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With the increasing demand on high-density integration and better performance of micro-nano optoelectronic devices, the operation temperatures are expected to significantly increase under some extreme conditions, posing a risk of degradation to metal-based micro-/nano-structured metasurfaces due to their low tolerance to high temperature. Therefore, it is urgent to find new materials with high-conductivity and excellent high-temperature resistance to replace traditional micro-nano metal structures. Herein, we have proposed and fabricated a thermally stable graphene assembly film (GAF), which is calcined at ultra-high temperature (~ 3000 ℃) during the reduction of graphite oxide (GO). Compared with micro-nano metals that usually degrade at around 550 ℃, the proposed GAF maintains a high extent of stability at an extremely high temperature up to 900 ℃. In addition, to make GAF a prime candidate to replace micro-nano metals, we have modified its fabrication process for improving its conductivity to 1.3 × 106 S/m, which is quite close to metals. Thus, micro-nano optoelectronic devices could retain high efficiency even when GAF replaces the crucial micro-nano metals. To verify the thermostability of optoelectronic devices composed of GAF, we have compared the high-temperature resistance performance of two structures capable of achieving plasmon-induced transparency (PIT) at the THz region, one using micro-nano metals (Aluminum) and the other GAF. The Al metasurface displayed a near-complete loss of PIT effects after a high-temperature treatment, while GAF could remain excellent PIT properties at above 900 ℃, thus enable to fulfil its optimum performance. Overall, the proposed thermostable metasurface provides new pathway for the construction of thermostable optoelectronic devices that can operate under ultra-high temperature scenario. [ABSTRACT FROM AUTHOR]

We study the plasmon-induced transparency (PIT) effect in the terahertz domain based on bulk Dirac semimetal (BDS) metamaterials constructed from a cut-wire and a C-shaped resonator pair. With the help of numerical simulations and coupled mode theory, we find that the depth of the transparent window can be adjusted by the coupling distance between the cut-wire and the C-shaped resonator pair. It is further shown that by changing the Fermi level of BDS, the PIT effect can be dynamically tuned without reconstructing the geometry. Simultaneously, the metamaterial structure has excellent sensing properties, which contributes to the theoretical design of bio-chemical sensors. Finally, we further analyze the PIT effect caused by a metamaterial system consisting of a cross-shaped and four C-shaped resonators. Under the illumination of normal incident light along the x- and y-axes in the polarization direction, the device not only produces a PIT effect but also exhibits a strong resonance response consistency for light in both polarization directions, indicating that it exhibits polarization independence for incident terahertz light. This work will provide potential design value for dynamically tunable polarization-independent sensing applications. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

Graphene can support surface plasmon polaritons (SPPs) in the terahertz band, and graphene SPP sensors are widely used in the field of terahertz micro- and nano-optical devices. In this paper, we propose an H-shaped graphene metasurface and investigate the plasmon-induced transparency (PIT) phenomenon in the proposed structure using the finite-difference time-domain (FDTD) method. Our results show that the Fermi energy levels, as well as certain shape parameters, can effectively modulate the PIT phenomenon in the proposed structure. Interestingly, changing some of these shape parameters can excite two dips into three. In terms of sensing performance, the maximum values of sensitivity and figure of merit (FOM) are 1.4028 THz/RIU and 17.97, respectively. These results offer valuable guidance for the use of terahertz optical graphene SPP sensors. [ABSTRACT FROM AUTHOR]

In this paper, by finite-difference-time-domain (FDTD) simulation, we numerically demonstrate a plasmon-induced transparency (PIT) effect based on a carbon nanotubes (CNT) metamaterial. The proposed CNT metamaterials are regular arrays of unit cells composed of a cut wire and two rectangular split ring resonators (RSRRs). Under the excitation of a TM-polarization terahertz wave, a single PIT transparency window is achieved for the symmetry CNT metamaterial. By breaking the symmetry of the proposed structure with different sizes of RSRRs, a double PIT window is realized. Results show that the PIT effect is originated from the destructive interference of the three bright modes. A coupled harmonic oscillator model is used to describe the destructive interference between the three bright modes, and the results agree well with the FDTD simulation. The effect of geometrical sizes, like coupling distance, splitting size, and opening direction between two RSRRS on the PIT window is analyzed in detail. Further, the slow light performance of the proposed asymmetric CNT metamaterial is investigated, and a maximum time delay of 1.71 ps is obtained. Finally, an extensive sensitivity of 1.12 THz/RIU is obtained. Therefore, the proposed CNT-based device exhibits numerous potential applications in THz slow light, sensors, and modulators. [ABSTRACT FROM AUTHOR]

