Uncooled Broadband Photodetection via Light Trapping in Conformal PtTe2-Silicon Nanopillar Heterostructures

Dirac semimetals have demonstrated significant attraction in the field of optoelectronics due to their unique bandgap structure and high carrier mobility. Combining them with classical semiconductor materials to form heterojunctions enables broadband optoelectronic conversion at room temperature. However, the low light absorption of layered Dirac semimetals substantially limits the device's responsivity in the infrared band. Herein, a three-dimensional (3D) heterostructure, composed of silicon nanopillars (SiNPs) and a conformal PtTe2 film, is proposed and demonstrated to enhance the photoresponsivity for uncooled broadband detection. The light trapping effect in the 3D heterostructure efficiently promotes the interaction between light and PtTe2, while also enhancing the light absorption efficiency of silicon, which enables the enhancement of the device responsivity across a broadband spectrum. Experimentally, the PtTe2-SiNPs heterojunction device demonstrates excellent photoelectric conversion behavior across the visible, near-infrared, and long-wave infrared (LWIR) bands, with its responsivity demonstrating an order-of-magnitude improvement compared to the counterparts with planar silicon heterojunctions. Under 11 μm laser irradiation, the noise equivalent power (NEP) can reach 1.76 nW·Hz-1/2 (@1 kHz). These findings offer a strategic approach to the design and fabrication of high-performance broadband photodetectors based on Dirac semimetals.

Tunable light trapping in the graphene metasurface

Graphene metasurfaces based on surface plasmon resonance can greatly enhance the interaction between light and matter at the nanoscale. At present, the resonance of graphene metasurfaces is widely used to enhance the absorption of atomic layer graphene, but little work has focused on the light field trapping capabilities it brings. In this paper, we numerically study the light trapping and manipulation of an asymmetric graphene metasurface. The designed device supports two resonant modes, and the multipole decomposition confirms that the electric dipole response dominates them. The calculated average electric field enhancement factor (EF) can reach 1206 and 1779, respectively. The near-field distribution indicates that the electric field is mainly localized in the graphene nanodisks. When the Fermi energy changes, the intensity and peak position of EF can be effectively regulated. In addition, when the polarization of the incident light is adjusted, the light field capture of the two modes is independently regulated. These results reveal that the graphene metasurface has significant light field capture and regulation ability, which provides a new idea for the realization of active regulation of high-performance low-dimensional optical devices.

Polarization-sensitive switchable display through critical coupling between graphene and a quasi-BIC

Enhanced light-matter interaction of a local field is of prime importance in optics as it can improve the performance of nanophotonic devices. Such enhancement can be achieved by utilizing the optical bound states in the continuum (BICs). In this study, a dielectric metasurface is proposed that could enhance the light-matter interactions in graphene. A symmetry-protected BIC was observed in such a metasurface, which could transform into a quasi-BIC with a high quality (Q-) factor when the in-plane symmetry is broken. As the graphene monolayer was introduced into the system, its absorption was enhanced by the quasi-BIC resonance. By optimizing the graphene Fermi energy and the asymmetry parameter of the metasurface to satisfy the critical-coupling condition, a tunable absorber could be achieved. The absorbing intensity could be efficiently modulated by varying the polarization direction of the incident light, the maximum difference of which was up to 95.4%. Also, further investigation showed that such a feature indicates potential application in digital switches and image displays, which could be switched by incident polarization only, and therefore without dependence on an additional structural change.

Accurate Time-Domain Modeling of Arbitrarily Shaped Graphene Layers Utilizing Unstructured Triangular Grids

The accurate modeling of curved graphene layers for time-domain electromagnetic simulations is discussed in the present work. Initially, the advanced properties of graphene are presented, focusing on the propagation of strongly confined surface plasmon polariton waves at the far-infrared regime. Then, the implementation of an unstructured triangular grid was examined, based on the Delaunay triangulation method. The electric-field components were placed at the edges of the triangles, while two different techniques were proposed for the sampling of the magnetic ones. Specifically, the first one suggests that the magnetic component is placed at the triangle’s circumcenter providing more accurate results, although instability may occur for nonacute triangles. On the other hand, the magnetic field was sampled at the triangle’s centroid, considering the second technique, ensuring the algorithm’s stability, but further approximations were required, leading to a slight accuracy reduction. Moreover, the updating equations in the time-domain were extracted via an appropriate approximation of Maxwell equations in their integral form. Finally, graphene was introduced in the computational domain as an equivalent surface current density, whose location matches the corresponding electric components. The validity of our methodology was successfully performed via the comparison of graphene surface wave propagation properties to their theoretical values, whereas the global error determination indicates the minimal triangle dimensions. Additionally, an instructive setup comprising a circular graphene scatterer was analyzed thoroughly, to reveal our technique’s advantages compared to the conventional staircase discretization.

Multiple interference theoretical model for graphene metamaterial-based tunable broadband terahertz linear polarization converter design and optimization

Terahertz (THz) polarization converters often working as modulators and switches have many applications in THz sensing, imaging and communication, but many of them still suffer from low polarization conversion efficiency, fixed and narrow polarization conversion band, and low output polarization purity, which are mainly due to the lack of theoretical model for THz polarization converter design and optimization. In order to solve the problem, we adopt multiple interference theory to successfully design and optimize a graphene metamaterial-based tunable broadband THz linear polarization converter: it achieves polarization conversion ratio (PCR) over 0.97, polarization azimuth angle of almost ±90° and rather low ellipticity within a broad polarization conversion band of 1.25 THz; and additionally, its polarization conversion band can be actively tuned by adjusting the graphene chemical potential and almost insensitive to the incident THz radiation angle below 50°. Considering the high performance of the optimal graphene metamaterial-based tunable broadband THz linear polarization converter, this work provides an optimal design offering a way in high-quality manipulation of THz radiation polarization; but more importantly, delivers a theoretical model for tunable THz polarization converter design and optimization.

