Plasmonically induced transparency in in-plane isotropic and anisotropic 2D materials

Utilization of smartphones for the evaluation of Gr/Ni nanostructures magnetically controlled based on optical fibers surface plasmons

In the suggested optical fiber-based magnetoplasmonic system, we investigated the magnetic properties of graphene/nickel nanostructures. The plasmonic mode changes under the magnetic field observed in the intensity diagrams over time. To be accessible, cheap, and portable, we used a smartphone as a detector and processor. Considering the ambient noise and the light source, it was reported that the intensity of the changes improved up to 5 times. Further, the clad corrosion experiment carried out by pure dimethyl ketone in an intensity modulation by a smartphone camera and 10 seconds suggested removing fluorine polymer clad.

Polarization-controlled and symmetry-dependent multiple plasmon-induced transparency in graphene-based metasurfaces

In this paper, we theoretically and numerically demonstrate a polarization-controlled and symmetry-dependent multiple plasmon-induced transparency (PIT) in a graphene-based metasurface. The unit cell of metasurface is composed of two reversely placed U-shaped graphene nanostructures and a rectangular graphene ring stacking on a dielectric substrate. By adjusting the polarization of incident light, the number of transparency windows can be actively modulated between 1 and 2 when the nanostructure keeps a geometrical symmetry with respect to the x-axis. Especially, when the rectangular graphene ring has a displacement along the y-direction, the number of transparency windows can be arbitrarily switched between 2 and 3. The operation mechanism behind the phenomena can be attributed to the near-field coupling and electromagnetic interaction between the bright modes excited in the unit of graphene resonators. Moreover, the electromagnetic simulations obtained by finite-difference time-domain (FDTD) method agree well with the theoretical results based on the coupled modes theory (CMT). In addition, as applications of the designed nanostructure, we also study the modulation degrees of amplitude, insertion loss and group index of transmission spectra for different Fermi energies, which demonstrates an excellent synchronous switch functionality and slow light effect at multiple frequencies. Our designed metasurface may have potential applications in mid-infrared optoelectronic devices, such as optical switches, modulators, and slow-light devices, etc.

Analogous plasmon-induced absorption based on an end-coupled MDM structure with area-cost-free cavities

We theoretically investigate an end-coupled metal-dielectric-metal (MDM) structure that achieves analogous plasmon-induced absorption (APIA) in an area-cost-free manner. First, a squared ring is set to end-couple with MDM input and output waveguides, generating three Lorentzian-like peaks in the spectrum. Then, two APIA windows as well as two Fano resonances can be induced via appropriately arranging two area-free cavities. Numerous numerical results demonstrate that the proposed structure has remarkable sensing and phase characteristics. Our proposed PIA-based MDM structure is promising in potential applications of bio-chemical sensing, slow light devices, optical switching, and chip-scale plasmonic devices.

High-Q and tunable analog of electromagnetically induced transparency in terahertz all-dielectric metamaterial

We propose a novel all-dielectric metamaterial (ADMM), to the best of our knowledge, with a simple structure to achieve the analog of electromagnetically induced transparency (EIT) in the terahertz range. The ADMM is constructed by unit cells with two same silicon bar resonators on a quartz substrate. By breaking the symmetrical array of silicon resonators, the guided-mode resonance can be excited in the substrate, and the destructive interference between a broadband electric-dipole resonance and a narrowband guided-mode resonance gives rise to an EIT-like response. The EIT window can reach a high quality factor (Q-factor) over 1500 by carefully adjusting the asymmetry degree within the unit cell. A dynamically tunable ADMM was further developed by employing photoactive doped silicon. By varying the carrier density of the doped silicon through optical pump, the strength of the EIT-like resonance can be actively modulated, enabling an on-to-off switch of the slow-light effect. The designed ADMM can achieve a high-Q EIT-like response and dynamic modulation, which may give potential applications in bio/chemical sensing, optical switching, and slow-light devices.

