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We propose a dynamically tunable surface plasmon polaritons (SPPs) waveguide system based on bulk Dirac semimetals (BDS) containing only a side-coupled T-shaped cavity. Plasmon-induced transparency (PIT) is achieved in the terahertz band by introducing a position offset. We have analytically investigated the spectral characteristics of PIT in this system, indicating that the larger the position offset, the higher the peak of the PIT window. The spectrum responses of PIT system can be flexibly regulated via transforming the geometric parameters of the structure. At the same time, it is particularly sensitive to the refractive index of the surrounding medium, which is promising for sensing devices. In addition, the resonance frequency and peak transmission can be actively adjusted by controlling the Fermi energy of the BDS without reconstructing the geometric structure. Moreover, the optical delay time near the PIT peak reaches 11.001 ps, which has good slow-light characteristics and is a candidate in the field of slow-light equipment. The structure we designed may have potential application value in the design of SPPs light-guide devices.

Ghost imaging is an unconventional imaging mechanism that utilizes the high-order correlation to reconstruct object’s image. Limited by the maximum refresh rate of DMD or SLM, the sampling efficiency of ghost imaging has been a major obstacle for practical application. In this paper, foveated ghost imaging based on deep learning (DPFGI) is proposed to generate non-uniform resolution speckle patterns according to the object detection results as the fovea point. We combine foveated speckle pattern inspired by the human visual system with GAN-based ghost imaging object detection system to realize selecting the region of interest for foveated imaging intelligently. The simulation and experimental results show that DPFGI can detect objects in undersampled images with higher accuracy and achieve higher PSNR in the fovea region compared with uniform-resolution ghost imaging, which opens new perspectives for more intelligent ghost imaging.

Solution cathode glow discharge-atomic emission spectrometry (SCGD-AES) has attracted wide attention globally in recent years. The technology has the advantages of on-line detection of various meta...

We propose a waveguide system based on the bulk Dirac semimetals material to achieve plasmon-induced absorption (PIA). Numerical results were verified by classical three-level system theory. The absorption window is attributed to the extreme destructive interference of the bright and dark modes. The dynamic controllability of the absorption window modulation ratio is achieved by adjusting the Fermi level. In addition, the plasmonic system is extremely sensitive to environmental refractive indices. In this way, the structure makes the absorption window not only vertically controllable, but also horizontally adjustable. Simultaneously, the group time delay of the PIA window can reach −57.2 ps, which is more advantageous for ultra-fast light devices. Finally, by studying the two-channel PIA waveguide system, more excellent design platforms are provided for optical transmission for practical multi-functional highly integrated plasmonic components.

We introduce a remarkably simple coplanar nanostructure consisting of two longer and one shorter graphene nanorods to support a Fano resonance, which arises from the coupling between bright mode and dark mode by breaking the geometrical symmetry. Simulation results, performed with the finite-difference time-domain method, show that the resonant frequency of the bright mode in the transmission spectrum can be independently tuned by changing the lateral offset of the shorter graphene nanorod, while changing its longitudinal offset can independently adjust the resonant frequency of the dark mode. Furthermore, Fano resonance spectrum can also be regulated actively by altering the Fermi energy of graphene as well as refractive index of the surrounding media. This letter proposes a unique modulation strategy of Fano resonance in metamaterials, assisting in developing potential in applications of sensor devices and some other plasmonic functional devices.

We demonstrate multiple Fano resonances in an integrated single dark mode hybrid metamaterial waveguide structure, which consists of three gold cut wires placed on a dielectric board waveguide. With a symmetrical pattern, the design shows double sharp Fano peaks with high $Q$ -factors of 950 and 216, which are attributed to the interaction between plasmon mode and quasi-guided modes. By breaking the structural symmetry, a quadrupole mode on middle vertical cut wire is excited and further results in multiple Fano resonances through both the diffracted waves and near-field coupling with bright–dark mode. Based on the above-mentioned properties, we show a high $Q$ -factor refractive index sensor with figure of merit values of 330 and 281. Interestingly, these $Q$ -factors of Fano peaks can be increased to 1357 and 361 to improve the sensing performance, by capping a silicon layer on the metal surface. This letter provides a way to obtain multiple high $Q$ -factor Fano resonances, which can widen channels for fabricating device in biochemical sensing.

