Plasmon-induced Transparency Window Research Articles

Symmetry-Engineered Dual Plasmon-Induced Transparency via Triple Bright Modes in Graphene Metasurfaces

Dynamical manipulation of plasmon-induced transparency (PIT) in graphene metasurfaces is promising for optoelectronic devices such as optical switching and modulating; however, previous design approaches are limited within one or two bright/dark modes, and the realization of dual PIT windows through triple bright modes in graphene metasurfaces is seldom mentioned. Here, we demonstrate that dual PIT can be realized through a symmetry-engineered graphene metasurface, which consists of the graphene central cross (GCC) and graphene rectangular ring (GRR) arrays. The GCC supports a bright mode from electric dipole (ED), the GRR supports two nondegenerate bright modes from ED and electric quadrupole (EQ) due to the C2v symmetry breaking, and the resonant coupling of these three bright modes induces the dual PIT windows. A triple coupled-oscillator model (TCM) is proposed to evaluate the transmission performances of the dual PIT phenomenon, and the results are in good agreement with the finite-difference time-domain (FDTD) method. In addition, the dual PIT windows are robust to the variation of the structural parameters of the graphene metasurface except for the y-directioned length of the GRR. By changing the carrier mobility of graphene, the amplitudes of the two PIT windows can be effectively tuned. The alteration of the Fermi level of graphene enables the dynamic modulation of the dual PIT with good performances for both modulation degree (MD) and insertion loss (IL).

Multi-channel switch and sensing applications based on multilayered annular graphene metamaterials at terahertz frequency

In this study, a novel silica-graphene–silica periodic graphene structure consisting of six graphene semi-rings is proposed. The structure is based on a three-layer graphene metamaterial with a semicircular ring that achieves a tunable double plasmon-induced transparency (PIT) effect. In the proposed structure, the double-PIT window can be switched simultaneously at multiple frequencies through the dynamic tunability of graphene. Besides, the sensitivities of the refractive index for the PIT windows are investigated with the maximum values of 1.42 THz RIU−1 and 1.09 THz RIU−1, respectively, indicating the structure’s performance as a terahertz sensor. Overall, it shows the potential of PIT effect in graphene metamaterials in controlling electromagnetic field responses. It has made positive contributions to the development of terahertz technology and related fields.

Terahertz refractive index sensor based on dual plasmon-induced transparency in a graphene metasurface

A novel graphene metasurface is proposed in this article, which is simulated by the finite difference time domain (FDTD) and found to exhibit a significant dual Plasmon-induced transparency (PIT) phenomenon in the terahertz frequency band. For further research, new transmission spectra are simulated using the coupled mode theory (CMT), and after comparison, it is found that the images were basically consistent and had a high degree of agreement. In addition, through observation and analysis of the PIT window, it is found that the PIT phenomenon can be effectively tuned by adjusting the Fermi level. Finally, the metasurface is proposed in this article, which has excellent sensing properties. After calculation and comparison of different refractive indices of surrounding media, it is found that the maximum sensitivity can reach 1.567 THz RIU−1 at a frequency of f = 4.8716 THz, with a figure of merit (FOM) of 23.855. Compared with other sensors of the same type, the sensor in this paper has great advantages. In summary, the graphene metasurface proposed in this article provides more theoretical support for manufacturing dynamically adjustable graphene sensors.

Double plasmon-induced transparency 3 bit graphene encoder

In this paper, a 3 bit encoder was proposed which was based on double plasmon-induced transparency (PIT) effect. This structure consists of the bright-bright mode and realizes a double PIT window in the 0.1–3 THz range. Electric field distributions reveal that the double PIT effect is originated from the strong destructive interference between three bright modes, the Lorentz oscillation coupling model is utilized to confirm the finite-integration time-domain (FITD) simulation regardless of the x-polarization direction or y-polarization direction. When the incident light is x-polarized, encoding frequencies were at 0.976 THz, 1.525 THz and 2.069 THz, respectively; while in y-polarization, encoding frequencies were at 1.579 THz, 1.935 THz and 2.589 THz, respectively. Results depict that the Modulation Depth (MD) could reach 98 % regardless of the x-polarization direction or the y-polarization direction. As an electro-optical switch, the maximum MD could reach ≥98 %, the minimum IL could reach 0.35 dB, the maximum ER could reach ≥14 dB. Besides the encoder, the proposed structure is of great importance for the design of optical switches, terahertz modulators and slow light devices.