Recent progress in two-dimensional materials for terahertz protection

With the wide applications of terahertz (THz) devices in future communication technology, THz protection materials are essential to overcome potential threats. Recently, THz metamaterials (MMs) based on two-dimensional (2D) materials (e.g., graphene, MXenes) have been extensively investigated due to their unique THz response properties. In this review, THz protection theories are briefly presented first, including reflection loss and shielding mechanisms. Then, the research progress of graphene and other 2D material-based THz MMs and intrinsic materials are reviewed. MMs absorbers in the forms of single layer, multiple layers, hybrid and tunable metasurfaces show excellent THz absorbing performance. These studies provide a sufficient theoretical and practical basis for THz protection, and superior properties promised the wide application prospects of 2D MMs. Three-dimensional intrinsic THz absorbing materials based on porous and ordered 2D materials also show exceptional THz protection performance and effectively integrate the advantages of intrinsic properties and the structural characteristics of 2D materials. These special structures can optimize the surface impedance matching and enable multiple THz scatterings and electric transmission loss, which can realize high-efficiency absorption loss and active controllable protection performance in ultra-wide THz wavebands. Finally, the advantages and existing problems of current THz protection materials are summarized, and their possible future development and applications are prospected.

Tunable multilayer-graphene-based broadband metamaterial selective absorber

We propose a tunable multilayer-graphene-based broadband metamaterial selective absorber using the finite-difference time domain. The simulation results reveal that the absorption spectra of the proposed metamaterial with the nano-cylinder and 30-layer graphene show high absorption (88.3%) in the range of 250-2300 nm, which covers the entire solar spectrum. Moreover, the graphene-based metamaterial has a low thermal emittance of 3.3% in the mid-infrared range (4-13 µm), which can greatly reduce the heat loss. The proposed metamaterial has a tunable cutoff wavelength, which can be tuned by controlling the Fermi level of graphene. In addition, our structure is an angle-insensitive absorber, and the device has the potential to be widely used in solar cell and thermal detectors.

Broadband polarization-insensitive and wide-angle solar energy absorber based on tungsten ring-disc array

Nowadays, solar energy is considered one of the most clean energy sources. In addition, the data from the literature tell us that its main radiation bandwidth is approximately 295-2500 nm. In this work, we proposed a novel kind of broadband solar energy absorber based on tungsten (W) to achieve broadband absorption of solar energy. A four-layer ring-disk structure (SiO2-SiO2-W) is employed in our design. A finite-difference time-domain (FDTD) simulation was used to ascertain the absorption performance of the absorber. The results demonstrate that a broadband solar energy absorption was realized, the bandwidth is of 1530 nm with an absorption efficiency of more than 90%, and an absorption efficiency of 97% was achieved in this region. The absorption spectra can be tuned through changing the structural and geometric parameters. Moreover, the absorber has excellent polarization independence and can be used under incident angles from 0° to 60°. The proposed solar energy absorber is simple to fabricate, and can be used for photothermal conversion, solar energy harvesting and utilization.

Chiral graphene plasmonic Archimedes' spiral nanostructure with tunable circular dichroism and enhanced sensing performance

Circular dichroism spectroscopy is frequently used to characterize the chiral biomolecules by measuring the absorption spectra contrast between the left-handed circularly polarized light and the right-handed circularly polarized light. Compared with biomolecules, chiral metal plasmonic nanostructures also produce a strong circular dichroism response in the range of near-infrared. However, due to the large damping rate, the non-adjustable resonant frequency of the conventional metals, the applications of chiral metal plasmonic nanostructures in the fields of photoelectric detection and chemical and biochemical sensing are restricted. Here, we present a chiral graphene plasmonic Archimedes' spiral nanostructure that displays a significant circular dichroism response under the excitation of two polarizations of circularly polarized light. By manipulating the material and geometric parameters of the Archimedes' spiral, the stronger circular dichroism responses and modulation of the resonant wavelength are achieved. The optimized plasmonic nanostructure has outstanding refractive index sensing performance, where the sensitivity and figure of merit reach 7000nm/RIU and 68.75, respectively. Our proposed chiral graphene plasmonic Archimedes' spiral nanostructure might find potential applications in the fields of optical detection and high performance of index sensing.

Tunable plasmon-induced transparency with a dielectric grating-coupled graphene structure for slowing terahertz waves

We present a tunable plasmon-induced transparency (PIT) structure that is composed of dielectric grating and a graphene system to manipulate terahertz (THz) waves. The graphene system consists of a graphene sheet and a graphene ribbon layer, with a spacer between them. By exploiting the diffraction coupling of THz wave with dielectric grating, graphene plasmonic resonance is efficiently excited on both graphene sheet and graphene ribbons. This leads to the surface plasmon mode of the graphene sheet and the localized plasmon mode of the graphene ribbons. The coupling between the two-plasmon modes via near-field destructive interference generates a strong PIT effect with slowing the group velocity of THz waves. A group delay over 0.2 ps and group index beyond 170 can be achievable. The group slowing effect is dynamically tunable with varying the Fermi level of graphene. The work suggests a promising scheme for on-chip graphene slow-wave devices at the THz regime.