In order to remarkably enhance its absorption capability, a tunable dual-band graphene-based perfect absorber is proposed. By using the finite-difference time-domain (FDTD) simulations, dual-band perfect absorption peaks are realized in the optical communication regime based on the critical coupling effect. By changing the lateral displacement of the two different holes, multiple absorption peaks could also be achieved. Furthermore, by manipulating the widths of air trenches, the wavelengths of absorption peaks could be shifted, while the values of absorption peaks are almost unchanged. Moreover, our proposed absorber also demonstrates polarization-dependence.

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.

Fourier single-pixel imaging (FSI) has been proven capable of acquiring excellent image quality when is sampled the fully information in Fourier domain. However, when the sampling measurements is limited, image reconstruction by applying inverse Fourier transform algorithm would result in the severe ringing effect and the loss of image details. In this report, we propose a new algorithm for FSI reconstruction based on compressed sensing theory, which utilizes joint discrete gradient and non-local self-similarity priors, thus substantially using the prior knowledge of natural images in reconstruction process. Both the results of computational simulations and experiments demonstrate the efficacy of the proposed algorithm.

General plasmonic systems to realize plasmonically induced transparency (PIT) effect only exist one single PIT mainly because they only allow one single coupling pathway. In this study, we propose a distinct graphene resonator-based system, which is composed of graphene nanoribbons (GNRs) coupled with dielectric grating-loaded graphene layer resonators, to achieve two switchable PIT effects. By designing crossed directions of the resonators, the proposed system exists two different PIT effects characterized by different resonant positions and linewidths. These two PIT effects result from two separate and polarization-selective coupling pathways, allowing us to switch the PIT from one to the other by simply changing the polarization direction. Parametric studies are carried to demonstrate the coupling effects whereas the two-particle model is applied to explain the physical mechanism, finding excellent agreements between the numerical and theoretical results. Our proposal can be used to design switchable PIT-based plasmonic devices, such as tunable dual-band sensors and perfect absorbers.

General two-dimensional (2D) material-based systems that achieve plasmonically induced transparency (PIT) are limited to isotropic graphene only through unidirectional bright–dark mode interaction. Moreover, it is challenging to extend these devices to anisotropic 2D films. In this study, we exploit surface plasmons excited at two crossed grating layers, which can be formed either by dielectric gratings or by the 2D sheet itself, to achieve dynamically tunable PIT in both isotropic and anisotropic 2D materials. Here, each grating simultaneously acts as both bright and dark modes. By taking isotropic graphene and anisotropic black phosphorus (BP) as proofs of concept, we reveal that this PIT can result from either unidirectional bright–dark or bidirectional bright–bright and bright–dark mode hybridized couplings when the incident light is parallelly/perpendicularly or obliquely polarized to the gratings, respectively. Identical grating parameters in isotropic (crossed lattice directions in anisotropic) layers produce polarization-independent single-window PIT, whereas different grating parameters (coincident lattice directions) yield polarization-sensitive double-window PIT. The proposed technique is examined by a two-particle model, showing excellent agreement between the theoretical and numerical results. This study provides insight into the physical mechanisms of PIT and advances the applicability and versatility of 2D material-based PIT devices.

Plasmonic metamaterials enable manipulation of light at subwavelength scales and exhibit unique optical functionalities. However, the realization of high-performance, large-range, and dynamically tunable optical absorbers based on plasmonic metamaterials remains challenging. Here, we propose and demonstrate a continuously tunable absorbers consisting of a zigzag array of bulk Dirac semimetals (BDS) meta-atoms and a metal reflector spaced by insulator layers. This structure exhibits a collective resonance formed by the electric dipole modes polarized along the long axis of each individual meta-atom, which allows us to precisely control this resonance frequency by fine-tuning the unit cell geometry and the Fermi energy levels of the BDS. In addition, the related physical mechanism behind this complete absorption can explained by employing coupled-mode theory (CMT) and mode-expansion theory (MET). Our results may arouse the investigations of the tunable metamaterials device based on the BDS.