Terahertz sensor based on plasmon-induced transparency in a carbon nanotube metamaterial

In this paper, the plasmon-induced transparency (PIT) effect based on a carbon nanotube (CNT) resonator structure is achieved. An array of two split ring resonators (SRRs) and a cut wire (CW) resonator are utilized to form the proposed metamaterial. A PIT transparency window is achieved under a TM polarization terahertz light. Results show that the PIT effect is originated from the near-field coupled of the bright mode and dark mode. A coupled harmonic oscillator model is used to describe the near-field coupling between the bright mode and subradiant mode, and the results agree well with the FDTD simulation. The effect of geometrical sizes, like structure period, the radius and the splitting degree of the split ring resonator, the length of the cut wire resonator, and the coupling distance on the PIT window is analyzed in detail. Besides, the sensing and slow light performance of the proposed CNT metamaterial are studied, a maximum sensitivity of 0.74 THz RIU, and a time delay of 0.54 ps are obtained. Therefore, the proposed CNT-based device can be applied to the PIT effect, near-infrared modulators, slow light devices, sensors, and other fields.

Active control of broadband plasmon induced transparency window and amplitude modulation based on embedded-vanadium-dioxide metamaterials

A Vanadium-Dioxide (VO2)-embedded metamaterial is designed to achieve amplitude modulation and on-off switch at plasmon induced transparency window. With conductivity of VO2 across the phase transition by thermal and electrical stimuli, mode switch between PIT and dipole mode can be achieved. The amplitude modulation depth and the maximum frequency width at half-maximum are 77.58% and 0.4 THz, respectively. Such VO2 embedded PIT modulator is useful for manipulating terahertz radiation in various practical applications.

A polarization-insensitive dual plasmon-induced transparency terahertz sensor based on graphene metamaterial

We present a polarization-insensitive dual plasmon-induced transparency (PIT) sensor based on graphene metamaterial in the terahertz band. The double PIT window results from the coupling of the bright mode and quasi-dark modes, and the theoretical results fit well with the simulated results by using the three-particle coupling system. The sensor exhibits excellent sensing performance with sensitivity and the figure of merit (FOM) of the two transmission peaks can reach 1.3 THz/RIU and 6.259 RIU−1. The sensor is insensitive to the changes in the polarization angle, and the transmission spectra of the sensor remain essentially constant in two polarization directions for incident angles less than 60°, which reduces the impact of the experimental environment on the results. The resonant frequency of the PIT window can be turned by adjusting the Fermi level of graphene. The sensor proposed in this paper is an active advance in improving the reliability of analyte detection results.

THz plasmon-induced transparency metasurfaces based on metal-graphene hybrid structure for high-sensitive amino acid disease-marker biosensing

Terahertz metasurface sensors have attracted great interest in recent years due to their fast and non-destructive detection. One of the main research hotspots is to further improve sensing sensitivity, i.e., to achieve accurate discrimination with minimal amounts of marker targets for fast disease diagnosis. Graphene is a two-dimensional material with unique properties, for example, its electrical properties can be manipulated by the contact biomolecules, providing excellent sensing performance. In this paper, a metal-graphene hybrid metasurface is proposed and demonstrated for high-sensitive biosensing based on the plasmon-induced transparency (PIT) resonances. Experimental results show that 0.8 ng of the tyrosine enhances the PIT window to 4.8-fold, with the limit of detection down to 10 ng/mL. And with increasing amino acid quantities, three amino acids can be high-effectively distinguished to diagnose tyrosinemia, phenylketonuria, and hyperglycinemia. This metasurface sensor has the advantages of high sensitivity, fast detection speed, label-free and steady properties, which has potential applications in the fields of trace molecular sensing and disease diagnosis.

Multiple plasmon-induced transparency based on black phosphorus and graphene for high-sensitivity refractive index sensing.

A hybrid bilayer black phosphorus (BP) and graphene structure with high sensitivity is proposed for obtaining plasmon-induced transparency (PIT). By means of surface plasmon resonance in the rectangular-ring BP structure and ribbon graphene structure, a PIT effect with high refractive index sensitivity is achieved, and the surface plasmon hybridization between graphene and anisotropic BP is analyzed theoretically. Meanwhile, the PIT effect is quantitatively described using the coupled oscillator model and the strong coherent coupling phenomena are analyzed by adjusting the coupling distance between BP and graphene, the Fermi level of graphene, and the crystal orientation of BP, respectively. The simulation results show that the refractive index sensitivity S = 7.343 THz/RIU has been achieved. More importantly, this is the first report of tunable PIT effects that can produce up to quintuple PIT windows by using the BP and graphene hybrid structure. The high refractive index sensitivity of the quintuple PIT system for each peak is 3.467 THz/RIU, 3.467 THz/RIU, 3.600 THz/RIU, 4.267 THz/RIU, 4.733 THz/RIU and 6.133 THz/RIU, respectively, which can be used for multiple refractive index sensing function.