In this paper, we designed a multi-layer borophene-based nanoribbons metasurface and achieved a dynamically tunable plasmon-induced transparency (PIT) effect. The numerical results indicate that the parallel multi-layer borophene nanoribbons (BNRs) support the PIT effect. This can be seen as a PIT effect caused by bright-bright mode excitation and coupling between upper and lower layer. The physical principles of the double-layer nanoribbons model can be theoretically demonstrated by using the two-particle model. Due to the heavier effective mass of borophene nanoribbons in the y-direction, when the placement direction of borophene nanoribbons is changed from the x-direction to the y-direction, the position of the PIT peak undergoes a significant red shift. By increasing the borophene carrier density in the model, the PIT window produces a blue shift. The PIT window can be tuned by changing the carrier concentration. Finally, the sensitivity and figure of merit (FOM) of the sensor are calculated, and the results are 55.3 THz/RIU and 86.58, respectively. The proposed structure has potential applications in the research and design of high-sensitivity sensors, optical switches, and optical storage. [ABSTRACT FROM AUTHOR]

A monolayer graphene metamaterial structure with a periodic pattern of transverse graphene strip and semi-enclosed graphene square ring is proposed. The dynamically tunable quadruple plasmon-induced transparency (PIT) effect is achieved in the terahertz band. The strong destructive interference between the bright mode and the dark mode, the electric field distributions of the composite graphene structures explain this quadruple-PIT phenomenon well. The results obtained by finite-difference time-domain (FDTD) simulation agree well with those calculated by the coupled mode theory (CMT). By modulating the Fermi level of graphene, two hexa-frequency asynchronous switches and two quadruple-frequency synchronous switches are realized, their maximum modulation depth is 99.9%, and its corresponding insertion loss and extinction ratio are 0.10 dB and 29.90 dB, respectively. These excellent performances exceed many similar graphene metamaterial structures. Therefore, our proposed quadruple-PIT graphene metamaterial structure has potential application value for the design of terahertz multi-channel switches. [ABSTRACT FROM AUTHOR]

This paper presents the design and numerical investigation of a novel, tunable, and highly efficient metal–insulator-metal (MIM) plasmonic device specifically designed for near-infrared (NIR) applications. The device leverages strategically placed stub resonators within a MIM waveguide. We introduce two small perturbations, a triangle and a rectangle, to achieve remarkable functional versatility. Comprehensive numerical analysis, employing the finite element method (FEM) and validated by the transmission line method (TLM), demonstrates the working principle and excellent agreement between the approaches. Our simulation-driven approach, utilizing a Genetic Algorithm (GA) for accelerated optimization, was crucial in achieving performance levels difficult or costly to reach through purely experimental methods. The GA enabled efficient exploration of a vast parameter space, iterative refinement of device configurations, and fine-tuning of geometric characteristics. This meticulous optimization allows us to control complex interactions within the simulated structure. The proposed device offers diverse functionalities based on adjusted geometrical parameters, including: A. Flat-top band-pass filtering: Achieving a maximum transmission efficiency of 95.8 % within a compact footprint of 420 nm × 540 nm. B. Dual-band band-pass filtering: Maintaining a high transmission efficiency of 88.4 % within a slightly larger size of 450 nm × 540 nm. C. Triple-band notch filtering: Exhibiting minimum transmission (below 1 %) at specific resonance wavelengths for targeted signal suppression. D. Plasmon-induced transparency (PIT) effect: Offering potential applications in various optical functionalities. And, E. Perfect absorption: Achieving a maximum absorption efficiency of 99.62 %, paving the way for efficient light harvesting and manipulation. This multifunctional plasmonic device excels in its combination of compactness, tunability, and diverse NIR functionalities. It holds promise for miniaturized optical components, integrated photonic circuits, and advanced light-matter interactions. Our findings contribute significantly to the advancement of compact, efficient, and readily manufacturable photonic technologies.

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).

This paper presents a metamaterial biosensor composed of dual-cut wires (DCWs) and quadruple split-ring resonators (QSRs), achieving polarization-independent plasmon-induced transparency (PIT) effects in the terahertz range. By leveraging the coupling between bright and dark modes, we observe a transparent window with a minimal loss at 1.22 THz. We investigate the physical mechanism of the PIT effect by analyzing the surface current distribution and electric fields. Simulations reveal that the PIT transparency shows a peak shift of up to 146.7 GHz with an analyte thickness of 14 μm. Moreover, as the refractive index of the analyte increases from 1.0 to 1.6, the biosensor's theoretical sensitivity is calculated to be 281.25 GHz/RIU. Furthermore, we explore the application of the proposed DCW/QSR biosensor for the detection of bacteriophage viruses. Our simulation results demonstrate that the DCW/QSR biosensor serves as an effective sensing platform for detecting viruses such as PRD1 and MS2. These findings underscore the potential of our high-sensitivity metamaterial biosensor, which holds great promise in the field of biosensing, offering a practical and cost-effective approach to label-free biomedical detection. [ABSTRACT FROM AUTHOR]

