Plasmon-induced Transparency Research Articles

Multifunctional plasmonic waveguide system based on coding metamaterials and inverse design

The benefits of low-power consumption, high throughput and high parallel processing suggest that optical computing has great potential for information processing. In this article, we propose a multifunctional plasmonic waveguide system (MPWS) to implement various optical functions based on metal coding metamaterials and inverse design technology. Due to the flexible and multivariant manipulation of electromagnetic waves with coding metamaterials, the MPWS can be utilized as an ultra-compact multifunctional component in integrated optical processors for optical computing. Because every optical component is designed by a same scheme, the MPWS avoids additional connection loss and waveguide coupling region caused by inconsistent interfaces and footprints. Driven by strong constraint of electromagnetic waves and overcoming the diffraction limit, the size of the core area in MPWS is only 400 nm × 500 nm. To demonstrate the effectiveness of the MPWS, we use it to implement four typical filtering functions and plasmon-induced transparency (PIT)-like effects. Especially, the Q-factor of PIT-like effects reaches to 58.13. Meanwhile, the simulation results optimized by inverse design technology show high consistency with that theoretically calculated by the combination of coupled-mode theory and the transfer-matrix method. Compared to previous works, our proposed MPWS not only provides a universal scheme to implement various optical information processing functions for optical computing but also shows the applications of inverse design on photonic devices.

Dynamic manipulation of plasmon induced transparency with parallel-orthometric graphene strips structure

We put forward an uncomplicated and novel graphene structure, which produces single and dual dynamic tunable plasmon induced transparency (PIT) in terahertz frequency through the coupling of bright-bright modes and bright-dark modes. The structure is studied theoretically by using the coupled mode theory (CMT), which is consistent with the finite difference time domain (FDTD) simulation one. The Fermi level of graphene is investigated to dynamically control the transmission of the PIT peak at the center frequency, and the modulation depth of this switch effect is up to 89%. Besides, the bandwidth and frequency of the PIT can also be altered through the Fermi level. Furthermore, multiple switching effects are achieved with the modulation depths of 57%, 67%, and 61%, respectively. We believe that this structure has a good application prospect in the communication and sensing areas.

A Tunable Terahertz Graphene Metamaterial Sensor Based on Dual Polarized Plasmon-Induced Transparency

We present a terahertz graphene metamaterial sensor based on dual polarized plasmon-induced transparency caused by the destructive interference between the bright mode and the two quasi-dark modes in the terahertz (THz) band. In addition, the sensor can achieve frequency modulation by adjusting the polarization direction of the THz wave and the Fermi level of graphene. Importantly, the sensor also has high sensitivity at each polarization direction, and the sensitivity of dual transmission peaks with x-polarization direction can reach about 1.1 THz/RIU (Refractive Index Unit). Furthermore, the theoretical results of the proposed three-particle model are in good agreement with the simulated transmission spectra. The sensor is also insensitive to the change of incident angles, and the transmission spectra of the sensor can remain roughly unchanged with the incident angle less than 60°, which is beneficial to the high-speed and high-sensitivity detection in a complex environment. Therefore, the proposed graphene metamaterial sensor exhibits numerous potential applications in THz biochemical sensing.

Dynamic modulation of dual plasmon-induced transparency based on coupled graphene grating-sheets

We propose to achieve dual plasmon-induced transparency effects through the couplings between a bright mode and two dark modes in a metamaterial system with two graphene sheets separated by periodic graphene gratings. We find that both the number and the positions of the transparent windows can be modulated by tuning the Fermi energy and coupling distance between graphene resonators, which allows us to design a high-efficiency switch with an on/off state modulation amplitude of 71.1%. A three-particle model-based theoretical model is applied to explain the simulated results and excellent agreement between them is found.