Ultrathin multi-band coherent perfect absorber in graphene with high-contrast gratings

High-contrast gratings (HCGs) can be designed as a resonator with high-quality factor and surface-normal emission, which are excellent characters for designing optical devices. In this work, we combine HCGs with plasmonic graphene structure to achieve an ultrathin five-band coherent perfect absorber (CPA). The presented CPA can achieve multi- and narrow-band absorption with high intensity under a relatively large incident angle. The good agreement between theoretical analysis and numerical simulated results demonstrates that our proposed HCGs-based structure is feasible to realize CPA. Besides, by dynamically adjusting the Fermi energy of graphene, we realize the active tunability of resonance frequency and absorption intensity simultaneously. Benefitting from the combination of HCGs and the one-atom thickness of graphene, the proposed device possesses an extremely thin feature. Our work proposes a novel method to manipulate coherent perfect absorption and is helpful to design tunable multi-band and ultrathin absorbers.

A near-ideal solar selective absorber with strong broadband optical absorption from UV to NIR

A near-ideal solar absorber, which is composed of a metal nanowire array and a planar multilayer system, is proposed and investigated. Both numerical and analytical results show that the proposed nanostructure can achieve over 90% optical absorption throughout the wavelength range of 300-1909 nm due to the coupled effect of multiple resonance modes, and can maintain a good absorption stability over a wide incident angle regardless of the polarization states. Meanwhile, for practical applications, the total photothermal conversion efficiency can reach 95.57% at operating temperature of 373.15 K, which is particularly useful in solar energy harvesting. The absorption performance is also strongly dependent on the diameter and height of nanowire as well as the thicknesses of dielectric layers, enabling the further improvement of both the operating bandwidth and absorption efficiency. Moreover, by adjusting the period of the multilayer or nanowire materials, the selective absorption properties of this nanostructure can be flexibly controlled to satisfy more spectral requirements. These features make the presented designs hold promise for a series of solar-dependent optical applications, such as photothermal energy generation and thermal emitters.

Inversion Method Characterization of Graphene-Based Coordination Absorbers Incorporating Periodically Patterned Metal Ring Metasurfaces

In this paper, we propose a tunable coordinated multi-band absorber that combines graphene with metal–dielectric–metal structures for the realization of multiple toward perfect absorption. The parametric inversion method is used to extract the equivalent impedance and explain the phenomena of multiple-peak absorption. With the change of the Fermi level, equivalent impedances were extracted, and the peculiarities of the individual multiple absorption peaks to change were determined. By changing the structure parameters of gold rings, we obtain either multiple narrow-band absorption peaks or a broadband absorption peak, with the bandwidth of 0.8 μm where the absorptance is near 100%. Therefore, our results provide new insights into the development of tunable multi-band absorbers and broadband absorbers that can be applied to terahertz imaging in high-performance coordinate sensors and other promising optoelectronic devices.

Switchable Electromagnetically Induced Transparency with Toroidal Mode in a Graphene-Loaded All-Dielectric Metasurface

Active photonics based on graphene has attracted wide attention for developing tunable and compact optical devices with excellent performances. In this paper, the dynamic manipulation of electromagnetically induced transparency (EIT) with high quality factors (Q-factors) is realized in the optical telecommunication range via the graphene-loaded all-dielectric metasurface. The all-dielectric metasurface is composed of split Si nanocuboids, and high Q-factor EIT resonance stems from the destructive interference between the toroidal dipole resonance and the magnetic dipole resonance. As graphene is integrated on the all-dielectric metasurface, the modulation of the EIT window is realized by tuning the Fermi level of graphene, engendering an appreciable modulation depth of 88%. Moreover, the group velocity can be tuned from c/1120 to c/3390. Our proposed metasurface has the potential for optical filters, modulators, and switches.

Complete optical absorption in hybrid halide perovskites based on critical coupling in the communication band

In order to remarkably enhance the absorption capability of (CH₃NH₃)PbI₃, a tunable narrow-band (CH₃NH₃)PbI₃-based perfect absorber based on the critical coupling with guided resonance is proposed. By using the finite-difference time-domain (FDTD) simulations, a complete absorption peak is achieved at the wavelength of 1310 nm. Moreover, we have compared the simulation results with theoretical calculations, which agree well with each other. By changing related structural parameters, the wavelength of absorption peak can be tuned effectively. Furthermore, the proposed absorber can tolerate a relatively wide range of incident angles and demonstrate polarization-independence. In addition to (CH₃NH₃)PbI₃, the complete optical absorption in the other halide perovskites can be realized by the same mechanism.

A Tunable Terahertz Metamaterial Absorber Composed of Hourglass-Shaped Graphene Arrays

In this paper, we demonstrate a tunable periodic hourglass-shaped graphene arrays absorber in the infrared (IR) and terahertz (THz) frequency bands. The effects of graphene geometric parameters, chemical potentials, periods, and incident angles on the pure absorption characteristics are studied by using the Finite Difference Time Domain (FDTD) method. In addition, this paper also analyzes the pure absorption characteristics of bilayer graphene arrays. The simulation results show that the maximum absorption reaches 38.2% for the monolayer graphene structure. Furthermore, comparing the bilayer graphene structure with the monolayer structure under the same conditions shows that the bilayer structure has a tunable dual-band selective absorption effect and has a higher maximum absorption of 41.7%. Moreover, it was found that there are dual-band tunable absorption peaks at 21.6 μm and 36.3 μm with the maximum absorption of 41.7% and 11%. The proposed structure is a convenient method which could be used in the design of graphene-based optoelectronic devices, biosensors, and environmental monitors.