A monolayer metamaterial consisting of a rectangle graphene strip and four L-shaped graphene blocks was proposed to achieve a quintuple plasmon-induced transparency (quintuple-PIT). The numerical simulation results based on the finite difference time domain method agree well with the calculated results of the coupled mode theory. By modulating the Fermi energy level of graphene, an octuple-frequency asynchronous switch and a sextuple-frequency synchronous switch are designed and studied, which have excellent amplitude modulation degree (up to 97.7%), extinction ratio (up to 16.41 dB), insertion loss (low to 5.4%), and dephasing time(low to 3.86 ps). Furthermore, the results show that the proposed metamaterials has polarization-sensitive characteristics due to their non-central symmetry. Further research shows that the group index is as high as 604 which has a good slow light effect and can be used for optical storage. Hence, the quintuple-PIT proposed structure in this paper has good application value in the expansion of multi-function optical switches and the research of high-quality optical memory. [ABSTRACT FROM AUTHOR]

This study proposes a terahertz metamaterial structure composed of a silicon–graphene–silicon sandwich, aiming to achieve quadruple plasmon-induced transparency (PIT). This phenomenon arises from the interaction coupling of bright–dark modes within the structure. The results obtained from the coupled mode theory (CMT) calculations align with the simulations ones using the finite difference time domain (FDTD) method. Based on the electric field distributions at the resonant frequencies of the five bright modes, it is found that the energy localizations of the original five bright modes undergo diffusion and transfer under the influence of the dark mode. Additionally, the impact of the Fermi level of graphene on the transmission spectrum is discussed. The results reveal that the modulation depths (MDs) of 94.0%, 92.48%, 93.54%, 96.54%, 97.51%, 92.86%, 94.82%, and 88.20%, with corresponding insertion losses (ILs) of 0.52 dB, 0.98 dB, 1.37 dB, 0.70 dB, 0.43 dB, 0.63 dB, 0.16 dB, and 0.17 dB at the specific frequencies, are obtained, achieving multiple switching effects. This model holds significant potential for applications in versatile modulators and optical switches in the terahertz range. [ABSTRACT FROM AUTHOR]

Machine learning is emerging as a new approach that provides more options for solving complex problems involving electromagnetic phenomena. This paper evaluates the application of machine learning to the design of graphene-based absorbers, which is a research challenge. Five machine learning algorithms — k -nearest neighbor regression (kNN), artificial neural network (ANN), decision tree (DT), extremely randomized trees (ETs) and random forest (RF) — are applied to realize the transmission spectrum prediction and reverse design of a graphene nanoribbon waveguide side-coupled absorber. The results show that all five algorithms are effective, with RF being the most accurate in the inverse design. Compared with previous work, the application of machine learning in the intelligent design of graphene absorbers is evaluated more comprehensively, providing a reference for the selection of machine learning algorithms for future inverse design problems. [ABSTRACT FROM AUTHOR]

We have numerically studied a tunable dual‐spectral plasmon‐induced transparency metamaterial structure based on Dirac semimetal films in the terahertz region. The structure was composed of three length‐variant parallel coplanar strips. The dual‐spectral plasmon‐induced transparency is mainly induced by the bright‐bright modes coupling between neighboring Dirac semimetal strips. By adjusting the Fermi energies of Dirac semimetal strips, the individual and synchronous modulation of the dual‐transparency window in resonance frequency, bandwidth and strength can be obtained, respectively. Simultaneous change of the Fermi energies of all the strips can achieve an overall frequency shift of the dual‐transparency window, with the frequency modulation depth of 21.7% and 20.0%, respectively. The maximum group delays of 5.26 and 3.57 ps for each window were calculated in our proposed structure. The simulation results demonstrated the potential of the proposed structure to expand the applications for slow‐light systems, filters, and switchers in the terahertz region. [ABSTRACT FROM AUTHOR]

A tunable and switchable plasmon-induced transparency (PIT) effect in the near-infrared region (NIR) is achieved in a borophene-based metasurface, which consists of a periodic array of parallel double-layer borophene nanoribbons (BNRs). The upper- and lower-layer BNRs fulfill two plasmonic bright modes exciting and coupling for PIT phenomenon generation. By changing borophene electron density, the PIT window can be not only tuned to varying resonance frequency, but also adjusted for switching modulation. The calculation results reveal that as the electron density increases from 2.4 × 1019 to 5.6 × 1019 m−2, the PIT resonance frequency correspondingly shifts from 150 to 220 THz, and a maximum amplitude modulation depth (MD) of the PIT window reaches 98.3% at 193.55 THz ( λ = 1.55 μ m ). Moreover, the slow light characteristics of the proposed metasurface are analyzed in detail using the well-controlled group delay. Such a switchable and broadband tunable metadevice can expand the applications for PIT effect in active slow light, plasmonic sensing, and optical modulator areas. [ABSTRACT FROM AUTHOR]

