Sensing analysis based on plasmon induced transparency in nanocavity-coupled waveguide

Quadruple Plasmon-Induced Transparency and Dynamic Tuning Based on Bilayer Graphene Terahertz Metamaterial

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.

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.

Ultra-high sensitivity sensing based on ultraviolet plasmonic enhancements in semiconductor triangular prism meta-antenna systems

Silicon (Si), germanium (Ge), and gallium arsenide (GaAs) are familiar semiconductors that always act in the role of optical dielectrics. However, these semiconductors also have plasmonic behaviors in ultraviolet (UV) ranges due to the strong interband transitions or valence electrons. And few studies are aimed at investigating plasmonic properties in the semiconductor at the nanoscale. In this work, we discuss UV plasmonics and sensing properties in single and dimer Si, Ge, and GaAs triangular prism meta-antenna systems. The results show that obvious local surface plasmon resonances (LSPRs) can be realized in the proposed triangular prism meta-antennas, and the resonant wavelength, electromagnetic field distribution, surface charge distribution, and surface current density can be effectively tuned by structural and material parameters. In addition, we also find that the Si triangular prism meta-antenna shows more intense plasmonic responses in UV ranges than that in the Ge or GaAs triangular prism nanostructures. Especially, the phase difference between the triangular prism nanostructure and light source can effectively regulate the symbol and value of the surface charge. Moreover, the great enhancement of electric field can be seen in the dimer triangular prism meta-antennas when the distance of the gap is g<5 nm, especially g=1 nm. The most interesting result is that the maximum of refractive index sensitivity s and figure of merit (FoM) are greatly enlarged in dimer triangular prism meta-antennas. Particularly, the sensitivity can reach up to 215 nm/RIU in the dimer GaAs triangular prism meta-antennas, which is improved more than one order of magnitude. These research results may play important roles in applications of the photo detecting, plasmonic sensing and disinfecting in UV ranges.

Tunable Fano Resonance and Enhanced Sensing in a Simple Au/TiO2 Hybrid Metasurface

We investigate Fano resonances and sensing enhancements in a simple Au/TiO₂ hybrid metasurface through the finite-different time-domain (FDTD) simulation and coupled mode theory (CMT) analysis. The results show that the Fano resonance in the proposed simple metasurface is caused by the destructive interaction between the surface plasmon polaritons (SPPs) and the local surface plasmon resonances (LSPRs), the quality factor and dephasing time for the Fano resonance can be effectively tuned by the thickness of Au and TiO₂ structures, the length of each unit in x and y directions, as well as the structural defect. In particular, single Fano resonance splits into multiple Fano resonances caused by a stub-shaped defect, and multiple Fano resonances can be tuned by the size and position of the stub-shaped defect. Moreover, we also find that the sensitivity in the Au/TiO₂ hybrid metasurface with the stub-shaped defect can reach up to 330 nm/RIU and 535 nm/RIU at the Fano resonance 1 and Fano resonance 2, which is more than three times as sensitive in the Au/TiO₂ hybrid metasurface without the stub-shaped defect, and also higher than that in the TiO₂ metasurface reported before. These results may provide further understanding of Fano resonances and guidance for designing ultra-high sensitive refractive index sensors.

Coupling effects in single-mode and multimode resonator-coupled system

We have proposed a simple metal-dielectric-metal (MDM) waveguide system side-coupled with single-mode and multimode resonators. This proposed structure can achieve a typical dual plasmon-induced transparency (PIT) effect in the transmission spectra. The two PIT peaks exhibit opposite evolution tendencies with the increase in the depth of stubs. A multimode-coupled mode theory (M-CMT), confirmed by simulated results, is originally introduced to investigate the coupling effects of the proposed structure. Compared to the previous reported multichannel filters, the proposed structure includes obvious advantages, such as structural simplicity and ease of fabrication. In addition, the sensing characteristics of the proposed structure based on PIT effects are discussed numerically. The results demonstrate that the proposed structure is suitable for applications in multichannel filters, optical switches, and sensors.

On-chip plasmon-induced transparency in THz metamaterial on a LiNbO3 subwavelength planar waveguide

We experimentally demonstrate on-chip plasmon-induced transparency at THz frequencies using a meta-structure deposited on a 50 μm-thick dielectric subwavelength waveguide. The obvious plasmon-induced transparency results from strong coupling between the respective modes of a cut wire and a double-gap split ring resonator. The simulation and experimental results are consistent. Based on our numerical simulations of the temporal evolution of plasmon-induced transparency, a π/2 phase difference at the transparency peak between the above two modes is observed, i.e., there is energy oscillating between them that exhibits Rabi oscillation-like behavior. In addition, at the transparency peak, a strong local-field enhancement effect and high transmission can be obtained simultaneously, which can be tuned by changing the separation between the cut wire and the double-gap split ring resonator. These results will facilitate the design of THz integrated photonic devices and serve as an excellent platform for nonlinear optics and sensing.