A Triple-Band Hybridization Coherent Perfect Absorber Based on Graphene Metamaterial

In this paper, a triple-band hybridization coherent perfect absorber based on graphene metamaterial is proposed, which consists of graphene concentric nanorings with different sizes and a metallic mirror separated by SiO2 layer. Based on the finite-difference time-domain (FDTD) solution, triple-band coherent perfect absorption is achieved at frequencies from 0.6 THz to 1.8 THz, which results from the surface plasmon resonance hybridization. The wavelength of the absorption peak can be rapidly changed by varying the Fermi level of graphene. Most importantly, the wavelength of the absorption peak can be independently tuned by varying the Fermi level of the single graphene nanoring. Moreover, the triple hybridization perfect absorber is angle-insensitive because of the perfect symmetry structure of the graphene nanorings. Therefore, our results may widely inspire optoelectronic and micro-nano applications, such as cloaking, tunable sensor, etc.

Coatings in Photovoltaic Solar Energy Worldwide Research

This paper describes the characteristics of contributions that were made by researchers worldwide in the field of Solar Coating in the period 1957–2019. Scopus is used as a database and the results are processed while using bibliometric and analytical techniques. All of the documents registered in Scopus, a total of 6440 documents, have been analyzed and distributed according to thematic subcategories. Publications are analyzed from the type of publication, field of use, language, subcategory, type of newspaper, and the frequency of the keyword perspectives. English (96.8%) is the language that is most used for publications, followed by Chinese (2.6%), and the rest of the languages have a less than < 1% representation. Publications are studied by authors, affiliations, countries of origin of the authors, and H-index, which it stands out that the authors of China contribute with 3345 researchers, closely followed by the United States with 2634 and Germany with 1156. The Asian continent contributes the most, with 65% of the top 20 affiliations, and Taiwan having the most authors publishing in this subject, closely followed by Switzerland. It can be stated that research in this area is still evolving with a great international scientific contribution in improving the efficiency of solar cells.

Tunable and Anisotropic Dual-Band Metamaterial Absorber Using Elliptical Graphene-Black Phosphorus Pairs

We numerically propose a dual-band absorber in the infrared region based on periodic elliptical graphene-black phosphorus (BP) pairs. The proposed absorber exhibits near-unity anisotropic absorption for both resonances due to the combination of graphene and BP. Each of the resonances is independently tunable via adjusting the geometric parameters. Besides, doping levels of graphene and BP can also tune resonant properties effectively. By analyzing the electric field distributions, surface plasmon resonances are observed in the graphene-BP ellipses, contributing to the strong and anisotropic plasmonic response. Moreover, the robustness for incident angles and polarization sensitivity are also illustrated.

A Tunable Dual-Band and Polarization-Insensitive Coherent Perfect Absorber Based on Double-Layers Graphene Hybrid Waveguide

A suspended monolayer graphene has only about 2.3% absorption rate in visible and infrared band, which limits its optoelectronic applications. To significantly increase graphene's absorption efficiency, a tunable dual-band and polarization-insensitive coherent perfect absorber (CPA) is proposed in the mid-infrared regime, which contains the silicon array coupled in double-layers graphene waveguide. Based on the FDTD methods, dual-band perfect absorption peaks are achieved in 9611 nm and 9924 nm, respectively. Moreover, due to its center symmetric feature, the proposed absorber also demonstrates polarization-insensitive. Meanwhile, the coherent absorption peaks can be all-optically modulated by altering the relative phase between two reverse incident lights. Furthermore, by manipulating the Fermi energies of two graphene layers, two coherent absorption peaks can move over a wide spectrum range, and our designed CPA can also be changed from dual-band CPA to narrowband CPA. Thus, our results can find some potential applications in the field of developing nanophotonic devices with excellent performance working at the mid-infrared regime.

Coordinated multi-band angle insensitive selection absorber based on graphene metamaterials

In this paper, we propose a tunable, multi-band, selective absorber composed of multiple layers. Each layer consisted of SiO₂/graphene/SiC, and a layer of silver was used as the ground plane of the entire structure. Simulation results show that we can passively and actively coordinate the resonant frequency of the perfect absorption peak by changing the geometric parameters of the array and the Fermi level of the graphene. The absorber is not sensitive to the angle of incidence and the direction of polarization. We propose a theoretical basis for the formation of multiple absorption peaks. The theoretical calculations are in good agreement with the simulation results. In addition, we simulated the three- and four-layer structures. The results show that in the terahertz (THz) band, composite structures of three and four layers can obtain three and four perfect absorption peaks, respectively. Our results provide new insights into the THz band of harmonizable multi-band absorbers that can be applied to THz imaging to coordinate sensors and other optoelectronic devices.

Graphene-Based Biosensors for Detection of Composite Vibrational Fingerprints in the Mid-Infrared Region

In this study, a label-free multi-resonant graphene-based biosensor with periodic graphene nanoribbons is proposed for detection of composite vibrational fingerprints in the mid-infrared range. The multiple vibrational signals of biomolecules are simultaneously enhanced and detected by different resonances in the transmission spectrum. Each of the transmission dips can be independently tuned by altering the gating voltage applied on the corresponding graphene nanoribbon. Geometric parameters are investigated and optimized to obtain excellent sensing performance. Limit of detection is also evaluated in an approximation way. Besides, the biosensor can operate in a wide range of incident angles. Electric field intensity distributions are depicted to reveal the physical insight. Moreover, another biosensor based on periodic graphene nanodisks is further proposed, whose performance is insensitive to the polarization of incidence. Our research may have a potential for designing graphene-based biosensor used in many promising bioanalytical and pharmaceutical applications.

