Plasmon-induced transparency with detuned ultracompact Fabry-Perot resonators in integrated plasmonic devices

Dynamically tunable multi-band plasmon-induced absorption based on multi-layer borophene ribbon gratings

In this paper, we propose a borophene-based grating structure (BBGS) to realize multi-band plasmon-induced absorption. The coupling of two resonance modes excited by upper borophene grating (UBG) and lower borophene grating (LBG) leads to plasmon-induced absorption. The coupled-mode theory (CMT) is utilized to fit the absorption spectrum. The simulated spectrum fits well with the calculated result. We found the absorption peaks exhibit a blue shift with an increase in the carrier density of borophene grating. Further, as the coupling distance D increases, the first absorption peak shows a blue shift, while the second absorption peak exhibits a red shift, leading to a smaller reflection window. Moreover, the enhancement absorption effect caused by the bottom PEC layer is also analyzed. On this basis, using a three-layer borophene grating structure, we designed a three-band perfect absorber with intensities of 99.83%, 99.45%, and 99.96% in the near-infrared region. The effect of polarization angle and relaxation time on the absorption spectra is studied in detail. Although several plasmon-induced absorption based on two-dimensional (2D) materials, such as graphene, black phosphorus, and transition metal dichalcogenides (TMDs), have been previously reported, this paper proposes a borophene-based metamaterial to achieve plasmon-induced perfect absorption since borophene has some advantages such as high surface-to-volume ratios, mechanical compliance, high carrier mobility, excellent flexibility, and long-term stability. Therefore, the proposed borophene-based metamaterial will be beneficial in the fields of multi-band perfect absorber in the near future.

Plasmonic nanosensor and pressure-induced transparency based on coupled resonator in a nanoscale system

Plasmonic nanosensors and the dynamic control of light fields are of the utmost significance in the field of micro- and nano-optics. Here, our study successfully demonstrates a plasmonic nanosensor in a compact coupled resonator system and obtains the pressure-induced transparency phenomenon for the first time to our knowledge. The proposed structure consists of a groove and slot cavity coupled in the metal-insulator-metal waveguide, whose mechanical and optical characteristics are investigated in detail using the finite element method. Simulation results show that we construct a quantitative relationship among the resonator deformation quantity, the applied pressure variation, and the resonant wavelength offset by combining the mechanical and optical properties of the proposed system. The physical features contribute to highly efficient plasmonic nanosensors for refractive index and optical pressure sensing with sensitivity of 1800 nm/RIU and 7.4 nm/MPa, respectively. Furthermore, the light waves are coupled to each other in the resonators, which are detuned due to the presence of pressure, resulting in the pressure-induced transparency phenomenon. It is noteworthy to emphasize that, unlike previously published works, our numerical results take structural deformation-induced changes in optical properties into account, making them trustworthy and practical. The proposed structure introduces a novel, to the best of our knowledge, approach for the dynamic control of light fields and has special properties that can be utilized for the realization of various integrated components.

Tunable electromagnetically induced absorption based on coupled-resonators in a compact plasmonic system

Electromagnetically induced absorption (EIA) exhibits abnormal dispersion and novel fast-light features, making it a crucial aspect of nanophotonics. Here, the EIA phenomenon is numerically predicted in a compact plasmonic waveguide system by introducing a slot resonator above a square cavity. Simulation results reveal that the EIA response can be easily tuned by altering the structure's parameters, and double EIA valleys can be observed with an additional slot resonator. Furthermore, the investigated structures demonstrate a fast-light effect with an optical delay of ∼ -1.0 ps as a result of aberrant dispersion at the EIA valley, which enable promising applications in the on-chip fast-light area. Finally, a plasmonic nanosensor with a sensitivity of ∼1200 nm/RIU and figure of merit of ∼16600 is achieved based on Fano resonance. The special features of our suggested structure are applicable in realization of various integrated components for the development of multifunctional high-performance nano-photonic devices.

Actively tunable plasmon-induced transparency in terahertz based on Dirac semimetal metamaterials

We numerically investigate a tunable plasmon-induced transparency based on bulk Dirac semimetal (BDS) metamaterial in the terahertz band. In the unit cell, the prominent transparent peak appears to be due to the interference between the cut wires (CWs) and split-ring resonators (SRRs). An active modulation via near-field coupling is obtained by varying the Fermi level of the BDS. Introducing photoactive silicon, it will be found that once the intensity of the pump light is adjusted, a tunable transparent peak will appear. Furthermore, by shifting the coupling distance between CWs and SRRs, the depth of the transparent peak will change accordingly. Finally, we place the structure in environments with different refractive indices, which will exhibit excellent sensitivity and facilitate the application of biochemical sensors. This simple and easy-to-fabricate metamaterial structure will have excellent potential applications in modulation, filters, and detection.

