a graphene microribbon array by more than five orders of magnitude compared to an infinite graphene sheet, under normal illumination of terahertz waves. As the Fermi level and period length of the ribbon array increase, the transmission obviously manifests a blue shift but denotes a red shift with an increase in ribbon width. The quality factor of resonance (and so the THG efficiency) also shows improvement with an increase in graphene Fermi level, carrier mobility and period length and is degraded by an increase in ribbon width. Generating new frequencies, terahertz signal processing, spectroscopy and so on are among the plethora of valuable potential applications envisioned to be developed based on the findings reported here.

Investigation of ablation of thin foil aluminum ribbon array at 1.5 MA

We present experimental studies of initiation and ablation of a thin foil aluminum ribbon array at the 1.5 MA current level. In contrast to the previous work, we employ ribbon arrays with different ribbon gap parameters to investigate how this affects plasma initiation and foil ablation. Gated narrowband ultraviolet imaging indicated that the current was disorderly distributed at early period of discharge. But later on, it became axially stable and azimuthally symmetrical even for load with a gap as small as 0.1 mm. Using magnetic field probes installed inside and outside the array, we also observed that precursor current at positions with a distance of less than 2.7 mm to the central axis for 4-mm-radius arrays decreased when ribbon gap became small. Results of 0.2 mm gap ribbon array showed an evidence that ribbons can be merged. These observations imply that thin foil ribbon arrays may have potential applications in z-pinch experiments on large scale pulsed power facilities.

3D Stretchable Arch Ribbon Array Fabricated via Grayscale Lithography.

Microstructures with flexible and stretchable properties display tremendous potential applications including integrated systems, wearable devices and bio-sensor electronics. Hence, it is essential to develop an effective method for fabricating curvilinear and flexural microstructures. Despite significant advances in 2D stretchable inorganic structures, large scale fabrication of unique 3D microstructures at a low cost remains challenging. Here, we demonstrate that the 3D microstructures can be achieved by grayscale lithography to produce a curved photoresist (PR) template, where the PR acts as sacrificial layer to form wavelike arched structures. Using plasma-enhanced chemical vapor deposition (PECVD) process at low temperature, the curved PR topography can be transferred to the silicon dioxide layer. Subsequently, plasma etching can be used to fabricate the arched stripe arrays. The wavelike silicon dioxide arch microstructure exhibits Young modulus and fracture strength of 52 GPa and 300 MPa, respectively. The model of stress distribution inside the microstructure was also established, which compares well with the experimental results. This approach of fabricating a wavelike arch structure may become a promising route to produce a variety of stretchable sensors, actuators and circuits, thus providing unique opportunities for emerging classes of robust 3D integrated systems.

Study of backward waves in multilayered structures composed of graphene micro-ribbons

In this paper, the multilayered structure composed of periodic arrays of graphene ribbons separated by the dielectric slabs is investigated for both the transverse electric (TE) and transverse magnetic (TM) polarizations. The transmission spectra show a stopband region between the low-THz frequency passband region and the higher-frequency one for TM polarization due to dual capacitive-inductive nature of the surface impedance corresponding to arrays of graphene ribbons. For TE polarization, the surface impedance is pure inductive; therefore, a passband region can be seen after the low-THz frequency stopband region. The in-plane permittivities are calculated based on the scattering parameters retrieval method. The negative permittivities are obtained in stopband regions corresponding to Bloch evanescent modes. This structure allows the propagation of backward waves in negative permittivity regime for TM polarization.

Fast Patterned Graphene Ribbons Via Soft–lithography

Patterned graphene with sub–microscale resolution is urgently required in its band–engineering and electronics applications. Several methods, such as conventional lithography, unzipping carbon nanotubes, chemical synthesis, assembling, shadow mask etching, have been proposed and demonstrated. Despite these advantages, an increasing demand for rapid, massively, high–throughput, and low–cost fabrication strategies for graphene ribbons continues to motivate research. Herein, we present an innovative approach for fabrication of graphene ribbon array by combining the soft–lithography with oxygen plasma etching. Large–scale, high–quality chemical vapor deposition graphene film was patterned into aligned ribbons with width in the range of several hundred nanometers to tens of micrometers (e.g., 100 nm∼30μm) and periods, according to the size of PDMS stamp and etching time. The morphology and quality of patterned graphene ribbons were characterized using optical microscopy, scanning electron microscopy and Raman spectroscopy. Electrical measurements on patterned graphene ribbons show a symmetric transport behavior, which could be an ideal platform for sensing. Further, temperature dependent conductance measurements reveal a thermal excitation effect. Our strategies open a new avenue to patterned graphene ribbons for multi–functional nanodevice engineering.