Bidirectional and dynamically tunable THz absorber with Dirac semimetal

Traditional absorbers are usually sandwich structures in which a metallic ground plane is employed to prevent the transmission. Such absorbers suffer from a major drawback that incident light can only irradiate from the front of the absorbers. In this paper, a novel absorber with bulk Dirac semimetal (BDS)-AlCuFe quasicrystals is proposed to realize bidirectional and dynamically tunable terahertz (THz) perfect absorption. The proposed structure consists of two layers of AlCuFe plates with rectangular apertures and a dielectric spacer. By adjusting transverse distance between the top and bottom rectangular apertures, perfect absorption could be realized under TM polarization. Simulation results show that perfect absorption can be obtained whether light irradiates from the front or back of the system, indicating a performance of bidirectional absorption. In addition, benefiting from the variable Fermi level of AlCuFe, the resonance frequency can be dynamically tuned in the THz range. Our work will stimulate more investigations on BDS-based bidirectional absorbers and optical modulators.

Numerical Study of Angle-Insensitive and Tunable Dual-Band THz Absorber Using Periodic Cross-Shaped Graphene Arrays

A dual-band terahertz (THz) absorber using the periodic cross-shaped graphene arrays is presented. It is shown that the dual-band light absorption enhancement of graphene results from the edge graphene plasmon (EGP) resonance, and the locations of the two absorption peaks can be precisely estimated by using the Fabry-Pérot (F-P) cavity model. Slight residual reflection remains at the two absorption peaks because the input impedance of the cross-arm cannot be perfectly matched with the free space impedance. In addition, the locations of the two absorption bands can be simultaneously tuned by changing the Fermi level of graphene, and they can be independently tuned by changing the width or the length of the cross-arm of graphene. Excellent angle-insensitivity dual-band absorption enhancement of graphene can be maintained for both the transverse electric (TE) and transverse magnetic (TM) polarizations.

Tunable optical angular selectivity in hyperbolic metamaterial via photonic topological transitions

An ultra-narrow angular optical transparency window based on photonic topological transition (PTT) is theoretically and numerically investigated in a low-loss hyperbolic metamaterial (HMM) platform, which consists of aligned metallic nanowires embedded indielectric host matrices. Our results indicate that, near the transition point of PTT, the designed system exhibits high-efficiency optical angular selectivity close to normal incidence by tailoring the topology of metamaterial's equi-frequency surface (EFS). Moreover, the operating wavelength (λ₀) is flexibly tunable by selecting appropriate material and geometrical parameters, which provides significant guidance for the later experimental design. Our method is further applied to super-resolution imaging, with a resolution of at least λ₀/4 and over a significant distances (>12λ₀). The HMM-supported angularly selective system could find promising applications for high-efficiency light manipulation and lensless on-chip imaging.

Hybrid Metal Graphene-Based Tunable Plasmon-Induced Transparency in Terahertz Metasurface

In this paper, we look at the work of a classical plasmon-induced transparency (PIT) based on metasurface, including a periodic lattice with a cut wire (CW) and a pair of symmetry split ring resonators (SSR). Destructive interference of the 'bright-dark' mode originated from the CW and a pair of SSRs and resulted in a pronounced transparency peak at 1.148 THz, with 85% spectral contrast ratio. In the simulation, the effects of the relative distance between the CW and the SSR pair resonator, as well as the vertical distance of the split gap, on the coupling strength of the PIT effect, have been investigated. Furthermore, we introduce a continuous graphene strip monolayer into the metamaterial and by manipulating the Fermi level of the graphene we see a complete modulation of the amplitude and line shape of the PIT transparency peak. The near-field couplings in the relative mode resonators are quantitatively understood by coupled harmonic oscillator model, which indicates that the modulation of the PIT effect result from the variation of the damping rate in the dark mode. The transmitted electric field distributions with polarization vector clearly confirmed this conclusion. Finally, a group delay t g of 5.4 ps within the transparency window is achieved. We believe that this design has practical applications in terahertz (THz) functional devices and slow light devices.

Polarization-insensitive and wide-angle broadband absorption enhancement of molybdenum disulfide in visible regime

In order to remarkably enhance the absorption capability of monolayer molybdenum disulfide (MoS2), a broadband MoS2-based perfect absorber, which is inspired by metamaterial, is proposed. By using the finite-difference time-domain (FDTD) simulations, the absorption of proposed MoS2-based absorber above 94% is achieved from 594 to 809 nm. Meanwhile, the average absorptions of monolayer MoS2 enhanced up to 27% and 67% are realized from 598 to 671 nm and 710 to 801 nm, respectively. By manipulating related structural parameters, the absorption spectrum can be further broadened and shifted in a wide wavelength range. Furthermore, the proposed absorber can tolerate a relatively wide range of incident angles and demonstrate polarization-independence.

A Tunable Plasmonic Refractive Index Sensor with Nanoring-Strip Graphene Arrays

In the present study, we design a tunable plasmonic refractive index sensor with nanoring-strip graphene arrays. The calculations prove that the nanoring-strip have two transmission dips. By changing the strip length L of the present structure, we find that the nanoring-strip graphene arrays have a wide range of resonances (resonance wavelength increases from 17.73 μm to 28.15 μm). When changing the sensing medium refractive index n_med, the sensitivity of mode A and B can reach 2.97 μm/RIU and 5.20 μm/RIU. By changing the doping level n_g, we notice that the transmission characteristics can be tuned flexibly. Finally, the proposed sensor also shows good angle tolerance for both transverse magnetic (TM) and transverse electric (TE) polarizations. The proposed nanoring-strip graphene arrays along with the numerical results could open a new avenue to realize various tunable plasmon devices and have a great application prospect in biosensing, detection, and imaging.