Dynamically tunable multiband plasmon-induced transparency effect based on graphene nanoribbon waveguide coupled with rectangle cavities system

A dynamically tunable multiband plasmon-induced transparency (PIT) effect in a series of rectangle cavities coupled with a graphene nanoribbon waveguide system is investigated theoretically and numerically by tuning the Fermi level of the graphene rectangle cavity. A single-PIT effect is realized using two different methods: one is the direct destructive interference between bright and dark modes, and the other is the indirect coupling through a graphene nanoribbon waveguide. Moreover, dual-PIT effect is obtained by three rectangle cavities side-coupled with a graphene nanoribbon waveguide. Results show that the magnitude of the dual-PIT window can be controlled between 0.21 and 0.74, and the corresponding group index is controlled between 143.2 and 108.6. Furthermore, the triple-PIT effect is achieved by the combination of bright–dark mode coupling and the cavities side-coupled with waveguide mechanism. Thus, sharp PIT windows can be formed, a high transmission is maintained between 0.51 and 0.74, and the corresponding group index is controlled between 161.4 and 115.8. Compared with previously proposed graphene-based PIT effects, the size of the introduced structure is less than 0.5 μm2. Particularly, the slow light effect is crucial in the current research. Therefore, a novel approach is introduced toward the realization of optical sensors, optical filters, and slow light and light storage devices with ultra-compact, multiband, and dynamic tunable.

Controlled and tunable plasmon-induced transparency based on graphene metasurfaces in atmospheric windows

Considering the development of plasmon-induced transparency functional devices and the research value of the atmospheric windows in many civilian and military applications, we propose a mid-infrared metasurface composed of a multilayer elliptical graphene and two graphene strips that can generate PIT, and the PIT can be dynamically tuned in the range of 3.67 μm–9.53 μm beyond monolayer graphene. The numerical results from Finite-Difference Time-Domain (FDTD) method are consistent with the analytical results from Lorentz oscillator theory. In addition, we can control the generation and vanishment of the PIT by changing the polarization of the source, achieving a triple-frequency asynchronous switch, and the performance index indicates that the modulation depth is maintained at about 90% at a relatively high Fermi energy level. Our work achieves a dynamically tunable and controllable PIT for applications including filters and optical switches in atmospheric windows.

Folding-assisted plasmonically induced transparency in coupled graphene nanodisks

Plasmonically induced transparency (PIT), a classical analog of electromagnetically induced transparency in quantum systems, could offer an efficient way to engineer light–matter interaction and thus achieve particular manipulation of light. To realize PIT, the crucial point is the design of two strongly coupled modes, with one bright and the other dark. Common practice often takes dipole resonance as the bright mode, and higher-order resonance as the dark one. Here we present an alternative scheme to realize PIT in coupled graphene nanodisks through mechanical folding, where only fundamental dipole resonances are involved, and the dark mode is derived from two nondegenerate dipole resonances in a folded nanodisk. Two parameters (the folding angle and rotating angle ) arising from folding can be utilized to tune the PIT. They determine two different PIT windows separately. By varying either one of them, single-window PIT can be switched to double-window PIT, and vice versa. In addition to mechanical operations, the proposed PIT can also be actively tuned by electrostatic gating. To properly understand the mechanism, a theoretical model of two coupled harmonic oscillators is presented, from which the predictions show good agreement with simulations.

Tunable Multi-band Switch with Uncoupled Graphene-based Metamaterial Patches

We present the tunable multiple plasmon-induced transparency (PIT) in the terahertz region by using a metamaterial made of two graphene bands and a graphene square ring. As the different modes of multiple PIT effects are independent of each other, the physical mechanism behind multiple PIT effects can be revealed by CMT theory. The PIT window changes significantly with the Fermi energy levels and structural parameters of graphene. The both three resonant frequencies increase linearly with the parameters and the Fermi energy changing, which can exhibit high sensitivities and figure of merit (FOM). Meanwhile, the amplitude modulation system can reach 99.63%, which can achieve excellent photoelectric switching. In addition, the group index can be as high as 2739. Therefore, the graphene-based metamaterial could be widely used in switches, modulators, excellent slow-light functional devices and filters in the terahertz region.