Gate-Tunable Plasmon-Induced Transparency Modulator Based on Stub-Resonator Waveguide with Epsilon-Near-Zero Materials

We demonstrate an electrically tunable ultracompact plasmonic modulator with large modulation strength (>10 dB) and a small footprint (~1 μm in length) via plasmon-induced transparency (PIT) configuration. The modulator based on a metal-oxide-semiconductor (MOS) slot waveguide structure consists of two stubs embedded on the same side of a bus waveguide forming a coupled system. Heavily n-doped indium tin oxide (ITO) is used as the semiconductor in the MOS waveguide. A large modulation strength is realized due to the formation of the epsilon-near-zero (ENZ) layer at the ITO-oxide interface at the wavelength of the modulated signal. Numerical simulation results reveal that such a significant modulation can be achieved with a small applied voltage of ~3V. This result shows promise in developing nanoscale modulators for next generation compact photonic/plasmonic integrated circuits.

Active Enhancement of Slow Light Based on Plasmon-Induced Transparency with Gain Materials

As a plasmonic analogue of electromagnetically induced transparency (EIT), plasmon-induced transparency (PIT) has drawn more attention due to its potential of realizing on-chip sensing, slow light and nonlinear effect enhancement. However, the performance of a plasmonic system is always limited by the metal ohmic loss. Here, we numerically report a PIT system with gain materials based on plasmonic metal-insulator-metal waveguide. The corresponding phenomenon can be theoretically analyzed by coupled mode theory (CMT). After filling gain material into a disk cavity, the system intrinsic loss can be compensated by external pump beam, and the PIT can be greatly fueled to achieve a dramatic enhancement of slow light performance. Finally, a double-channel enhanced slow light is introduced by adding a second gain disk cavity. This work paves way for a potential new high-performance slow light device, which can have significant applications for high-compact plasmonic circuits and optical communication.

Design of broadband graphene-metamaterial absorbers for permittivity sensing at mid-infrared regions

In this paper, a tunable broadband metamaterial absorber (MA) based on graphene is investigated theoretically and numerically at mid-infrared regions. Compared with the previously reported multiband graphene-based MAs, a broad bandwidth of 11.7 THz with the absorption over 90% is obtained in the proposed MA, which is composed of a Jerusalem cross (JC) metal encrusting into the slot graphene layer in the top layer. The results show that the origin of broadband absorption is caused by coupling effect between metal and graphene, and this effect is explained by the two-mode waveguide coupling theory. The tunability of MA is achieved via changing the external gate voltage to modify the Fermi energy of graphene. Further results show that the proposed MA can be used as the permittivity sensor with a high absorption. This work indicates that the proposed MA has the potential applications with respect to sensors and infrared absorbers.

Plasmon-induced transparency based on a triangle cavity coupled with an ellipse-ring resonator

In this paper, a novel compact plasmonic system is introduced to realize the phenomenon of plasmon-induced transparency. The proposed device consists of a triangle defect coupled with an ellipse-ring resonator based on a metal-insulator-metal platform. By the finite-difference time-domain method, the transmission characteristics are numerically studied in detail. In order to verify the simulation results, the coupled mode theory is utilized. In the following, the effect of geometrical parameters, namely, the major and minor radii of the ellipse-ring and the gap between cavities, are investigated. Moreover, the fundamental factors of transmission spectra including intrinsic Drude loss and refractive index of dielectric region are studied. As a result, the transmission peak is obtained near 70% and the full width at half-maximum is close to 28 nm. The sensitivity and figure of merit of the proposed structure are 860 nm/RIU and 31.6  RIU^-1, respectively. The mentioned compact structure has the ability and potential to be used in integrated optical circuits like slow light devices, nanoscale filters and nanosensors.

Novel oscillator model with damping factor for plasmon induced transparency in waveguide systems

We introduce a novel two-oscillator model with damping factor to describe the plasmon induced transparency (PIT) in a bright-dark model plasmonic waveguide system. The damping factor γ in the model can be calculated from metal conductor damping factor γ_c and dielectric damping factor γ_d. We investigate the influence of geometry parameters and damping factor γ on transmission spectra as well as slow-light effects in the plasmonic waveguide system. We can find an obvious PIT phenomenon and realize a considerable slow-light effect in the double-cavities system. This work may provide guidance for optical switching and plasmon-based information processing.