Surface enhanced perfect absorption in metamaterials with periodic dielectric nanostrips on silver film

Integrated dielectric metamaterials with plasmonic structures can cause drastic optical resonances and strengthen the capacity of light absorption. Here, we describe the optical properties of silicon nanoarrays on a thin silver film for extreme light confinement at subwavelength nanoscales. We attain the nearly total absorption in silicon nanostrips, which support magnetic quadruple Mie-type resonances in the visible regions. The Mie resonant field of the dielectric nanostrip engages the screening response of the silver film, resulting in plasmon resonance configuration and thus achieving perfect light absorption in the dielectric nanostrip. Moreover, we can attain similar results in other nanostructures, such as silicon cylinder and rhombus column arrays. Because it can sustain hybridized plasmon modes and magnetic modes, the combined system will benefit the application of solar energy accumulation.

Plasmon-phonon-polariton modes and field enhancement in graphene-coated hexagon boron nitride nanowire pairs

Both plasmon-phonon-polariton (SPP-PHP) modes and phonon-polariton (PHP) modes supported in graphene-coated hexagon boron nitride (h-BN) single nanowire are presented. The field distributions of the lowest 5 order modes of SPP-PHP modes supported in graphene-coated hexagon boron nitride nanowire pairs (SPP-PHP-GHNP) and the lowest 5 order modes of PHP modes supported in graphene-coated hexagon boron nitride nanowire pairs (GHNP) are also demonstrated and analyzed, respectively. The results of numerical calculation show that SPP-PHP-GHNP mode 0 owns the strongest confinement and lowest loss among the lowest 5 order modes of SPP-PHP-GHNP. Furthermore, the field enhancement of SPP-PHP-GHNP mode 0 can reach over 10⁵ by controlling the geometry parameters of GHNP. Meanwhile, the influence of tuning the Fermi level of graphene on the field enhancement is also presented in the paper. The proposed structure may improve the development of graphene-h-BN-based optoelectronic devices.

Tunable Multipolar Fano Resonances and Electric Field Enhancements in Au Ring-Disk Plasmonic Nanostructures

We theoretically research the characteristics of tunable multipolar Fano resonances in novel-designed Au ring-disk plasmonic nanostructures. We systematically study some structural parameters that influence the multipolar Fano resonances of the nanostructures. Adjustment of the radius (R₁ and R₂) of the Au ring, the radius (R₃) of the Au disk and the thickness (H) of the Au ring-disk can effectively adjust the multipolar Fano resonances. The complex field distributions excited by a Au ring-disk can produce dark resonance modes. At the frequency of the multipolar Fano resonances, strong localized field distributions can be obtained. The Fano resonances exhibit strong light-extinction properties in Au ring-disk nanostructures, which can be applied to an optical tunable filter and optical switch.

Towards high performance hybrid two-dimensional material plasmonic devices: strong and highly anisotropic plasmonic resonances in nanostructured graphene-black phosphorus bilayer

Plasmonics of two-dimensional materials have attracted increasing attention in the past few years. It provides a platform for strong light-matter interactions and enables a variety of novel applications in the infrared and terahertz ranges. In this paper, we study the plasmonic properties of a graphene-black phosphorus (G-BP) bilayer. It exhibits both strong and highly anisotropic plasmonic responses that performs beyond individual graphene and black phosphorus films. Polarization dependent, anisotropic perfect absorption can be realized in this type of two-dimensional plasmonic nanostructures with moderate doping levels. This type of hybrid architecture opens a new door for high performance two-dimensional material plasmonic devices.

Approaching perfect absorption of monolayer molybdenum disulfide at visible wavelengths using critical coupling

A simple perfect absorption structure is proposed to achieve the high efficiency light absorption of monolayer molybdenum disulfide (MoS₂) by the critical coupling mechanism of guided resonances. The results of numerical simulation and theoretical analysis show that the light absorption in this atomically thin layer can be as high as 98.3% at the visible wavelengths, which is over 12 times more than that of a bare monolayer MoS₂. In addition, the operating wavelength can be tuned flexibly by adjusting the radius of the air hole and the thickness of the dielectric layers, which is of great practical significance to improve the efficiency and selectivity of the absorption in monolayer MoS₂. The novel idea of using critical coupling to enhance the light-MoS₂ interaction can be also adopted in other atomically thin materials. The meaningful improvement and tunability of the absorption in monolayer MoS₂ provides a good prospect for the realization of high-performance MoS₂-based optoelectronic applications, such as photodetection and photoluminescence.

Dynamic tailoring of electromagnetic behaviors of graphene plasmonic oligomers by local chemical potential

In the mid-infrared and terahertz (THz) regime, graphene supports tunable surface plasmon resonance (SPR) by controlling the chemical potential, which promotes light-matter interaction at the selected wavelength, showing exceptional promise for optoelectronic applications. In this article, we show that the electromagnetic (EM) response of graphene oligomers can be substantially modified by the modification of the local chemical potential, strengthening or reducing the intrinsic plasmonic modes. The effect mechanism is corroborated by a graphene nanocluster composed of 13 nanodisks with D6h symmetry; by transforming to D3h symmetry, the effect mechanism was retained and more available plasmonic resonance modes appeared. The intriguing properties open a new way to design nanodevices made of graphene oligomers with highly efficient photoresponse enhancement and tunable spectral selectivity for highly accurate photodetection.