In this paper, a hybrid strategy incorporating finite element computation, artificial neural network (ANN) method and Random Forest (RF) algorithm is proposed for computation of the thermal residual stresses (TRS) in SiC/SiC composites with multi-layered interphases. The finite element method is adopted to calculate the TRS of the unidirectional SiC/SiC composites. The ANN with the structure of Random Forest (RFANN) is employed to approximate the non-linear relationship between the multi-layered interphases thicknesses and the TRS of the SiC/SiC composites. The well-trained RFANN is finally used to compute the TRS of a unidirectional SiC/SiC composite with six layers of interphases.

We propose a simple plasmonic waveguide system based on two-detuned resonators, which demonstrates multiple detuned-resonator induced transparencies at visible and near-infrared region. The performance of electromagnetic responses can be agile manipulated by tuning the asymmetry degree of the structure and the width of the split gap. Three transmission dips exist with the symmetrical design while three peaks emerge between the dip-position of the transmission spectra with two detuned resonators. The physical mechanism behind the plasmon-induced transparency (PIT) resonance is revealed as being attributed to the constructive interference between the confined modes in the detuned resonators. The former tend to the role of two coupled radiative oscillators. The work may open up avenues for the control of light in highly integrated optical circuits.

Solution cathode glow discharge–atomic emission spectrometry (SCGD–AES) has attracted increasing attention in recent years in water and environmental analysis. In this article, the traditional spec...

The investigation of plasmon induced transparency (PIT) resonances is performed in a system based on defective and complete silver nanostrips. The response of metasurface systems to electromagnetic fields under normal incidence is endowed by unique features, including some exquisite spatial phenomena. These phenomena are embodied in confined light-matter hybrid modes where the defective silver nanostrip excites superradiant dipole and quadrupole modes, resulting in a PIT window. Moreover, multispectral PIT resonances are observed, which involves quadrupole mode as superradiant to excite the proximity nanostrip. It can reveal much about the defective metasurface structure, such as exact tuning of the gap distance between these two nanostrips, variation in defect parameters and modulating in electric polarization angles of excitation. Still, our research realizes multiple PIT resonances in one nanostructure, which may have interesting applications of plasmonic switching devices in highly integrated circuits.

In this paper, the surface plasmon resonance (SPR) of rectangular cavity is investigated in Ag-Si-SiO2 under near-infrared light incident by finite-difference time-domain (FDTD). Between the surface plasma and the thickness of the silver film, the length of the silver film, the thickness of silicon, and the position of silicon, these relationships will be studied in a rectangular cavity, which is surrounded by silver film, two rectangular nanoscale silicons, and silicon dioxide. The surface plasmon resonance of the RCM is independent of the distance between the two silicons when the transverse silver film length l is fixed and of height d = 10k (k = 1, 2, 3, 4, and 5) nm of rectangular silicon and thickness t (t = 6, 7, 8, 9, 10, 11, and 12 nm) of silver film while the length l = 600 nm. The SPR is a lateral length dependence of silver film. The plasma resonance of the two first-order modes is distributed symmetrically on both sides of the local field in RCM, and it is irrelevant with the position of the vibration mode and the thickness of the silver film. On the basis of the above research, we propose to create a guided-wave resonance that can be directly observed from the normal incidence transmission spectra and electric field distribution. This structure can be used as a highly tunable optical sensitive element that can be tuned with a small change of transverse silver film length.

In this paper, we propose a structure composed of two graphene waveguides and dual coupled graphene ring resonators (GRRs) to achieve a plasmon-induced absorption (PIA) effect. A three-level plasmonic system and a temporal coupled mode theory (CMT) are utilized to verify the simulation results. Moreover, a double-window-PIA effect can be conveniently attained by introducing another GRR with proper parameters to meet more specific acquirement in optical modulation process. The pronounced PIA resonances can be tuned in a number of ways, such as by adjusting the coupling distance between the GRRs and the couplings between the GRR and the waveguide, and tuning the radius and the Fermi energy of the GRRs. Besides, the produced PIA effect shows a high group delay up to − 1 . 87 ps, exhibiting a particularly prominent fast-light feature. Our results have potential applications in the realization of THz-integrated spectral control and graphene plasmonic devices such as sensors, filters, ultra-fast optical switches and so on.