In this study, a novel multilayer terahertz metamaterial with four graphene sub-structures is designed to achieve dynamically tunable triple plasmon-induced transparency (PIT) due to interference among bright-bright modes. Coupled mode theory (CMT) and finite-difference time-domain (FDTD) simulations show a high degree of consistency in their results. Interestingly, enhanced transmission dips are observed when various graphene structures couple with the two longitudinal graphene strips at the bottom layer. Based on the dynamic modulation characteristics of graphene, the metamaterial demonstrates seven-band optical switching capabilities, achieving modulation depths (MD) of 94.6%, 88.1%, 90.8%, 90.8%, 90.4%, 86.6%, and 87.5%. Additionally, the proposed graphene metamaterial also shows excellent slow-light effects, with a group index reaching up to 1034. This proposed terahertz metamaterial holds significant theoretical implications for the development of dynamically integrated terahertz optoelectronic devices. • A multilayer terahertz graphene metamaterial structure has been proposed for achieving dynamically tunable triple PIT. • The optical switching effect can be achieved by adjusting the Fermi energy level. • Enhanced transmission dips are observed when different graphene structures are coupled with two graphene strips. [ABSTRACT FROM AUTHOR]

In this work, a PIT-like transparency is realized via the strong coupling of plasmonic dipole and epsilon-near-zero (ENZ) mode. Numerical simulations with the finite difference time-domain method (FDTD) demonstrate that triple transparent windows are achieved. The tunability of PIT resonant frequency and transmission amplitude can be achieved by changing the length or gap of the gold nanorods and the carrier concentration of ITO. Finally, it is found that the sensitivity of the model with the change of refractive index of background materials has reached 330.57 THz/RIU, and the figure of merit (FOM) has reached 27.54/RIU. This provides a theoretical reference for the application of the model in optical storage, filtering, and ultra-sensitive infrared band sensor design. [ABSTRACT FROM AUTHOR]

Numerical investigations were conducted of the plasmonically induced transparency (PIT) effect observed in a metal–insulator–metal waveguide coupled to asymmetric three-rectangle resonators, wherein, of the two PIT peaks that were generated, one PIT peak fell while the other PIT peak rose. PIT has been widely studied due to its sensing, slow light, and nonlinear effects, and it has a high potential for use in optical communication systems. To gain a better understanding of the PIT effect in multi-rectangle resonators, its corresponding properties, effects, and performance were numerically investigated based on PIT peak fluctuations. By modifying geometric parameters and filling dielectrics, we not only realized the off-to-on PIT optical response within single or double peaks but also obtained the peak fluctuation. Furthermore, our findings were found to be consistent with those of finite element simulations. These proposed structures have wide potential for use in sensing applications. [ABSTRACT FROM AUTHOR]

A Germanium (Ge) metastructure with switching features from broadband plasmon‐induced transparency (PIT) and electromagnetically induced absorption (EIA) is presented, which forms a broadband PIT through the near‐field coupling between the localized plasmon resonance and the Mie resonance employing four gold cut wires as bright plasmon resonators and four C‐shaped dielectric rings as dark plasmon resonators. A wide transparent window above 0.9 is achieved covering from 0.622 to 0.823 THz with a relative bandwidth of 27.8%. In addition, through phase modulation, a magnetic dipole is generated by the constructive interference of the near‐field coupling of the three resonators to realize the PIT to EIA conversion. In addition, a grid‐like metastructure is introduced to realize the PIT to EIA conversion by phase modulation. To enhance the EIA bandwidth, another layer of C‐shaped reflection plate resonator is introduced to increase the dark mode loss, and a layer of frequency selective surface composed of cross‐shaped resonators is also used. Thus under the dual effect, a wide absorption window above 0.75 is achieved covering from 0.647 to 0.756 THz with a relative bandwidth of 15.5%. [ABSTRACT FROM AUTHOR]

We propose a hybrid metamaterial whose unit cell is consisted of a silicon ring and a silver T-shaped antenna. We use finite-difference time-domain to simulate the electromagnetically-induced transparency (EIT)-like effect and slow-light performance of the metamaterial. Then, we also study the influence of different parameters, and finally achieve a group refractive index of 2253 slow-light effect based on an EIT-like effect. Moreover, we use two-dimensional material WS2 to tune the slow-light effect. This tuning method brings forward a research idea for the slow-light tuning of active metamaterials. [ABSTRACT FROM AUTHOR]