Hybrid Density Functional Study on the Photocatalytic Properties of Two-dimensional g-ZnO Based Heterostructures

In this work, graphene-like ZnO (g-ZnO)-based two-dimensional (2D) heterostructures (ZnO/WS₂ and ZnO/WSe₂) were designed as water-splitting photocatalysts based on the hybrid density functional. The dependence of photocatalytic properties on the rotation angles and biaxial strains were investigated. The bandgaps of ZnO/WS₂ and ZnO/WSe₂ are not obviously affected by rotation angles but by strains. The ZnO/WS₂ heterostructures with appropriate rotation angles and strains are promising visible water-splitting photocatalysts due to their appropriate bandgap for visible absorption, proper band edge alignment, and effective separation of carriers, while the water oxygen process of the ZnO/WSe₂ heterostructures is limited by their band edge positions. The findings pave the way to efficient g-ZnO-based 2D visible water-splitting materials.

Enhanced dual-band absorption of molybdenum disulfide using a plasmonic perfect absorber

In order to remarkably enhance its absorption capability, a tunable dual-band MoS₂-based perfect absorber inspired by metal-insulator-metal (MIM) metamaterial is proposed. By using the finite-difference time-domain (FDTD) simulations, dual-band perfect absorption peaks are realized in the visible light regime, and the absorptions of monolayer MoS₂ are enhanced up to 57% and 80.5% at the peak wavelengths. By manipulating related structural parameters, the peak wavelengths of MoS₂ absorption can be separately tuned in a wide wavelength range. Furthermore, the proposed absorber can tolerate a relatively wide range of incident angles and demonstrate polarization-dependence.

Hybridization-induced broadband terahertz wave absorption with graphene metasurfaces

Electromagnetic (EM) wave absorption plays a vital role in photonics. While metasurfaces are proposed to absorb EM waves efficiently, most of them exhibit limited bandwidth and fixed functionalities. Here, we propose a broadband and tunable terahertz (THz) absorber based on a graphene-based metasurface, which is constructed by a single layer of closely patterned graphene concentric double rings and a metallic mirror separated by an ultrathin SiO₂ layer. Plasmonic hybridization between two graphene rings significantly enlarges the absorption bandwidth, which can be further tuned by gating the graphene. Moreover, the specific design also makes our device insensitive to the incident angle and polarization state of impinging EM waves. Our results may inspire certain wave-modulation-related applications, such as THz imaging, smart absorber, tunable sensor, etc.

Broad-spectrum enhanced absorption of graphene-molybdenum disulfide photovoltaic cells in metal-mirror microcavity

The optical absorption of graphene-molybdenum disulfide photovoltaic cells (GM-PVc) in wedge-shaped metal-mirror microcavities (w-MMCs) combined with a spectrum-splitting structure was studied. Results showed that the combination of spectrum-splitting structure and w-MMC can enable the light absorption of GM-PVcs to reach about 65% in the broad spectrum. The influence of processing errors on the absorption of GM-PVcs in w-MMCs was 3-14 times lower than that of GM-PVcs in wedge photonic crystal microcavities. The light absorption of GM-PVcs reached 60% in the broad spectrum, even with the processing errors. The proposed structure is easy to implement and may have potentially important applications in the development of ultra-thin and high-efficiency solar cells and optoelectronic devices.

Plasmonic Absorption Enhancement in Elliptical Graphene Arrays

In this paper, we come up with a wavelength tunable absorber which is made up of periodically elliptical graphene arrays in the far-infrared and terahertz regions. Through simulation, we find that we can increase the length of long axis of the ellipse, raise the incidence angles of TM- and TE-polarization (TM- and TE-polarization indicate the direction of the incident electric field along the direction of the x and the y axis, respectively.) within certain limits, and increase the chemical potential of graphene, so as to enhance the absorption of light in the elliptical graphene arrays. We also compare the absorption spectra of the original structure and the complementary structure, and find that the absorption of the original structure is higher than that of the complementary structure. In the end, we study the changes in the absorption rate of the double layer structure of the elliptical array with the increase in the thickness of SiO₂. The elliptical array structure can be applied to tunable spectral detectors, filters and sensors at far-infrared and terahertz wavelengths.

Polarization-independent enhancement of graphene plasmons by coupling with the dipole-like near field of the metallic split-mesh structure

The localized electric field enhancement of graphene plasmon modes is limited by the duty cycle of graphene, the frequency, the absorption and the scattering rate. To obtain higher detectivity, higher field enhancement is required. While the absorption can be no larger than 100%, the scattering is an intrinsic limitation, and the frequency is designated, the duty cycle is the only parameter that can be designed freely to achieve high field enhancement. By etching graphene into periodic structures, i.e. reducing the duty cycle of graphene, the localized electric field can be enhanced as a result of the reduction of the active region. However, too small a duty cycle will weaken the coupling efficiency, which will reduce the absorption, and then the localized electric field can hardly be further enhanced. In this work, we propose to use the metallic split-mesh structure which will focus the incident radiation at the ends of the metallic bars. The absorption and the electric field will be greatly enhanced by placing graphene structures below the small holes formed by the metallic split-mesh structure.

Enhancing the localized electric field of graphene plasmons with a metallic split-mesh structure by more than an order of magnitude.