Analytical investigation of unidirectional reflectionless phenomenon near the exceptional points in graphene plasmonic system

We propose a two-dimensional array made of a double-layer of vertically separated graphene nanoribbons. The transfer matrix method and coupled mode theory are utilized to quantitatively depict the transfer properties of the system. We present a way to calculate the radiative and the intrinsic loss factors, combined with finite-difference time-domain simulation, conducting the complete analytical analysis of the unidirectional reflectionless phenomenon. By adjusting the Fermi energy and the vertical distance between two graphene nanoribbons, the plasmonic resonances are successfully excited, and the unique phenomena can be realized at the exceptional points. Our research presents the potential in the field of optics and innovative technologies to create advanced optical devices that operate in the mid-infrared range, such as terahertz antennas and reflectors.

Dynamic modulation of electric and magnetic toroidal dipole resonance and light trapping in Si-GSST hybrid metasurfaces

The weak coupling of a toroidal dipole (TD) to an electromagnetic field offers great potential for the advanced design of photonic devices. However, simultaneous excitation of electric toroidal dipoles (ETDs) and magnetic toroidal dipoles (MTDs) is currently difficult to achieve. In this work, we propose a hybrid metasurface based on Si and phase transition material G e 2 S b 2 S e 4 T e 1 (GSST), which is formed by four Si columns surrounding a GSST column and can simultaneously excite two different TD (ETD and MTD) resonances. We also calculated the electric field distribution, magnetic field distribution, and multipole decomposition of the two resonances, and the results show that the two modes are ETD resonance and MTD resonance, respectively. The polarization characteristics of these two modes are also investigated, and the average field enhancement factor (EF) of the two modes is calculated. The dynamic modulation of the relative transmission and EF is also achieved based on the tunable properties of the phase change material GSST. Our work provides a way to realize actively tunable TD optical nanodevices.

Analysis of the water-soluble vitamins based on MIM waveguide structure and Fano resonance

Fano resonance (FR) is extremely sensitive to extremely small changes in the surrounding environment. We first propose an optical nano-refractive index sensor based on Fano resonance, which is applied to the identification of water-soluble vitamins B1, B5 and B6 and the measurement of the concentration of vitamin B1. The sensor can be used to rapidly identify pure vitamins B1, B5, and B6 at a concentration of 1 g/50 mL at 25 °C based on the relationship between the wavelength shift in the FR line spectrum and the refractive index. This work shows that the sensitivity of the sensor can reach 1327.5 nm/RIU, the sensor can be used to rapidly identify vitamins B1, B5, and B6 through changes in refractive index under certain conditions. Moreover, rapid calculation of vitamin B1 solution concentration is achieved based on the relationship between different concentrations of vitamin B1 solution and their corresponding refractive indexes and wavelength shifts in their FR line spectrums, which is an important step for the application of the designed MIM waveguide structures to the fields of biology, chemistry, and medicine.

Lattice relaxation effects on the collective resonance spectra of a finite dipole array

Applying lattice parameter relaxation on a finite photonic crystal can adjust the smoothness of its surface lattice resonance spectral peak.

Fluctuation of Plasmonically Induced Transparency Peaks within Multi-Rectangle Resonators

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.

Plasmonic structure: toward multifunctional optical device with controllability

Multifunctional plasmonic components are the foundation for achieving a flexible and versatile photonic integrated loop. A compact device that can transform between multiple different functions is presented. The proposed structure consists of a resonator with a rotatable oval core coupled with three waveguides. The temporal coupled-mode theory and finite-difference time-domain method reveal that embedding of the elliptical core alters the original resonance mode, and the rotation of the core can manipulate field distribution in the cavity. Specifically, two switchable operating wavelengths are obtained, and the wavelengths can be adjusted by modifying the structural parameters of the elliptical core. Ultimately, a multifunctional optical device with signal controllability can be realized through the rotation of the embedded rotor: power splitter with selectable wavelengths and splitting ratios; bandpass filter with controllable output ports, wavelengths, and transmissions; demultiplexer with tunable output ports and transmissions; and switch with variable output ports, wavelengths, and transmissions. The fabrication tolerance of the device is investigated, considering waveguide width and coupling distance. This multifunctional plasmonic device is of great significance for the design and implementation of optical networks-on-chips.

Optical-fibre characteristics based on Fano resonances and sensor application in blood glucose detection

We propose an optical-fibre metal-insulator-metal (MIM) plasmonic sensor based on the Fano resonances of surface plasmon polaritons (SPPs). Its structure consists of a coupling fibre that connects C-shaped and rectangular cavities and a main fibre that contains a semi-circular resonator. When incident light passes through the main fibre, it excites SPPs along the interface between the metal and medium. The SPPs at the resonator induce Fano resonances, owing to the coupling effect. The results show that the designed optical-fibre MIM plasmonic sensor could flexibly tune the number of Fano resonances by adjusting the structure and geometric parameters to optimise the sensing performance. The full width at half maximum of the Lorentzian resonance spectra formed by the electric and magnetic fields reached 23 nm and 24 nm, respectively. The wavelength of the Fano resonance shifted as the refractive index changed; thus, the proposed sensor could realise the application of sensing and detection. The highest sensitivity achieved by the sensor was 1770 nm/RIU. Finally, we simulated the designed sensor to human blood-glucose-level detection, and observed that the resonance wavelength would increase with the increase of glucose concentration. Our study shows that optical fibres have broad application prospects in the field of electromagnetic switching and sensing.