This paper presents a metamaterial biosensor composed of dual-cut wires (DCWs) and quadruple split-ring resonators (QSRs), achieving polarization-independent plasmon-induced transparency (PIT) effects in the terahertz range. By leveraging the coupling between bright and dark modes, we observe a transparent window with a minimal loss at 1.22 THz. We investigate the physical mechanism of the PIT effect by analyzing the surface current distribution and electric fields. Simulations reveal that the PIT transparency shows a peak shift of up to 146.7 GHz with an analyte thickness of 14 μm. Moreover, as the refractive index of the analyte increases from 1.0 to 1.6, the biosensor’s theoretical sensitivity is calculated to be 281.25 GHz/RIU. Furthermore, we explore the application of the proposed DCW/QSR biosensor for the detection of bacteriophage viruses. Our simulation results demonstrate that the DCW/QSR biosensor serves as an effective sensing platform for detecting viruses such as PRD1 and MS2. These findings underscore the potential of our high-sensitivity metamaterial biosensor, which holds great promise in the field of biosensing, offering a practical and cost-effective approach to label-free biomedical detection.

In this research, a metamaterial structure composed of graphene sandwich structure is put forward to achieve triple plasmon-induced transparency. This phenomenon is generated by brightâ€"bright modes interaction in the proposed structure, and the results calculated though couple mode theory are consistent with the simulation ones using finite difference time domain method. In addition, the effects of graphene Fermi level and scattering rate on the transmission spectrum are also discussed. It is found that the five-fold switching effects are achieved with the modulation depths (MDs) of 92%, 91%, 95%, 90% and 94%, respectively. What is more, the different graphene strip can also be adjusted by changing Fermi levels to achieve the switching effects with the MDs of 98% and 97%. This model has the prospect to be applied in multi-functional modulators and optical switches in terahertz band. [ABSTRACT FROM AUTHOR]

In this paper, we design a multifunctional micro-nano device with a hybrid metamaterial-waveguide system, which leads to a triple plasmon-induced transparency (PIT). The formation mechanisms of the three transparent peaks have their own unique characteristics. First, PIT-I can be switched into the BIC (Friedrich–Wintge bound state in continuum), and the quality factors (Q-factors) of the transparency window of PIT-I are increased during the process. Second, PIT-II comes from near-field coupling between two bright modes. Third, PIT-III is generated by the near-field coupling between a low-Q broadband bright mode and a high-Q narrowband guide mode, which also has a high-Q transparent window due to the guide mode. The triple-PIT described above can be dynamically tuned by the gate voltage of the graphene, particularly for the dynamic tuning of the Q values of PIT-I and PIT-III. Based on the high Q value of the transparent window, our proposed structure can be used for highly sensitive refractive index sensors or devices with prominent slow light effects. [ABSTRACT FROM AUTHOR]

Manipulating spoof surface plasmon polaritons (SPPs) effectively has promising applications in developing ultracompact plasmonic circuits. Research on versatile components in planar spoof SPP waveguides has attracted widespread attention over the decades. In particular, cascaded chip-scale devices play an important role in enhancive buffering, highly-sensitive sensing and nonlinear interaction. Herein, we propose a novel strategy to manipulate the switching and slow wave features of spoof SPPs on the basis of plasmon-induced transparency (PIT) spectral responses. A combined module consisting of split square-ring resonators and electric-LC resonators is loaded with a spoof SPP waveguide along the propagating trajectory, where the destructive interference between the two resonators contributes to the PIT behavior and the group delay of the spoof SPPs. A modulation contrast of up to 16.2 dB is achieved for the spoof SPP transmission at the PIT peak frequency. The measurements are consistent with both full-wave simulations and theoretical calculations. Cascaded capabilities performed by installing multiple PIT modules at an interval of an equivalent wavelength are accomplished in the planar spoof SPP waveguide. The proposed strategy opens up fascinating prospects on highly integrated plasmonic circuits and networks.

A relatively simple metasurface structure is proposed to achieve a dynamically tunable slow light effect. The metasurface consists of two horizontal graphene strips and two vertical continuous graphene strips. Strong interference between the bright and dark modes enables the metasurface to generate a significant triple plasmon-induced transparency (PIT). Varying the coupling distance between the horizontal strips, double-PIT and triple-PIT can be transformed into each other. During the reduction of the transparent window, electromagnetic waves undergo strong phase changes, resulting in a higher group index. After structural optimization, the maximum time delay and group index can be as high as 2.624 ps and 3934, surpassing comparable slow light devices. The proposed patterned graphene metasurface provides theoretical guidance for designing high-performance slow light devices for optical storage, nonlinear optics and quantum optics. [Display omitted] • Based on graphene strips for achieving triple plasmon-induced transparency (PIT) effect. • The PIT effect can be dynamically regulated by the Fermi levels of graphene. • Varying the coupling distance between the two horizontal graphene strips, the triple-PIT can evolve into a dual-PIT. • The proposed metasurface exhibits outstanding slow-light performance with a group index up to 3934. [ABSTRACT FROM AUTHOR]