Total absorption of light in monolayer transition-metal dichalcogenides by critical coupling

Transition-metal dichalcogenides with exceptional electrical and optical properties have emerged as a new platform for atomic-scale optoelectronic devices. However, the poor optical absorption resists their potential applications. The novel method of critical coupling with guided resonances is proposed to realize total absorption of light in monolayer MoS₂ both theoretically and numerically. Simulated results illustrate that the perfect absorption with critical coupling is achieved by choosing suitably the ration of the hole radius to the period of the photonic crystal slab, and that the tunability of absorption peaks is obtained by a small change in the period and the thickness of the slab. Intriguingly, such device manifests the unusual polarization-insensitive feature and the good absorption stability over a wide angle range of incidence. The total absorption in monolayer MoSe₂, WS₂, and WSe₂ is realized handily by the same principle. Hence, our results may open up new possibilities for improving the light-matter interaction in monolayer transition-metal dichalcogenides and find utility in wavelength-selective photoluminescence and photodetection.

Dynamically Tunable Plasmon-Induced Transparency in On-chip Graphene-Based Asymmetrical Nanocavity-Coupled Waveguide System

A graphene-based on-chip plasmonic nanostructure composed of a plasmonic bus waveguide side-coupled with a U-shaped and a rectangular nanocavities has been proposed and modeled by using the finite element method in this paper. The dynamic tunability of the plasmon-induced transparency (PIT) windows has been investigated. The results reveal that the PIT effects can be tuned via modifying the chemical potential of the nanocavities and plasmonic bus waveguide or by varying the geometrical parameters including the location and width of the rectangular nanocavity. Further, the proposed plasmonic nanostructure can be used as a plasmonic refractive index sensor with a sensing sensibility of 333.3 nm/refractive index unit (RIU) at the the PIT transmission peak. Slow light effect is also realized in the PIT system. The proposed nanostructure may pave a new way towards the realization of graphene-based on-chip integrated nanophotonic devices.

Greatly enhanced light emission of MoS2 using photonic crystal heterojunction

We present theoretical study on developing a one-dimensional (1D) photonic crystal heterojunction (h-PhC) that consists of a monolayer molybdenum disulfide (MoS2) structure. By employing the transfer matrix method, we obtained the analytical solution of the light absorption and emission of two-dimensional materials in 1D h-PhC. Simultaneously enhancing the light absorption and emission of the medium in multiple frequency ranges is easy as h-PhC has more modes of photon localization than the common photonic crystal. Our numerical results demonstrate that the proposed 1D h-PhC can simultaneously enhance the light absorption and emission of MoS2 and enhance the photoluminescence spectrum of MoS2 by 2-3 orders of magnitude.

Tunable ultra-high-efficiency light absorption of monolayer graphene using critical coupling with guided resonance

We numerically demonstrate a novel monolayer graphene-based perfect absorption multi-layer photonic structure by the mechanism of critical coupling with guided resonance, in which the absorption of graphene can significantly reach 99% at telecommunication wavelengths. The highly efficient absorption and spectral selectivity can be obtained with designing structural parameters in the near-infrared region. Compared to previous works, we achieve the complete absorption of single-atomic-layer graphene in the perfect absorber with a lossless dielectric Bragg mirror, which not only opens up new methods of enhancing the light-graphene interaction, but also makes for practical applications in high-performance optoelectronic devices, such as modulators and sensors.

Dynamically controllable plasmon induced transparency based on hybrid metal-graphene metamaterials

Novel hybrid metal-graphene metamaterials featuring dynamically controllable single, double and multiple plasmon induced transparency (PIT) windows are numerically explored in the terahertz (THz) regime. The designed plasmonic metamaterials composed of a strip and a ring with graphene integration generate a novel PIT window. Once the ring is divided into pairs of asymmetrical arcs, double PIT windows both with the spectral contrast ratio 100% are obtained, where one originates from the destructive interference between bright-dark modes, and the other is based on the interaction of bright-bright modes. Just because the double PIT windows are induced by two different mechanisms, the continuously controllable conductivity and damping of graphene are employed to appropriately interpret the high tunability in double transparency peaks at the resonant frequency, respectively. Moreover, multiple PIT windows can be achieved by introducing an additional bright mode to form the other bright-bright modes coupling. At the PIT transparent windows, the dispersions undergo tremendous modifications and the group delays reach up to 43 ps, 22 ps, and 25 ps, correspondingly. Our results suggest the existence of strong interaction between the monolayer graphene layer and metal-based resonant plasmonic metamaterials, which may hold widely applications in filters, modulators, switching, sensors and optical buffers.

Toroidal resonance based optical modulator employing hybrid graphene-dielectric metasurface

In this paper, we demonstrate the combination of a dielectric metasurface with a graphene layer to realize a high performance toroidal resonance based optical modulator. The dielectric metasurface consists of two mirrored asymmetric silicon split-ring resonators (ASSRRs) that can support strong toroidal dipolar resonance with narrow line width (~0.77 nm) and high quality (Q)-factor (~1702) and contrast ratio (~100%). Numerical simulation results show that the transmission amplitude of the toroidal dipolar resonance can be efficiently modulated by varying the Fermi energy EF when the graphene layer is integrated with the dielectric metasurface, and a max transmission coefficient difference up to 78% is achieved indicating that the proposed hybrid graphene/dielectric metasurface shows good performance as an optical modulator. The effects of the asymmetry degree of the ASSRRs on the toroidal dipolar resonance are studied and the efficiency of the transmission amplitude modulation of graphene is also investigated. Our results may also provide potential applications in optical filter and bio-chemical sensing.