Graphene-enhanced hybrid terahertz metasurface sensor for ultrasensitive nortriptyline sensing and detection.

Graphene is a two-dimensional material with unique physical and chemical properties, whose excellent biocompatibility has also attracted widespread attention in the field of biosensing and medical detection. Graphene provides a novel solution for dramatically improving the sensitivity of terahertz metasurface sensors, since the electrical conductivity can be modified by contact with biomolecules. In this paper, a metal-graphene hybrid metasurface is proposed and demonstrated for high-sensitive nortriptyline sensing based on the plasmon-induced transparency (PIT) resonances. The π-π stacks between nortriptyline and graphene lead to an increase in the Fermi level of graphene and a decrease in the conductivity, thus enhancing the PIT resonance. Experimental results show that the peak-to-peak amplitude magnitude of the PIT window is enhanced up to 3.4-fold with 1 ng nortriptyline analyte, and the minimum detection limit is extended down to 0.1 ng. But no significant change is observed from the samples without graphene as a comparative experiment, which demonstrates that the presence of graphene greatly enhances the bonding to the drug molecules and improves the sensing sensitivity. This metasurface sensor has the advantages of high sensitivity, fast detection speed, label-free and steady properties, which has potential applications in the fields of trace molecular sensing and disease diagnosis.

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.

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.

Plasmon-induced transparency in a reconfigurable composite valley photonic crystal.

We propose a new kind of reconfigurable topological valley photonic crystal (TVPC), and a novel topological waveguide can be formed by constructing a domain wall between two TVPCs with opposite valley-Chern indices. The topological waveguide mode in the composite TVPC has large group refractive index. A topologically protected coupled waveguide cavity system is then designed by introducing a hexagonal ring cavity at the center of the straight domain wall of a combined TVPC, in which a narrow plasmon induced transparency window rises at 3.8848 GHz with a Q-factor of 1387 and a maximum group refractive index as high as 186. We propose a notch filter with a resonant frequency of 3.8852 GHz and a very high Q-factor of 10224. By changing the refractive index of liquid crystals via an external voltage applied between two parallel metal plates, the filter can be switched between band-pass and band-stop based on the reconfigurable topological interface state.

Quadruple plasmon-induced transparency and tunable multi-frequency switch in monolayer graphene terahertz metamaterial

A monolayer graphene metamaterial composed of a graphene block and four graphene strips, which has the metal-like properties in terahertz frequency range, is proposed to generate an outstanding quadruple plasmon-induced transparency (PIT). Additional analyses show that the forming physical mechanism of the PIT with four transparency windows can be explained by strong destructive interference between the bright mode and the dark mode, and the distributions of electric field intensity and electric field vectors under the irradiation of the incident light. Coupled mode theory and finite-difference time-domain method are employed to study the spectral response characteristics of the proposed structure, and the theoretical and simulated results are in good agreement. It is found that a tunable multi-frequency switch and excellent optical storage can be achieved in the wide PIT window. The maximum modulation depth is up to 99.7%, which corresponds to the maximum extinction ratio of 25.04 dB and the minimum insertion loss of 0.19 dB. In addition, the time delay is as high as 0.919 ps, the corresponding group refractive index is up to 2755. Thus, the proposed structure provides a new method for the design of terahertz multi-frequency switches and slow light devices.

Dynamically tunable plasmon-induced transparency effect based on graphene metasurfaces

Plasmon-induced transparency (PIT) is theoretically explored for a graphene metamaterial using finite-difference time-domain numerical simulations and coupled-mode-theory theoretical analysis. In this work, the proposed structure consists of one rectangular cavity and three strips to generate the PIT phenomenon. The PIT window can be regulated dynamically by adjusting the Fermi level of the graphene. Importantly, the modulation depth of the amplitude can reach 90.4%. The refractive index sensitivity of the PIT window is also investigated, and the simulation results show that a sensitivity of 1.335 THz RIU−1 is achieved. Additionally, when the polarization angle of the incident light is changed gradually from 0° to 90°, the performance of the structure is greatly affected. Finally, the proposed structure is particularly enlightening for the design of dynamically tuned terahertz devices.