Slow Light Effect and Tunable Channel in Graphene Grating Plasmonic Waveguide

A graphene plasmon waveguide composed of silicon grating substrate and a silica separator is proposed to generate the slow-light effect. A bias voltage is applied to tune the optical conductivity of graphene. The tunability of the slow-light working channel can be achieved due to the adjustable bias voltage. With an increase in the bias voltage, the working channel exhibited obvious linear blue-shift. The linear correlation coefficient between the working channel and the bias voltage was up to 0.9974. The average value of the normalized delay bandwidth product (NDBP) with different bias voltages was 3.61. In addition, we also studied the tunable group velocity at a specific working channel. Due to the tunability of this miniaturized waveguide structure, it can be used in a variety of applications including optical storage devices, optical buffers and optical switches.

Multi-Band Analogue Electromagnetically Induced Transparency in DoubleTuned Metamaterials

A multi-band analogue electromagnetically induced transparency (A-EIT) metamaterial is proposed. The structure is composed of liquid crystal (LC) layer and a graphene strips layer on both sides of silicon dioxide. The transmission spectrum and electric field distribution of only one graphene strip and two graphene strips have been studied. As a bright mode, the graphene strip is coupled with adjacent graphene strip to realize the A-EIT effect. When multiple graphene strips are coupled with each other, the multi-band A-EIT is obtained due to the electric dipole resonances of the four strips. The results show that the multiband A-EIT effect can be tuned by voltage on LC and graphene layer, respectively. Moreover, changing the incident angle of the electromagnetic wave has had little influence on the transmission window in the low frequency band, it is meaning that the A-EIT effect with insensitive to the incident angle can be obtained. Each transmission window has a high maximum transmittance and figure of merit (FOM). The multi-band A-EIT effect can widen the application on sensor and optical storage devices.

Dynamical manipulation of a dual-polarization plasmon-induced transparency employing an anisotropic graphene-black phosphorus heterostructure

Dynamical tunable plasmon-induced transparency (PIT) possesses the unique characteristics of controlling light propagation states, which promises numerous potential applications in efficient optical signal processing chips and nonlinear optical devices. However, previously reported configurations are sensitive to polarization and can merely operate under specific single polarization. In this work we propose an anisotropic PIT metamaterial device based on a graphene-black phosphorus (G-BP) heterostructure to realize a dual-polarization tunable PIT effect. The destructive interference coupling between the bright mode and dark modes under the orthogonal polarization state pronounced anisotropic PIT phenomenon. The coupling strength of the PIT system can be modulated by dynamically manipulating the Fermi energy of the graphene via the external electric field voltage. Moreover, the three-level plasmonic system and the coupled oscillator model are employed to explain the underlying mechanism of the PIT effect, and the analytical results show good consistency with the numerical calculations. Compared to the single-polarization PIT devices, the proposed device offers additional degrees of freedom in realizing universal tunable functionalities, which could significantly promote the development of next-generation integrated optical processing chips, optical modulation and slow light devices.

Quadruple plasmon-induced transparency of polarization desensitization caused by the Boltzmann function

This study proposes a graphene metamaterial desensitized to the polarized angle to produce tunable quadruple plasmon-induced transparency (PIT). As a tool employed to explain the PIT, n-order coupled mode theory (CMT) is deduced for the first time and closely agrees with finite-difference time-domain (FDTD) simulations according to the quadruple PIT results in the case of n = 5. Additionally, the response of the proposed structure to the angle of polarized light is investigated. As a result, the Boltzmann function satisfied by the response of graphene strips to the polarization direction of incident light is proposed for the first time. Its property of polarization desensitization can be attributed to structural centrosymmetry, and conjugated variety which the Boltzmann functions result in. Therefore, a quintuple-mode modulation based on simultaneous electro-optical switch is realized by tuning Fermi levels within graphene. Its modulation degrees of amplitude and dephasing times are obtained. Given that the slow-light property is an important application of PIT, the n-order group index is thereby obtained. Hence, not only do the insights gained into polarization-desensitization structure provide new ideas for the design of novel optoelectronic devices, but also the results from the n-order CMT offer new research progress and references in theory.