A novel nonreciprocal graphene waveguide with Kerr nonlinear material in mid-infrared region is proposed and the corresponding transmission characteristics are investigated. In the proposed graphene waveguide, a transmission contrast ratio up to 19.2 dB can be achieved between forward and backward transmission by using the finite-difference time-domain (FDTD) method, revealing an excellent nonreciprocal effect. Moreover, the proposed nonreciprocal graphene waveguide can also be flexibly controlled as forward transmission or backward transmission at different wavelengths by adjusting the structural parameters and input intensity. Therefore, our results can offer a new possibility and important application in highly integrated optical circuits.

A novel nonreciprocal MIM waveguide with Kerr nonlinear material in near-infrared region is proposed and the corresponding transmission characteristics are investigated. In the proposed MIM waveguide, a transmission contrast ratio up to 49.2 dB can be achieved between forward and backward transmission by using the finite-difference time-domain (FDTD) method, revealing an excellent nonreciprocal effect. Moreover, the proposed nonreciprocal MIM waveguide can also be flexibly controlled as forward transmission or backward transmission at different wavelengths. Therefore, our results can offer a new possibility and important application in highly integrated optical circuits.

We numerically and theoretically demonstrate a plasmon-induced transparency (PIT) at the mid-infrared region with finite-difference time-domain method. The system consists of an optically bright dipole mode and a dark quadrupole mode, which are supported by the graphene nanoring and graphene nanostrips, respectively. The coupling between the two modes introduces transparency window and large group delays. The pronounced PIT resonance can be easily modified by adjusting the geometric parameters and the Fermi level of graphene nanostructure. Our results suggest that the demonstrated PIT effect may be applicated in the slow-light device, active plasmonic switching, and optical sensing.

An approach to capture light efficiently into graphene surface plasmons by patterning the sinusoidal dielectric metasurfaces above and below the graphene sheet is proposed. The presence of plasmonic resonance is demonstrated by means of an analytical model based on the transformation optics through the extraction of effective graphene conductivity, which is further revealed via a numerical study of the optical spectra as a function of grating parameters at the subwavelength scale. Besides, the resonant position is found to be sensitive to the dielectric contacted with graphene. These findings can deepen our understanding of plasmon resonances and pave the way to the design of graphene plasmonic devices like refractive index sensors.

The perfect plasmon-induced absorption (PIA) effect is achieved in a simple plasmonic system, composed of two rectangle cavities side-coupled to the metal-insulator-metal (MIM) waveguide with a barrier. The transmission properties of the system are calculated by the finite-difference time-domain (FDTD) method and theoretically explained by the three-level plasmonic system. More interestingly, the broadband absorption window will be splitted into double windows by adding another rectangle cavity resonator, and the physical mechanism is presented. At the same time, multi-switching effect is also discovered. Undoubtedly, the proposed novel structures have potential application in highly integrated optical circuit and can be simply fabricated.

We propose the idea of a tunable nearly perfect absorber based on graphene metamaterial consisting of a periodically arranged graphene ribbons (bright mode) and the inverse slots in the thin graphene film (dark mode). The absorption peak has a narrow linewidth of 160 nm and the absorbance approaching to 97.12%. The fascinating absorption feature not merely due to plasmonically induced absorption arising from destructive interference between the bright and dark mode but also the metamaterial absorption. We have testified numerically that the electric fields are highly confined inside the slot. The mid-IR absorption spectrum can be dynamically tuned by a small change in the chemical potential of graphene, in addition to varying geometrical parameters. Our paper considers a situation that represents an intermediate plasmonic coupling regime, between near-field and far-field coupling. Such absorber possesses potential for applications in mid-IR nanophotonic devices.

An optical waveguide for working in mid-infrared range is highly desirable. However, such a waveguide is required to yield a long propagation distance and deep subwavelength confinement. Although the graphene-based surface plasmon polariton (SPP) waveguides have exhibited some potential in this regard, their propagation lengths are currently insufficient (∼100 μm). In this paper, we propose a graphene-based cylindrical long-range SPP (LRSPP) waveguide, which is composed of a cylindrical silicon nanowire core surrounded by an inner graphene layer, a silica layer, and an outer graphene layer from inside to out. First, the electromagnetic field equation and the dispersion equation of this waveguide are derived. Then, combining these derived equations with the numerical simulation, the propagation properties of the LRSPP and the short-range SPP modes are analyzed. The results show that the proposed waveguide with the LRSPP fundamental mode has a notable advantage for simultaneously achieving a larger propagation length (∼10 μm) and a deep subwavelength confinement ( $\sim10^{- 5}\,A_{0}$ , where $A_{0}$ is the diffraction-limited mode area). This waveguide has potential for application in high-density photonic integrated circuits in the mid-infrared range.