A double hexagonal single 'S' metamaterial (DHSSM), exhibiting plasmon-induced transparency (PIT) in its spectral response is studied in this paper. Amplitude modulation of the PIT window is achieved by varying the azimuth angle of substructure 'S'. Theoretical investigations into the PIT effect are conducted through numerical simulations. Moreover, an equivalent coupling circuit and Lorentz model are constructed to elucidate the PIT modulation mechanism. The results reveal that the PIT physical mechanism of the DHSSM has originated in destructive interference between bright-bright modes, which is directly excited by terahertz waves on the double hexagonal split rings and the 'S' substructure. For slow optical propagation performance, the structure has a high group delay (up to 41.92 ps) while allowing for the adjustment of the transparency window amplitude. The proposed metamaterial holds promising prospects for applications in slow light devices, switches, and filters within the terahertz frequency range. • Coupling between bright modes produces electromagnetically induced transparency at terahertz band. • The equivalent coupling circuit and Lorentz model reveal the amplitude modulation mechanism. • Strong scattering results in high group delay. • Amplitude modulation can be realized while maintaining high group delay. [ABSTRACT FROM AUTHOR]

Based on coupled-mode theory (CMT) and the finite-difference time-domain (FDTD) approach, we propose a graphene metasurface-based and multifunctional polarization beam splitter that is dynamically tunable. The structure, comprising two graphene strips at the top and bottom and four triangular graphene blocks in the center layer, can achieve triple plasma-induced transparency (PIT). In a single polarization state, the computational results reveal that synchronous or asynchronous six-mode electro-optical switching modulation may be performed by modifying the Fermi levels of graphene, with a maximum modulation degree of amplitude (MDA) of 97.6% at 5.148 THz. In addition, by varying the polarization angle, a polarization-sensitive, tunable polarization beam splitter (PBS) with an extinction ratio and insertion loss of 19.6 dB and 0.35 dB at 6.143 THz, respectively, and a frequency modulation degree of 25.2% was realized. Combining PIT with polarization sensitivity provides a viable platform and concept for developing graphene metasurface-based multifunctional and tunable polarization devices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Biosensors have proved immensely useful in numerous vital areas for detection and testing. Here, a novel plasmonic nanosensor, based on loaded slot cavity, is proposed and studied. We conduct a detailed analysis of the influence of the load’s parameters on the transmission characteristics based on the finite element method and surface charge and current model. Simulation results reveal that the existence of the load can cause the resonant wavelength to have a linear or nonlinear red shift, and yield an enhanced plasmonic nanosensor with sensitivity about ${S}={2900}$ nm/RIU and a detection limit about 1 $\times \,\,10^{\text {-3}}$. In addition, the proposed structure is well used in actual biosensing for blood plasma concentration, glucose concentration, ethanol temperature and diseased cell detection with high sensitivity. Finally, an extended structure with detuned loaded slot cavities is proposed to realize PIT/multi-PIT and slow light effect. The special features of our proposed structure are applicable in the realization of various integrated components for the development of high-performance plasmonic biosensor. [ABSTRACT FROM AUTHOR]

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.

Recently reported metamaterial analogues of plasmon-induced transparency are very promising in designing novel optical components, such as slow-light devices and optical switches. However, most of the previous works mainly focus on manipulating the PIT effects using planar structures in the free space under normal incidence, where the resonances are mostly induced by the incident electric fields. Here, we present a PIT analogue in a waveguide system in the terahertz regime by inserting coupled split-ring resonator pairs into a tapered parallel plate waveguide. Different from the free-space cases, the incident magnetic fields can also play an important role here. By carefully arranging the relative orientations of the coupled split-ring resonators in each pair, giant modulation of the PIT effect is observed. This straightforward approach may enable novel devices for integrated terahertz devices.