Refractive index sensor with alternative high performance using black phosphorus in the all-dielectric configuration

We theoretically propose a nonplasmonic optical refractive index sensor based on black phosphorus (BP) and other dielectric materials in the infrared band. Due to the anisotropic property of BP, the proposed sensor can achieve alternative sensitivity and figure of merit (FOM) in its different crystal directions. The high sensitivity and FOM are attributed to the strong magnetic resonance in the all-dielectric configuration. The coupled-mode theory (CMT) is used to verify the simulation results and reveal the physical mechanism. Furthermore, influences of the sample and the incident angle on the performance of the sensor are also discussed. Our design utilizes a simple dielectric structure with a BP monolayer, which exhibits great potential for the future high-performance sensor with low cost.

Polarization independent and non-reciprocal absorption in multi-layer anisotropic black phosphorus metamaterials

The polarization independent and non-reciprocal absorption is particularly crucial for the realization of non-reciprocal absorption devices. Herein, we proposed and studied the absorption response of two- and three-layer anisotropic black phosphorus (BP) metamaterials by using the finite-difference time-domain (FDTD) simulation and radiation oscillator theory (ROT) analysis. It is shown that, due to unequal surface plasmon resonant modes excited in zigzag (ZZ) and armchair (AC) directions of the anisotropic BP layer, tunable polarization independent and dependent absorption can be achieved for the proposed multi-layer anisotropic BP metamaterials with AC-AC, AC-ZZ, ZZ-AC, AC-AC-φ, AC-ZZ-φ, and ZZ-AC-φ configurations. Especially, the polarization independent absorption also can be realized for odd-layer BP nanostructures. Unlike previous reports, polarization independence only can be achieved in the even-layer BP nanostructure. Moreover, tunable non-reciprocal absorption with the extremely large non-reciprocal degree (NRD) is also found in the case of AC-ZZ and ZZ-AC configurations and AC-ZZ-φ and ZZ-AC-φ configurations. These results may open up the possibility of realizing tunable polarization independent and non-reciprocal plasmonic devices based on 2D materials.

Planar metamaterial sensor with graphene elliptical rings in transmission mode

A periodic planar metamaterial sensor in the terahertz band based on surface plasmon polariton resonances is proposed and studied. The unit cell includes four half-elliptical graphene rings located on a three-layer substrate including a SiO₂ layer, an air gap, and another SiO₂ layer. The embedded air gap between the two layers of SiO₂ improves the sensitivity of the sensor. Parametric study is performed, and the effects of the dimensions of the elliptical rings, the air gap thickness, and the Fermi energy of graphene on resonant frequency, sensitivity, and figure of merit (FoM) are investigated and graphically illustrated. The parameters of the sensor are optimized to provide a high sensitivity with a suitable FoM. By changing the refractive index of the sensing environment from 1.2 to 2, maximum sensitivity of 21.1 µm/RIU with FoM 5.14 is provided. The performance of the sensor is compared with previous works, and it is shown that a considerable improvement in sensitivity is achieved. The proposed sensor is suitable for biosensing applications.

Slow-light analysis based on tunable plasmon-induced transparency in patterned black phosphorus metamaterial

In this paper, a tunable plasmon-induced transparency (PIT) structure based on a monolayer black phosphorus metamaterial is designed. In the structure, destructive interference between the bright and dark modes produces a significant PIT in the midinfrared band. Numerical simulation and theoretical calculation methods are utilized to analyze the tunable PIT effect of black phosphorus (BP). Finite-difference-time-domain simulations are consistent with theoretical calculations by coupled mode theory in the terahertz frequency band. We explored the anisotropy of a BP-based metasurface structure. By varying the geometrical parameters and carrier concentration of the monolayer BP, the interaction between the bright and dark modes in the structure can be effectively adjusted, and the active adjustment of the PIT effect is achieved. Further, the structure's group index can be as high as 139, which provides excellent slow-light performance. This study offers a new possibility for the practical applications of BP in micro-nano slow-light devices.