We present a multi-band terahertz absorber formed by periodic square metallic ribbon with T-shaped gap and a metallic ground plane separated by a dielectric layer. It is demonstrated that absorption spectra of the proposed structure consist of four absorption peaks located at 1.12, 2.49, 3.45, and 3.91 THz with high absorption coefficients of 98.0, 98.9, 98.7, and 99.6%, respectively. It is demonstrated that the proposed absorber has the tunability from single-band to broadband by changing the length of square metallic ribbon and we can also select or tune the frequencies which we want to use by changing polarization angles. Importantly, the quality factor Q at 3.91 THz is 30.1, which is 5.6 times higher than that of 1.12 THz. These results indicate that the proposed absorber has a promising potential for devices, such as detection, sensing, and imaging.

We present a high-performance refractive index sensor based on Fano resonance with a figure of merit (FOM) about 56.5 in all-dielectric metasurface which consists of a periodically arranged silicon rings with two equal splits dividing them into pairs of arcs of different lengths. A Fano resonance with quality factor ~133 and spectral contrast ratio ~100% arises from destructive interference of two antiphase electric dipoles in the two arcs of the split-ring. We can turn on and/or off the Fano resonance with a modulation depth nearly 100% at the operating wavelength of 1067 nm by rotating the polarization of incident light. We believe that our results will open up avenues for the development of applications using Fano resonance with dynamically controllability such as biochemical sensors, optical switching, and modulator.

In this paper, we present a swarm intelligence design technology based on a workshop of meta-synthetic engineering, including the architecture, the decision-making process of swarm intelligence design based on a meta-synthetic workshop, and the design resource delivery technology involved in the design. We conclude the paper with a discussion of future research.

Solution-cathode glow discharge-atomic emission spectrometry (SCGD-AES) with a portable spectrometer was used to detect metal elements in solution. A modified pulsation damper was designed and optimized to improve discharge plasma stability. The effects of pulsation damper parameters as well as the discharge current and solution flow rate were investigated. The plasma stability of Na, K and Li elements under optimized parameters with the modified pulsation damper was discussed and they showed good stability over a long time. The standard deviation (SD) of background intensity was low, and the detection limits for Zn, Mg, Ag, Pb, Sr, Na, Li, K, Rb and Cs were 72, 2.1, 1.2, 35, 86, 0.04, 0.16, 0.14, 0.38 and 1.1 μg L−1, respectively. Ag, Zn and Pb elements in a certified reference material were quantitatively analyzed, and the measurement results obtained by SCGD-AES agreed well with the reference values. Specifically, the precision (relative standard deviation of 10 measured values) of detection was lower than 1.85%. The results show that the SCGD-AES system with the modified pulsation damper can accurately and stably detect the content of metal elements in water.

Dyes are toxic and considered to be extremely hazardous to natural environments. Hence, adsorbents to remove dyes from contaminated water are needed. To develop adsorbents with a high adsorption capacity for different dyes, easy separation, and low cost, a novel dye adsorbent was prepared by activating fly ash with NaOH. The adsorbent morphology, structure, and specific surface area were characterized using scanning electron microscopy, X-ray powder diffraction, and surface area measurements using N2 adsorption-desorption. The adsorption abilities of the synthesized adsorbents were examined based on methylene blue and acid fuchsin adsorption from water. The capabilities of the adsorbents as a function of adsorbent use, dye type, dye concentration, time, and pH were investigated and compared. The results for methylene blue and acid fuchsin adsorption were modeled using pseudo-second order kinetics and the Langmuir adsorption isotherm, respectively. These modified adsorbents synthesized from fly ash may provide a promising solution to purify dye-contaminated waste water with the advantages of high efficiency and low cost.

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.