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 n m . 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. [ABSTRACT FROM AUTHOR]

A plasmon-induced transparency (PIT) effect based on an asymmetric graphene loop structure has been proposed and investigated in this paper. The microstructure consists of a pair of graphene square loops and a dielectric substrate. The calculated results show that the transparency peak can be produced at 5.68 THz by the frequency detuning between two different graphene square loops. The geometric parameters of microstructure, such as the coincidence degree between two square loops, the length and the width of two square loops, will affect the position of PIT-window. Moreover, by adjusting the Fermi level of graphene through external gate voltage, the PIT-window can be dynamically tuned. Importantly, the PIT-window in graphene metamaterials can also serve as the amplitude modulator at the fixed frequency and the refractive index sensor. In addition, an improved microstructure is proposed for realizing the multi-PIT-window. The amplitude modulation of multi-PIT-window can be adjusted up to 53% by controlling the coupling distance, which has certain application prospects in the fields of double-channel filters, optical switches, and modulators. [ABSTRACT FROM AUTHOR]

Black phosphorus (BP), as a new type of two-dimensional material, has drawn considerable interest because of its distinct physics and electronic characteristics. In this work, we theoretically present a BP-based metamaterial, unit cell of which is composed of a rectangular BP nano-patch and two parallel BP strips. The research results indicate that tunable anisotropic plasmon-induced transparency (PIT) effect can be achieved in the presented metamaterials when the polarization of incident light is along armchair and zigzag directions of BP crystal, respectively. Moreover, the spectra responses and group delay accompanied by the PIT effect can be actively controlled by adjusting the carrier density and geometric parameters. The electromagnetic simulation results calculated by finite-difference time-domain method show good agreement with the coupled Lorentz oscillator model. Our proposed nanostructure provides a new path for designing photonic devices such as slow light and photodetector in the mid-infrared region. [ABSTRACT FROM AUTHOR]

Slow light devices with buffering capability play a critical role in all-optical signal processing. In this paper, multiple slow light phenomena are implemented based on plasmon-induced transparency (PIT) in our device. The device mainly consists of dual tooth cavities coupled with stub resonators, respectively. Temporal coupled-mode theory model illustrates that the triple PIT phenomena can be achieved based on different formation mechanisms. The simulation results calculated by the finite-difference time-domain method reveal that significant slow light response occurs at two wavelength regions. In addition, the parameters of structure have an important influence on PIT response and slow light characteristics. Moreover, the separate manipulation of wavelength, transmission and group index at transparency peak can be achieved in different slow light channels by adjusting the structural parameters. This plasmonic device is of great significance for the design of optical networks on chips. [ABSTRACT FROM AUTHOR]

Identification of biomedical molecules by terahertz (THz) sensing is a quite promising noninvasive technique. Here, we present a THz graphene metamaterial with plasmon-induced transparency (PIT) for high sensitivity biosensing. The strong PIT effect is generated by the destructive interference between the plasmon modes excited by two graphene strips. For testing different biomedical analytes, a distinct frequency shift at PIT resonance can be observed due to the effect of surrounding refractive index variation. The metamaterial scheme for noninvasive biomedical sensing can achieve the sensitivity as high as 2.6 THz/RIU. Particular, it can not only detect the phase of red blood cell infected by malaria but also the concentration of ethanol aqueous solution. The underlying mechanism is discussed for understanding the THz sensing operation. • THz graphene metamaterial for highly noninvasive sensitivity biosensing is present. • The metamaterial can achieve the sensitivity as high as 2.6 THz/RIU. • The metamaterial can defect the different phases of red blood cell. • The metamaterial provides a technique for biomolecular detection and disease diagnosis. [ABSTRACT FROM AUTHOR]

The plasmon-induced transparency (PIT) in planar metasurface comprising of resonators having one S-shaped structure and a rectangular strip (both made of silver) was investigated. It was found that the S-shaped component of metasurface induces a single bright mode, whereas the rectangular strip manifests two bright modes with strong coupling of incidence electromagnetic waves. When both the metasurface components were placed in the proximity, the PIT-like phenomenon was observed. The effects of rotational angle of the metasurface as well as the angle of incidence on the transmission spectra (of the metasurface) were also investigated. Both the transverse electric (TE) and transverse magnetic (TM) waves were used to explore the PIT effect. It was noticed that the PIT window could be easily tuned by altering the horizontal distance between the resonator components (i.e., the S-shaped structure and rectangular strip). It is expected that the proposed metasurface structure would be prudent in switching, ultrafast sensing and optical applications.

Abstract The plasmon-induced transparency (PIT), which is destructive interference between the superradiation mode and the subradiation mode, is studied in patterned graphene-based terahertz metasurface composed of graphene ribbons and graphene strips. As the results of finite-difference time-domain (FDTD) simulation and coupled-mode theory (CMT) fitting, the PIT can be dynamically modulated by the dual-mode. The left (right) transmission dip is mainly tailored by the gate voltage applied to graphene ribbons (stripes), respectively, meaning a dual-mode on-to-off modulator is realized. Surprisingly, an absorbance of 50% and slow-light property of 0.7 ps are also achieved, demonstrating the proposed PIT metasurface has important applications in absorption and slow-light. In addition, coupling effects between the graphene ribbons and the graphene strips in PIT metasurface with different structural parameters also are studied in detail. Thus, the proposed structure provides a new basis for the dual-mode on-to-off multi-function modulators.