Self-reference plasmonic sensors based on double Fano resonances

High-sensitivity plasmonic refractive index sensors show great applications in the areas of biomedical diagnostics, healthcare, food safety, environmental monitoring, homeland security, and chemical reactions. However, the unstable and complicated environments considerably limit their practical applications. By employing the independent double Fano resonances in a simple metallic grating, we experimentally demonstrate a self-reference plasmonic sensor, which significantly reduces the error contributions of the light intensity fluctuations in the long-distance propagation and local temperature variations at the metallic grating, and the detection accuracy is guaranteed. The numerical simulation shows that the two Fano resonances have different origins and are independent of each other. As a result, the left Fano resonance is quite sensitive to the refractive index variations above the metal surface, while the right Fano resonance is insensitive to that. Experimentally, a high figure of merit (FOM) of 31 RIU^-1 and a FOM* of 860 RIU^-1 are realized by using the left Fano resonance. More importantly, by using the right Fano resonance as a reference signal, the influence of the light intensity fluctuations and local temperature variations is monitored and eliminated in the experiment. This simple self-reference plasmonic sensor based on the double Fano resonances may find important applications in highly-sensitive and accurate sensing under unstable and complicated environments, as well as multi-parameter sensing.

Theoretical analysis of ultrahigh figure of merit sensing in plasmonic waveguides with a multimode stub

We propose an expanded coupled mode theory to analyze sensing performance in a plasmonic slot waveguide side-coupled with a multimode stub resonator. It is confirmed by the finite-difference time-domain simulations. Through adjusting the parameters, we can realize figure of merit (FOM) of ∼59,010, and the sensitivity S can reach to 75.7. Compared with the plasmonic waveguide systems in recent Letters, our proposed structure has the advantages of easy fabrication, compactness, sensitivity, and high FOM. The proposed theory model and findings provide guidance for fundamental research of the integrated plasmonic nanosensor applications.

Influential and theoretical analysis of nano-defect in the stub resonator

We investigate a classic optical effect based on plasmon induced transparency (PIT) in a metal-insulator-metal (MIM) bus waveguide coupled with a single defective cavity. With the coupled mode theory (CMT), a theoretical model, for the single defective cavity, is established to study spectral features in the plasmonic waveguide. We can achieve a required description for the phenomenon, and the theoretical results also agree well with the finite-difference time-domain (FDTD) method. Our researches show that the defect’s position and size play important roles in the PIT phenomenon. By adjusting the position and size of the defect, we can realize the PIT phenomenon well and get the required slow light effect. The proposed model and findings may provide guidance for fundamental research of the control of light in highly integrated optical circuits.

Tunable high quality factor in two multimode plasmonic stubs waveguide

We numerically investigate the optical characteristics of a metal-dielectric-metal (MDM) waveguide side-coupled with two identical multimode stub resonators. Double plasmon-induced transparency (PIT) peaks with narrow full width at half maximum (FWHM) and high quality factor (Q-factor) can be observed in this structure. The Q-factors of PIT peaks in two stub resonators system are larger than those in single stub resonator system. A multimode coupled-radiation oscillator theory (MC-ROT), which is derived from ROT, is proposed to analyze the spectral response in the multimode system for the first time. The analytical results are confirmed by the finite-difference time-domain (FDTD) simulation results. We can also find that the Q-factors of the two PIT peaks have an opposite evolution tendency with the change of the stubs parameters and the maximum can reach to 427. These results may provide some applications for ultrasensitive sensors, switches and efficient filters.

Tunable nanoplasmonic sensor based on the asymmetric degree of Fano resonance in MDM waveguide

We first report a simple nanoplasmonic sensor for both universal and slow-light sensing in a Fano resonance-based waveguide system. A theoretical model based on the coupling of resonant modes is provided for the inside physics mechanism, which is supported by the numerical FDTD results. The revealed evolution of the sensing property shows that the Fano asymmetric factor p plays an important role in adjusting the FOM of sensor, and a maximum of ~4800 is obtained when p = 1. Finally, the slow-light sensing in such nanoplasmonic sensor is also investigated. It is found that the contradiction between the sensing width with slow-light (SWS) and the relevant sensitivity can be resolved by tuning the Fano asymmetric factor p and the quality factor of the superradiant mode. The presented theoretical model and the pronounced features of this simple nanoplasmonic sensor, such as the tunable sensing and convenient integration, have significant applications in integrated plasmonic devices.

Unidirectional reflectionless propagation in plasmonic waveguide-cavity systems at exceptional points

We design a non-parity-time-symmetric plasmonic waveguide-cavity system, consisting of two metal-dielectric-metal stub resonators side coupled to a metal-dielectric-metal waveguide, to form an exceptional point, and realize unidirectional reflectionless propagation at the optical communication wavelength. The contrast ratio between the forward and backward reflection almost reaches unity. We show that the presence of material loss in the metal is critical for the realization of the unidirectional reflectionlessness in this plasmonic system. We investigate the realized exceptional point, as well as the associated physical effects of level repulsion, crossing and phase transition. We also show that, by periodically cascading the unidirectional reflectionless plasmonic waveguide-cavity system, we can design a wavelength-scale unidirectional plasmonic waveguide perfect absorber. Our results could be potentially important for developing a new generation of highly compact unidirectional integrated nanoplasmonic devices.