The difficulty in infrared and radar fusion target detection lies in the pre-processing steps such as sample reduction and feature reduction of heterogeneous sensors. For the problem of data volume redundancy, HVS sample reduction is proposed to reduce the amount of data in the infrared data set. Infrared and radar joint feature vectors are constructed through nearest neighbor data association. For the problem of feature redundancy, a weighted mutual information feature selection method based on prior information is proposed according to the difference of feature sources. The fusion data set is then used for classification. The experiments show that the HVS-based sample reduction and the prior weighted feature selection achieved a higher detection probability and a shorter detection time, and improved the robustness in the case of large differences between the training set and the test set.

The constructive interference of bright and dark plasmonic modes results in plasmon-induced absorption (PIA) effect. Here, we theoretically investigate PIA effect, which is realized by the constructive interference between a Fabry-Perot (F-P) resonance mode and a graphene quasi-guided mode. Numerical simulation reveals at least three advantages of our structure over previous ones. First, the extinction ratio can reach ~ 99.999%, resulting in the ultra-high figure of merit* (FOM*) as high as 106. Second, the intensity of this pronounced PIA effect can be optimized by adjusting the coupling distance. Third, the resonance frequency can be easily tunable by tuning the graphene Fermi level. This system may have potential applications in dynamically optical switching and biochemical sensing. Electronic supplementary material The online version of this article (10.1186/s11671-019-3121-9) contains supplementary material, which is available to authorized users.

In the infrared small target detection, the clutter formed by buildings, trees and protruding clouds is densely distributed and difficult to filter out. The hysteresis threshold detection algorithm utilizes the geometric features of small target to reduce false alarms. Images are filtered in multiple scales, the location and scale of the points of interest are extracted by non-maximum suppression. To determine the connection state of the focus and clutter, local gradient second-order origin moment is proposed to eliminate strong edges. The hysteresis threshold segmentation is performed to exclude stubborn false alarms and detect small targets. Experiments show that the proposed algorithm has a significant effect in removing false alarms, and achieves both the high detection probability and low false alarm probability.

Fourier single-pixel imaging (FSI) usually achieves an enhanced imaging speed via undersampling; however, many details are lost in the reconstructed image obtained by frequency truncation due to the lack of the high-frequency part, and the image contains ringing artefacts due to the lack of an expression ability, thus reducing the image quality. To improve the quality of real-time imaging, a fast image reconstruction framework based on the Wasserstein generative adversarial network (WGAN) and gradient penalty (GP) is proposed. Under the common constraints of the resistance loss and content loss, confrontation training is first conducted between the generator and the discriminator in the framework, and then, an additional generator is connected to improve the fidelity of the reconstructed image based on the generation of the confrontation network. The model can be used to restore high-frequency details and denoise low-quality images that are undersampled. The simulation and experimental results show that the FSI using the GAN method, which achieved a compression rate of 95–98 %, is superior to conventional imaging in terms of the quality at a low sampling rate; therefore, the proposed scheme has potential practical applications.

A dynamically tunable anisotropic narrowband absorber based on monolayer black phosphorous (BP) is proposed in the terahertz (THz) band. The proposed absorber consists of a monolayer BP and a silicon (Si) grating, which is placed on a silica (SiO2) isolation layer and a gold (Au) substrate. The benefit from the critical coupling mechanism with guided resonance is the efficiency of the absorption can reach 99.9% in the armchair (AC) direction and the natural anisotropy of BP makes it only 87.2% in the zigzag (ZZ) direction. Numerical and theoretical studies show that the absorption efficiency of the structure is operatively controlled by critical coupling conditions, including the geometric parameters of the Si grating, the electron doping of BP and the angle of incident light, etc. More importantly, in the absence of plasmon response, this structure greatly enhances the interaction between light and matter in monolayer BP. In particular, there are several advantages in this structure, such as extremely high-efficiency absorption, excellent tunability, outstanding intrinsic anisotropy and easy manufacturing, which will show unusual and promising potential applications in the design of BP-based tunable high-performance devices.