Dual-Spectral Plasmon-Induced Transparent Terahertz Metamaterial with Independently Tunable Amplitude and Frequency

A bifunctional tunable metamaterial composed of pattern metal structure, graphene, and strontium titanate (STO) film is proposed and studied numerically and theoretically. The dual plasmon-induced transparency (PIT) window is obtained by coupling the bright state cut wire (CW) and two pairs of dark state dual symmetric semiring resonators (DSSRs) with different parameters. Correspondingly, slow light effect can also be realized. When shifting independently, the Fermi level of the graphene strips, the amplitudes of the two PIT transparency windows and slow light effect can be tuned, respectively. In addition, when independently tuning the temperature of the metamaterial, the frequency of the dual PIT windows and slow light effect can be tuned. The physical mechanism of the dual-PIT was analyzed theoretically by using a three-harmonic oscillator model. The results show that the regulation function of the PIT peak results from the change of the oscillation damping at the dark state DSSRs by tuning conductivity of graphene. Our design presents a new structure to realize the bifunctional optical switch and slow light.

Dynamically tunable terahertz sensors based on dual-layered graphene metamaterial

Plasmon-induced transparency (PIT) based sensors have attracted various attention for application in environmental monitoring, medical samples detection, food safety, and biochemical applications due to it is capable of detecting the minute changes in the refractive index. However, restricted by the limitation of the structure complexity and low sensing performance, the practical large-scale application of PIT-based sensors still remains a great challenge. Herein, we propose a simple dual-layer graphene metamaterial in which the continuity of the graphene layer is retained, providing a lot of convenience for the manufacturing of the device. Coupled mode theory (CMT) is used to demonstrate the physical mechanism of PIT. Particularly, The PIT windows delivered by this device can be modulated effectively by simply changing the Fermi energy and the parameter of the graphene strips. What is more, the relationship between the sensing performance and parameters of the device is described by charts. It is found that the device exhibits a high sensitivity of 15739 nm/RIU and a maximum figure of merit (FOM) of 469. The current work may pave the way for the further research on the applications and designs of nanosensors used in highly integrated optical circuits.

Enhanced second-order nonlinearity and tunable plasmon induced transparency in noncoplanar Dirac semimetal system

Plasmon Induced Transparency (PIT) effect possesses the characteristics of sharp and pronounced spectral response and considerable group delay. In this work, a PIT system consists of two noncoplanar Dirac semimetal particles (DSPs) has been studied by finite-difference time-domain (FDTD) method. Tailoring of this PIT system can be achieved by structural parameters, Fermi energy (EF), refractive index (RI) of dielectric medium and polarization angle of incident light. Benefitted from its smaller particle size, the bandwidth of PIT window in this DSPs system is up to 3.94 THz, which is much broader than in bulk Dirac semimetal PIT system (about 0.36 THz). An efficient second order nonlinearity of this DSPs system is obtained for its intensive localized field, non-centrosymmetric structureunique and unique electromagnetic mode. The second-order nonlinear conversion efficiency in this system is up to 10−6 to 10−5 for its noncoplanar structure. This noncoplanar DSPs system can also achieve approximate linear light intensity modulation effect of its second harmonic waves (SHWs). This developed noncoplanar DSPs structure could provide guidance for the design of high integration Dirac semimetal THz devices.

Dynamically tunable plasmon-induced transparency in waveguide based on Dirac semimetal

We report a numerical and theoretical study of the plasmon-induced transparency (PIT) effect in bulk Dirac semimetal (BDS) waveguide. This plasmon waveguide system is composed of a bus waveguide side coupled with two stub photoactive silicon resonators. The coupled mode theory (CMT) is introduced to analyze this system, and the theoretical results are perfectly consistent with the numerical calculation results. The numerical results show that the spectral characteristics of the structure depend on the coupling distance between the two stub photoactive silicon resonators. Intriguingly, the resonance characteristics of the transparent window can be actively controlled by adjusting the Fermi energy of the BDS or the intensity of the incident pump light. Finally, the group time delay of the PIT window can reach 3.7 ps, which is more suitable for applications with slow-light devices. Such a simple and multi-functional plasmon system is more conducive to the potential applications of composite high-performance devices.