Multiple and tunable plasmon induced transparency with L-shape graphene strips structure at terahertz frequency

Firstly, in this paper, the proposed structure is composed of a transverse graphene rectangular cavity and a vertical graphene rectangular cavity, so as to produce the plasmon-induced transparency (PIT) phenomenon. The PIT of graphene metamaterials is theoretically explored by using the finite-difference time-domain (FDTD) simulation and the coupled-mode-theoretical (CMT) analysis. Then, by adjusting the Fermi level of graphene, the PIT window is dynamically adjusted, so that the structure can be used as an optical switch. Furthermore, the refractive index sensitivity of the PIT window is investigated and the simulation results show that a sensitivity of 2.725 THz/RIU is achieved. In addition, multiple PIT windows are also generated by adding additional matched L-type graphene strips. Finally, the proposed structure may enlighten the developments of dynamically tuned terahertz devices.

Nonlinear modulation of terahertz waves based on a MAPbI3/Gold/Si Hybrid Plasmon-Induced Transparency (PIT) metasurface

As a kind of artificial electromagnetic device, metasurface is applicable in many areas for its distinctive characteristics. Plasmon-Induced Transparency (PIT) is a typical application of metasurface, with special electromagnetic characteristics such as high Q-factor and slow light effect. However, most of the current work has paid little attention to the ability of nonlinear manipulation of PIT metasurface. Using the excellent photoconductivity effect of MAPbI3 Perovskite, it is possible to design optically controlled nonlinear modulation devices based on a hybrid PIT metasurface and achieve an effective manipulation of terahertz waves. In this paper, a MAPbI3-Gold-Si hybrid metasurface is designed and fabricated to achieve a high Q-factor PIT effect. The experiment implies that the PIT metasurface can be actively and nonlinearly modulated with high efficiency after spin-coating the MAPbI3 film. In addition, the hybrid PIT metasurface is compatible with integrated circuit (IC) technology, hence it may provide a potential way in the region of terahertz communication, active filters, highly sensitive sensors and slow-light devices.

Unified model for plasmon-induced transparency with direct and indirect coupling in borophene-integrated metamaterials.

We propose, both numerically and theoretically, a uniform model to investigate the plasmonically induced transparency effect in plasmonic metamaterial consisting of dual-layer spatially separated borophene nanoribbons array. The dynamic transfer properties of light between two borophene resonators can be effectively described by the proposed model, with which we can distinguish and connect the direct and indirect coupling schemes in the metamaterial system. By adjusting the electron density and separation of two borophene ribbons, the proposed metamaterials enable a narrow band in the near-infrared region to reach high transmission. It provides a new, to the best of our knowledge, platform for optoelectronic integrated high-performance devices in the communication band.

Tunable narrow transparency windows induced by the coupled quasi-guided modes in borophene plasmonic nanostructure

A grating-assisted triple-layer continuous borophene sheets structure is presented to investigate the phase-coupled plasmon-induced transparency (PIT) effect at the near-infrared communication band. In the proposed structure, the diffraction grating is utilized to couple the normal incident waves, which excites the quasi-guided mode in triple-layer continuous borophene sheets. Compared to the patterned structure, the requirement of complicated processing techniques and extra doping configurations for tuning carrier density can be avoided by using the continuous borophene sheet. Additionally, the narrower transparency windows are shown in the spectral responses via the phase coupling between two detuned quasi-guided modes with high quality factors. The transparency windows can be efficiently tuned by adjusting the carrier densities in borophene sheets or the separation distances between the borophene sheets. The analytical results based on temporal coupled mode theory and transfer matrix method are consistent with the numerical simulations via finite-difference time-domain calculations. The proposed scheme provides the promising design of a tunable PIT component that is compatible with optical communication technology.

Graphene and metal hybridized terahertz metasurfaces toward tunable plasmon-induced transparency effects

Tunable metasurfaces have attracted much attention recently, owing to their capability of actively controlling electromagnetic waves. Here, we propose tunable graphene-metal hybridized terahertz (THz) metasurfaces exhibiting plasmon induced transparency (PIT) effects, driven alternately by metal and graphene resonators. The proposed meta-molecule comprises a central metal (graphene) strip placed orthogonally to two side graphene (metal) strips. Then, near-field coupling between the strips is probed by translating the central strip along the length of the side strips and the evolution of the PIT effect is investigated for orthogonal polarizations of probing THz. The coupled oscillator-based analytical model explains the coupling mechanism in these metasystems. We observe the strongest coupling and the most prominent PIT effect for edge-to-edge coupling (threshold position) between the central and side strips. Further, active PIT response is achieved by altering graphene's Fermi energy. These metasurfaces can contribute in designing compact and active metasurface-based hybrid devices for THz domain.