In this letter, the metal–insulator–metal plasmonic waveguides shoulder-coupled two concentric nanorings are investigated both numerically and theoretically. Based on the three-level system, the extreme destructive interference between bright and dark resonators gives rise to the distinct plasmonically induced absorption response with the abnormal dispersion and novel fast-light feature. By the same principle, the dramatic plasmonically induced transparency effect with slow-light characteristic is obtained in the waveguide-resonator system with side-coupling configuration. All simulated results are confirmed by the coupled mode theory. Proposed structures will open a new avenue for controlling the speed of a light signal in ultra-compact integrated devices.

A novel narrow-band plasmonic filter in terahertz (THz) region based on optical Tamm plasmon (OTP) in dual-section InSb slot waveguide is proposed, and the corresponding transmission characteristics are investigated. This filter consists of two InSb layers and two insulator layer sections separated by an InSb film. Each insulator layer section contains three insulator periodic units structured by alternately two insulators with different refractive indices. In the filter, due to the OTP is considered as a confined electromagnetic mode which exists at the boundary between two medium with high reflection, an OTP wave could be formed on the InSb-insulator interfaces and confined in the waveguide. Thus, a narrow-band transmission peak with 3 dB bandwidth of 0.017 THz in THz region around 0.54 THz is investigated by using the finite-difference time-domain (FDTD) method. Moreover, since the permittivity of InSb can be modified by varying the temperature, the performance of proposed plasmonic filter could be controlled by thermal tuning. Therefore, excellent tunable narrow-band filtering performance is obtained.

A tunable Fano-like resonance in the hybrid construction between graphene nanorings and graphene grating is proposed. The simulation results reveal that the Fano-like resonance is generated between two bright modes, which are different from the previous studies of coupling between the plasmonics bright and dark modes. The adjustability of this Fano resonance is simply researched by changing the geometry parameters of the composite structure and the Fermi-energy of graphene. Also, the dual-Fano resonance can be realized by only decreasing the thickness of the substrate spacer without employing extra resonance mode.

Tunable plasmon-induced transparency (PIT) is realized for the mid-infrared region only by using two parallel graphene nanostrips. The weak hybridization between the two bright modes results in the novel PIT optical response. The performance of the PIT system can be controlled by changing the geometry parameters of graphene nanostrips. At the same time, the resonance frequency of transparency window can be dynamically tuned by varying the Fermi energy of the graphene nanostrips via electrostatic gating instead of re-fabricating the nanostructures. Moreover, a figure of merit (FOM) value as high as 12 is achieved in the proposed nanostructures based on the performed sensitivity measures. Such proposed graphene-based PIT system may open up avenues for the development of compact elements such as tunable sensors, switchers, and slow-light devices.

A novel five-band terahertz metamaterial absorber based on three nested split-ring resonators is proposed. It is found that the structure has five distinctive absorption bands whose peaks are average over 99%. Different from previous reports by combing the resonances of the complex structure to obtain the multi-band responses, the proposed five-band absorber uses the hybrid of the LC resonance and dipolar response of the patterned structure, thus making the proposed structure quite easy to be fabricated. Furthermore, the sensing performance of the absorber is analyzed in terms of the over layer and the surrounding index.

The phenomenon of plasmon-induced transparency (PIT) is realized a in surface plasmon polariton waveguide at near-infrared frequencies. The right-angled slot and rectangle cavity placed inside one of the metallic claddings are respectively utilized to obtain bright and dark modes in a typical bright-dark mode waveguide. A PIT transmission spectrum of the waveguide is generated due to the destructive interference between the bright and dark modes, and the induced transparency peak can be manipulated by adjusting the size of the bright and dark resonators and the coupling distance between them. Subsequently, spectral splitting based on the PIT structure is studied numerically and analytically. Simulation results indicate that double electromagnetically induced transparency (EIT)-like peaks emerge in the broadband transmission spectrum by adding another rectangle cavity, and the corresponding physical mechanism is presented. Our novel plasmonic structure and the findings pave the way for new design and engineering of highly integrated optical circuit such as nanoscale optical switching, nanosensor, and wavelength-selecting nanostructure.