Abstract Plasmon-induced transparency is an efficient way to mimic electromagnetically induced transparency, which can eliminate the opaque effect of medium to the propagating electromagnetic wave. We proposed an aperture-side-coupled asymmetric bowtie structure to realize on-chip plasmon-induced transparency in optical communications band. The plasmon-induced transparency results from the strong coupling between the detuned bowtie triangular resonators. Either of the resonator works as a Fabry-Perot cavity with compact dimensions. The transparent peak wavelength can be easily controlled due to its strong linear relation with the resonator height. The ratio of absorption valley to the transparent peak can be more than 10 dB. Moreover, with excellent linearity of shifting wavelength to sensing material index, the device has great sensing performance and immunity to the structure deviations.

On-chip plasmon-induced transparency (PIT) possessing the unique properties of controlling light propagation states is a promising way to on-chip ultrafast optical connection networks as well as integrated optical processing chips. On-chip PIT has attracted enormous research interests, the latest developments of which have also yield progress in nanophotonics, material science, nonlinear optics, and so on. This review summarizes the realization methods, novel configurations, diversiform materials, and the improved performance indexes. Finally, a brief outlook on the remaining challenges and possible development direction in the pursuit of the application of a practical on-chip photonic processor based on PIT is also afforded.

It is necessary to explore the potential applications of surface plasmon polaritons (SPPs) on graphene metasurfaces and enhance the performance of their on–off modulators and slow‐light effect. A simple monolayer graphene metasurface is developed for achieving the plasmon‐induced transparency (PIT) phenomenon in the terahertz range. PIT can be effectively tuned by the positional parameter, the Fermi level, and the carrier mobility using the finite‐difference time‐domain (FDTD). Moreover, it is found that a quad‐frequency on–off modulator and an outstanding slow‐light effect can be realized in this proposed metasurface. Interestingly, modulation depth (MD), insertion loss (IL), ΔT, and the group index are of 98.90%, ≈0.05 dB, 0.97, and 3383, respectively. Hence, the results show that the proposed metasurface has excellent performance for on–off modulators and a slow‐light effect, which can provide theoretical guidance for designing terahertz micro‐nanophotonic devices. [ABSTRACT FROM AUTHOR]

The bulk Dirac semimetal (BDS) is an interesting material, similar to graphene, which can dynamically adjust its optical properties via a variation in its Fermi energy or electrical voltage. In this work, a BDS-based plasmonic device, which enables tunable terahertz plasmon-induced transparency, was proposed and designed. By using the finite element method, the surface plasmon polariton and plasmon-induced transparency of this device were systematically investigated. The results demonstrate that the plasmoninduced transparency of such device can be dynamically tuned by varying its Fermi energy. When the Fermi energy changes from 55 meV to 95 meV, the maximum group delay time of the device increases from 13.2 ps to 21 ps. In the case of a cascading device, the maximum group delay time can be further pushed up to 44.57 ps. The influence of the ambient refractive index on the optical properties of the proposed device was also considered and investigated. [ABSTRACT FROM AUTHOR]

A planar metamaterial structure has been designed to obtain surface plasmon-induced-transparency (PIT) effect, which is composed of a nanodisk sandwiched between double rods. From theoretical perspective, the three-level plasmonic system has been utilized to analyze the near-field coupling mechanism. Besides, the simulation results show that PIT resonance can be tailored by structure parameters. Particularly, when the length of the rods is more than twice of the diameter of the disk, two PIT windows are generated simultaneously, based on the coupling between the bright mode of the nanodisk and two multipole dark modes of rods. Furthermore, a metamaterial structure, composed by a disk and two pairs of nanorods, is suggested to achieve the polarization-independent plasmon-induced-transparency effect. As a result, this work shows great application prospect in the area of compact optical devices, such as multiband tunable filters, plasmonic switches, and slow light devices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We propose dynamically switchable multispectral plasmon-induced transparency (PIT) with high modulation depth in a three-dimensional metamaterial standing on a flexible substrate. The proposed metamaterial is composed of a pair of metal–insulator–metal (MIM) nano-cut-wires and a pair of insulator–metal–insulator (IMI) nano-cut-wires. Results show that two PIT windows can be achieved because of the near-field coupling between the dipole supported by the IMI nano-cut-wire and two quadrupoles supported by the MIM structures. These two PIT windows can be blue-shifted or even flipped over by stretching the substrate along one direction, or be switched off by stretching along the other direction. A classical coupled oscillator model is developed to quantitatively describe and explain these results. We expect this work will find promising applications in multispectral sensors, slow light devices and nonlinear optical devices. [ABSTRACT FROM AUTHOR]