Tunable plasmon-induced transparency in graphene-based plasmonic waveguide for terahertz band-stop filters

Terahertz (THz) tunable filters have great advantages in miniaturizing integrated and multiband communication systems. Here, THz band-stop filter with switchable single/double plasmon-induced transparency (PIT) based on metal-dielectric-metal waveguide consisting of a ring-stub cavity and two graphene ribbons (GRs) on stub sidewalls is proposed. The switchable characteristics of single and dual-PIT are investigated numerically and theoretically by the finite-difference time-domain and the radiating multi-oscillator theory, displaying good correspondence. The single-PIT is excited by the destructive interference between bright mode and dark mode, which possesses significant tunability of resonant frequency and transmission amplitude due to the existence of GRs. When independently regulating chemical potentials of GRs on the left and right sidewalls, the dual-PIT emerges. And the filter based on dual-PIT switches from single stopband to dual-stopband or even multi-stopband filtering. Besides, the band-stop filtering performance of the tunable PIT can be further optimized by increasing the number of ring-stub cavities. The tunable PIT in graphene-based plasmonic waveguide holds potential for THz multiband communications and subwavelength plasmonic devices, such as filters, switches, modulators.

Theoretical Design of Graphene-Based Bi-Functional Tunable Terahertz Metasurfaces

Tunable multifunctional metasurface has wide application such as optical electromagnetics and material science. In this paper, a terahertz (THz) metasurface based on double graphene split-ring resonators (GSRRs) are theoretically demonstrated, integrating dual-band absorption and plasmon-induced transparency (PIT) filtering effect. The structure is composed of a monolayer of graphene arrays with periodic patterns and a metal ground surface partitioned by a silicon dioxide dielectric layer. When the initial structure of unit cell is three-layer sandwich structure (bottom metal plate), its dual-frequency absorption spectra appears two peaks at 2.50 THZ and 3.38 THz, which are 99.98% and 97.94%, respectively. Then the mechanism of double band absorption is explained by analyzing the distribution of surface current and electric intensity of the absorbent material. When the initial arrangement of the cell is a double layer structure (without the bottom metal plate), the PIT effect will occur when the incident wave is y-polarized. And in a certain range to achieve more than 90% of the transmission. In addition, CST simulations demonstrate that the designed model supports changing the operating frequency by adjusting the Fermi energy of graphene The dual-function terahertz metasurface proposed in this work has broad application prospects in broadband communication, terahertz imaging and industrial sensors.

Tunable plasmon-induced transparency with coupled L-shape graphene metamaterial

A graphene-based metamaterial structure, consisting of a graphene strip (GS) and a L-shaped graphene-rectangular block (GRB), is proposed to generate the plasmon-induced transparency (PIT) effect. The potential physical properties of the PIT effect are analyzed by using the coupled mode theory (CMT). The PIT has the unique characteristics of controlling light propagation through the static and dynamic regulations, resulting in a prospective switching application. The performance of the optical switch is evaluated through different parameters, including the geometric size, Fermi level and polarization angle. The modulation depth of the amplitude can reach 74.9% with a specific Fermi level, while the maximum polarization extinction ratio can reach 11.34 dB. Furthermore, dual and triple PIT effects are achieved by the designing and optimizing the structure. The maximum multiple switching effect is obtained with a modulation depth of 97.3%, which has a promising prospect in terahertz optical switches.