A single- and dual-band convertible perfect absorber based on bulk Dirac semimetal (BDS), in which BDS disk array embedded in a dielectric layer deposited on the metal substrate, is proposed. In the use of new materials, we integrate two different modes in a simple structure which not complicatedly patterns the structure of BDS. The simulated results show that the full width at half maximum (FWHM) of mode f 1 and mode f 2 are Δ f = 4 × 1 0 − 2 THz and 2.4 × 1 0 − 2 THz, respectively. And the absorption spectra of our absorber can be well described by the coupled mode theory (CMT). Comprehensive analyses show that, for dual-band absorber, the two absorption peaks can be attributed to the localized cavity resonance and the inter-cell electric resonance, separately. It is interesting to find that double-band absorber can be converted to single-band absorber when the radius of BDS disk is chosen appropriately. Unlike traditional metal-based absorber, absorption frequency of the proposed absorber can be adjusted by varying Fermi energy of BDS from 20 to 50 meV while maintaining high absorption efficiency over 92%. Moreover, the proposed absorber is robust to incident angles from 0°to 60°. Thus, this paper may provide a guideline to investigate single- and dual-band convertible absorber with simple structure based on BDS.

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.

Gradient ghost imaging (GGI) is a new imaging method that can directly obtain the edge information of an unknown object from the correlation of light intensity fluctuations. However, this technique faces the problems of poor directionality and continuity. In this paper, we propose a multidirectional gradient ghost imaging scheme (MGGI), where Kirsch operator with 8-directional masks is first used to generate structured illuminations, and then a fast algorithm based on the connection of the 8 masks is applied in the sampling process. Compared with GGI based on Sobel operator, both simulation and experimental results demonstrate that the proposed method has the advantages of imaging quality and edge continuity even in a noisy environment. Furthermore, the MGGI does not increase the imaging time and thus promotes the practical development of ghost imaging.

Here we study the possibility to achieve polarization-independent optical absorption in stacked anisotropic 2D material nanostructures (NSs). Focusing on black phosphorus, we demonstrate that by crossly stacking even-layered NSs, surface plasmons resonant in the two lattice directions are complementary excited, leading to polarization-independent absorption at any layer distance. This property is numerically validated using full electromagnetic simulations and theoretically predicted by a two-particle model. Our proposal can open up the possibility of anisotropic 2D materials to develop polarization-independent plasmon devices such as sensors and absorbers.

A broadband and polarization-insensitive optical absorber based on van der Waals (vdW) material h-BN is proposed in the mid-infrared regime. By utilizing the finite-difference time-domain (FDTD) simulation, the broadband absorption spectrum above 95% is achieved from 6490 to 7295 nm. Moreover, the proposed structure can maintain the excellent performance and broadband absorption in a wide range of incident angles. To illustrate the physical mechanism of proposed absorber, we investigate the magnetic field distributions in the each h-BN structural unit at different wavelengths, and comprehend it with the dispersion relation. Thus, we believe that our research can find some important applications for thermal emitters and photovoltaic devices with outstanding properties working at mid-infrared regime. Keyword: vdW material, Absorption, Mid-infrared regime

A subwavelength terahertz plasmonic waveguide based on bulk Dirac semimetal (BDS)-insulator-metal (BIM) structure is investigated, which indicates that there is an optimized frequency range with the better confinement as well as lower loss. A broadband mode confinement up to λ 0/15 with a relatively low loss of 1.0 dB/λ 0 can be achieved. We also show that two silicon ribbons introduced into the BIM waveguide can form a dynamically tunable filter tailoring terahertz surface plasmon polaritons in deep-subwavelength scale, which can be further exploited for the design of ultra-compact THz plasmonic devices with dynamical tunability. Our results may also provide potential applications in optical filtering.

In this paper, a bulk Dirac semimetals (BDSs) based tunable narrowband absorber at terahertz frequencies is proposed and it has the attractive property of being polarization-independent at normal incidence because of its 90° rotational symmetry. Numerical results show that the absorption bandwidth is about 1.469e-2 THz and the total quality factor Q, defined as Q = f0/Δf, reaches about 94.6, which can be attributed to the low power loss of the guided mode resonance in the dielectric layer. The simulation results are analyzed with coupled mode theory. Interestingly, on the premise of maintaining the absorbance at a level greater than 0.95, the absorption frequency can be tuned from 1.381 to 1.395 THz by varying the Fermi energy of BDSs from 50 to 80 meV. Our results may also provide potential applications in optical filter and bio-chemical sensing.

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.