Dynamically controllable multi-switch and slow light based on a pyramid-shaped monolayer graphene metamaterial

Graphene, a new two-dimensional (2D) material, has attracted considerable attention in recent years because of the metallic characteristics at terahertz frequencies. The phase coupling of multilayer graphene-coupled grating structures is normally used to realize multiple plasmon-induced transparency (PIT) spectral responses. However, the device becomes more complicated with the increase in the number of graphene layers. In this work, we propose a five-step-coupled pyramid-shaped monolayer graphene metamaterial and predict a dynamically controllable PIT with four transparency peaks for the first time in the monolayer graphene metamaterial. A tunable multi-switch and good slow light effect is predicted over the wide PIT window, and the maximum modulation depth is high up to 16.89 dB, which corresponds to 97.95%, while the time delay of the induced transparent window is as high as 0.488 ps, where the corresponding group refractive index is 586. The electric field distributions and quantum level theory are used to explain the physical mechanism of the PIT with four transparency peaks. The coupled mode theory (CMT) is employed to establish the mathematical model of the PIT with four transparency peaks, and the consistency between the simulated and the calculated results is nearly perfect. We believe that the pyramid-shaped monolayer graphene metamaterial could be useful in efficient filters, switches, and slow light devices.

Dual-Band Plasmonic Perfect Absorber Based on the Hybrid Halide Perovskite in the Communication Regime

Due to the weak absorption of (CH3NH3)PbI3 in the communication regime, which restricts its optoelectronic applications, we design a adjustable dual-band perfect absorber based on the (CH3NH3)PbI3 to significantly enhance its absorption capability. Since the localized plasmon (LP) mode and surface plasmon (SP) mode are excited in the structure, which can both greatly enhance light absorption of the (CH3NH3)PbI3 layer, dual-band perfect absorption peaks are formed in the communication regime, and the absorption of (CH3NH3)PbI3 layer is increased to 43.1% and 64.2% at the dual-band absorption peaks by using finite-difference time-domain (FDTD) methods, respectively. By varying some key structural parameters, the dual-band absorption peaks of (CH3NH3)PbI3 can be separately shifted in a wide wavelength region. Moreover, the designed absorber can keep good performance under wide angles of incidence and manifested polarization correlation. Furthermore, not just for (CH3NH3)PbI3, the physical mechanism in this absorber can also be utilized to strengthen the absorption of other halide perovskites.

Terahertz multifunction switch and optical storage based on triple plasmon-induced transparency on a single-layer patterned graphene metasurface

A terahertz metasurface consisting of a graphene ribbon and three graphene strips, which can generate a significant triple plasmon-induced transparency (triple-PIT), is proposed to realize a multifunction switch and optical storage. Numerical simulation triple-PIT which is the result of destructive interference between three bright modes and a dark mode can be fitted by coupled mode theory (CMT). The penta-frequency asynchronous and quatary-frequency synchronous switches can be achieved by modulating the graphene Fermi levels. And the switch performance including modulation depth (83.5% < MD < 93.5%) and insertion loss (0.10 dB < IL < 0.26 dB) is great excellent. In addition, the group index of the triple-PIT can be as high as 935, meaning an excellent optical storage is achieved. Thus, the work provides a new method for designing terahertz multi-function switches and optical storages.

Polarization-sensitive triple plasmon-induced transparency with synchronous and asynchronous switching based on monolayer graphene metamaterials

A monolayer graphene metamaterial comprising four graphene strips and four graphene blocks is proposed to produce triple plasmon-induced transparency (PIT) by the interaction of three bright modes and one dark mode. The response of the proposed structure is analyzed by using couple mode theory and finite-difference time-domain simulations, with the results of each method showing close agreement. A quadruple-mode on-to-off modulation based on synchronous or asynchronous switching is realized by tuning the Fermi levels in the graphene, its modulation degrees of amplitude are 77.7%, 58.9%, 75.4%, and 77.6% corresponding to 2.059 THz, 2.865 THz, 3.381 THz, and 3.878 THz, respectively. Moreover, the influence of the polarized light angle on triple-PIT is investigated in detail, demonstrating that the polarization angle affects PIT significantly. As a result, a multi-frequency polarizer is realized, its polarization extinction ratios are 4.2 dB, 7.8 dB, and 12.5 dB. Combined, the insights gained into the synchronous or asynchronous switching and the polarization sensitivity of triple-PIT provide a valuable platform and ideas to inspire the design of novel optoelectronic devices.