Symmetry-protected bound states in the continuum in graphene nanoribbons

Bound states in the continuum (BICs) have emerged as a significant design principle for producing systems with high-quality (Q) factor states to enhance light–matter interactions. As a particular case, symmetry-protected BICs are flexible to be designed, commonly by utilizing two identical lossless dielectric elements. Herein, different from previous studies, we propose symmetry-protected BICs in a plasmonic structure of two contacting graphene nanoribbons (GNRs), in which two GNRs are not identical and lossy. We show that BICs are achieved when two GNRs are perpendicular to each other, and as the vertical GNR deviates from the vertical direction (inversion symmetry breaking), it will evolve into quasi-BICs, with a new resonance dip appearing in the transmission spectrum. The spectrum curve can be well described by the coupled-mode theory, from which the variation of two fundamental states is clearly seen. Since in the presence of internal loss, the Q-factor of quasi-BICs does not follow the linear formula that is generally valid for symmetry-protected BICs. Alternatively, an extended formula is derived, which predicts exactly the behavior of the Q-factor of quasi-BICs. Besides BICs, the structure can also support plasmonically induced transparency (PIT) like effects, through rotating the vertical GNR to a particular angle. Therefore, a mechanically tunable switch, from BIC to PIT, is achieved here. Our work demonstrates an alternative scheme for BICs, and a new degree of freedom for tuning plasmonic coupling related effects.

Tunable terahertz group slowing effect with plasmon-induced transparency metamaterial.

We present a tunable plasmon-induced transparency (PIT) metamaterial for manipulating the group velocity of terahertz (THz) waves. The metamaterial is composed of metal split rings and photoconductive silicon strips. The strong PIT effect with slowing down THz waves is generated by the bright-bright mode coupling between the high-order plasmon mode and the lattice surface mode via electromagnetic destructive interference. By varying the conductivity of silicon strips, the group slowing performance is dynamically tunable. The group delay can achieve beyond 20 ps with the group index as high as 592, showing the promising application for THz signal manipulation.

Quantum coherence-assisted optical properties and drag of SPPs on quantum dots and resonantly-coupled dots-metal plasmonic interfaces via interbands tunneling and Fano resonance

Owing to their peculiar properties of great fundamental and practical implications, surface plasmons and exciton-plasmons interaction have attracted immense interest in the recent decades. In this regard, quantum dots and dots-metal waveguides and interfaces are promising candidates. In this article, we report first theoretical demonstration on the detailed optical and optotransport properties and surface plasmon polaritons (SPPs) dragging via quantum dots interbands tunneling-induced quantum coherence. For the required properties of SPPs, we proposed a resonantly coupled dots-metal plasmonic interface, which comprises of ensemble of three-level double quantum dot molecules (DQDMs) placed on the flat surface of gold thin film. For the theoretical results, we first solved analytically the density-matrix equation for the eigenvalues of the DQDMs in the Λ− type configuration. In view of the dressed-states picture, we interpreted the results further with regard to EIT (electromagnetically induced transparency), PIT (plasmonic induced transparency) and Fano interference. In particular, we elaborate on the role of interplay of the quantum dots and the coupled plasmonic resonances on the optical, optical-cum transport properties and drag of SPPs. We predict ultrafast superluminal group velocity for SPPs of the order 1.5×1010 m/s, quality factor of 38.48, confinment factor 1.52 and the optimum drag for SPPs of order 0.2 micro meter/radian.

Dynamically Tunable Plasmon-Induced Transparency in Parallel Black Phosphorus Nanoribbons

In this paper, we investigate the dynamically tunable plasmon-induced transparency (PIT) effects in parallel black phosphorus nanoribbons (BPNRs). The proposed structures consist of several parallel BPNRs having different lengths in one transverse period. The periods of all proposed structures are set as \(P = 500 \, nm\). To our best knowledge, these proposed structures have not been reported in published papers. It is the first time that simulates PIT effects by this way. The results show that the BPNRs can be regarded as bright modes. Single-band, double-band, triple-band, and multi-band PIT effects based on the bright-bright mode coupling between parallel BPNRs are achieved. The physical mechanism of the single-band model can be explained theoretically by the radiating two-oscillator (RTO) model. Due to the heavier effective mass in the zigzag (ZZ) direction of the BP, the frequencies of the transparent peaks are shifted to lower frequencies when the placement directions of BPNRs are changed from the X-direction to the Y-direction. Furthermore, the resonant frequencies of the transparent windows in each model can be tuned by changing the relaxation rates of the BPNRs. The frequencies of the transparent windows are blue-shifted as the relaxation rates are increased. Finally, the corresponding sensors based on single-band PIT effect show high sensitivities of 7.35 THz/RIU. Our study has potential applications for improving the design of multiple-band filters, sensors, and on–off switcher.