Numerical investigation of an ultra-broadband, wide-angle, and polarization-independent metasurface light absorber

In this paper, we propose and numerically investigate an ultra-broadband, wide-angle, and polarization-independent metasurface absorber based on periodic hexagon-latticed titanium (Ti) nanoring arrays over a continuous Ti film. The proposed absorber can achieve more than 90% absorptivity under normal incidence, ranging from 350 to 1453 nm, and the average absorption is up to 95.6%. Additionally, the absorptivity still remained beyond 70% when the incident angles varied from 0° to 60°. The simulations of electric field distributions indicate that the broadband absorption performance can be ascribed to the superposition of the localized surface plasmon resonance (LSPR) originated from the nanopillars and nanoholes, respectively. The proposed approach is simple and inexpensive, and the metal material is optional. Therefore, we believe that the proposed absorber will be a candidate for many potential applications, such as thermophotovoltaic cells, thermal emitters, and optoelectronic devices.

Radiative thermal switch driven by anisotropic black phosphorus plasmons

Black phosphorus (BP), as a two-dimensional material, has exhibited unique optoelectronic properties due to its anisotropic plasmons. In the present work, we theoretically propose a radiative thermal switch (RTS) composed of BP gratings in the context of near-field radiative heat transfer. The simply mechanical rotation between the gratings enables considerable modulation of radiative heat flux, especially when combined with the use of non-identical parameters, i.e., filling factors and electron densities of BP. Among all the cases including asymmetric BP gratings, symmetric BP gratings, and BP films, we find that the asymmetric BP gratings possess the most excellent switching performance. The optimized switching factors can be as high as 90% with the vacuum separation d=50 nm and higher than 70% even in the far-field regime d=1 µm. The high-performance switching is basically attributed to the rotatable-tunable anisotropic BP plasmons between the asymmetric gratings. Moreover, due to the twisting principle, the RTS can work at a wide range of temperature, which has great advantage over the phase change materials-based RTS. The proposed switching scheme has great significance for the applications in optoelectronic devices and thermal circuits.

Optical Fermi level-tuned plasmonic coupling in a grating-assisted graphene nanoribbon system

A novel graphene-based grating-coupled metamaterial structure is proposed, and the optical response of this structure can be obviously controlled by the Fermi level, which is theoretically regulated by the electric field of an applied voltage. The upper graphene monolayer can be intensely excited with the aid of periodic grating and thus it can be considered a bright mode. Meanwhile, the lower graphene monolayer cannot be directly excited, but it could be indirectly activated by the help of bright mode. The plasmonic polaritons resulting from the light-graphene interaction resonance can lead to a destructive interference effect, leading to a plasmonic induced transparency. This structure has a simple construction and retains the integrity of graphene. In the meantime, it can achieve a good tuning effect by adjusting the voltage regulation of microstructure array and it can obtain an outstanding reflection efficiency. Thus, this graphene-based metamaterial structure with these properties is very suitable for the plasmonic optical reflector. In contacting with the characteristics of material, the group delay of this device can reach to 0.3ps, which can well match the slow light performance. Therefore, the device is expected to make some contribution in optical reflection and slow light devices.

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.

Multifunctional and tunable trigate graphene metamaterial with "Lakes of Wada" topology

Many plasmon-induced transparency (PIT) metamaterials previously reported had limited functions. Their tunabilities were realized by complex discrete structures, which greatly increased the difficulty and cost of device fabrication and adversely affected their resonance characteristics. It is an open question to adjust the Fermi levels of many graphene patterns with only a few in-plane electrodes. We propose and numerically study a novel electrically tunable and multifunctional trigate graphene metamaterial (TGGM) based on the concept of "Lakes of Wada". Benefiting from the trigate regulation, our proposed TGGM turns out to exhibit excellent characteristics, that can not only be used for terahertz band-stop filter, terahertz refractive index sensor, near-field optical switch, slow-light device, but also for double PIT window metamaterial with broad transparency windows and large tunable frequency range.

Two Switchable Plasmonically Induced Transparency Effects in a System with Distinct Graphene Resonators

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.

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.