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

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

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.

Plasmonic-Induced Transparencies in an Integrated Metaphotonic System

In this contribution, we numerically demonstrate the generation of plasmonic transparency windows in the transmission spectrum of an integrated metaphotonic device. The hybrid photonic–plasmonic structure consists of two rectangular-shaped gold nanoparticles fully embedded in the core of a multimode dielectric optical waveguide, with their major axis aligned to the electric field lines of transverse electric guided modes. We show that these transparencies arise from different phenomena depending on the symmetry of the guided modes. For the TE0 mode, the quadrupolar and dipolar plasmonic resonances of the nanoparticles are weakly coupled, and the transparency window is due to the plasmonic analogue of electromagnetically induced transparency. For the TE1 mode, the quadrupolar and dipolar resonances of the nanoparticles are strongly coupled, and the transparency is originated from the classical analogue of the Autler–Townes effect. This analysis contributes to the understanding of plasmonic transparency windows, opening new perspectives in the design of on-chip devices for optical communications, sensing, and signal filtering applications.

All-optical frequency-dependent magnetic switching in metal-insulator-metal stub structures

Many attempts to switch magnetization with optical pulses were based on free-space coupling schemes of circularly polarized light pulses, so-called all-optical helicity-dependent magnetic switching; however, waveguide coupling schemes are promising for on-chip all-optical magnetic switching. Metal-insulator-metal (MIM) stub structures provide a promising platform for highly integrated photonic circuits, thanks to their compact size, on-chip compatibility, and ease of fabrication. We found clockwise and counterclockwise ring-like modes in the MIM stub structure, which can act as effective magnetic fields with two opposite directions due to the inverse Faraday effect. Effective magnetic field spectra inside the MIM stub have dual resonant peaks at which the effective magnetic field intensity reaches its extreme values with opposite signs, corresponding to binary magnetic states. Switching between the binary magnetic states can be achieved by altering the optical pump frequency. The all-optical frequency-dependent magnetic switching in the MIM stub may provide a chip-compatible and ultracompact tool for ultrafast switching of magnetic order.

A multichannel color filter with the functions of optical sensor and switch

This paper reports a multichannel color filter with the functions of optical sensor and switch. The proposed structure comprises a metal-insulator-metal (MIM) bus waveguide side-couples to six circular cavities with different sizes for filtering ultra-violet and visible lights into individual colors in the wavelength range of 350-700 nm. We used the finite element method to analyze the electromagnetic field distributions and transmittance properties by varying the structural parameters in detail. The designed plasmonic filter takes advantage of filtering out different colors since the light-matter resonance and interference between the surface plasmon polaritons (SPPs) modes within the six cavities. Results show that the designed structure can preferentially select the desired colors and confine the SPPS modes in one of the cavities. This designed structure can filter eleven color channels with a small full width at half maximum (FWHM) ~ 2 nm. Furthermore, the maximum values of sensitivity, figure of merit, quality factor, dipping strength, and extinction ratio can achieve of 700 nm/RIU, 350 1/RIU, 349.0, 65.04%, and 174.50 dB, respectively, revealing the excellent functions of sensor performance and optical switch, and offering a chance for designing a beneficial nanophotonic device.

Low-Threshold Nanolaser Based on Hybrid Plasmonic Waveguide Mode Supported by Metallic Grating Waveguide Structure

A high Q-factor of the nanocavity can effectively reduce the threshold of nanolasers. In this paper, a modified nanostructure composed of a silver grating on a low-index dielectric layer (LID) and a high-index dielectric layer (HID) was proposed to realize a nanolaser with a lower lasing threshold. The nanostructure supports a hybrid plasmonic waveguide mode with a very-narrow line-width that can be reduced to about 1.79 nm by adjusting the thickness of the LID/HID layer or the duty ratio of grating, and the Q-factor can reach up to about 348. We theoretically demonstrated the lasing behavior of the modified nanostructures using the model of the combination of the classical electrodynamics and the four-level two-electron model of the gain material. The results demonstrated that the nanolaser based on the hybrid plasmonic waveguide mode can really reduce the lasing threshold to 0.042 mJ/cm², which is about three times lower than the nanolaser based on the surface plasmon. The lasing action can be modulated by the thickness of the LID layer, the thickness of the HID layer and the duty cycle of grating. Our findings could provide a useful guideline to design low-threshold and highly-efficient miniaturized lasers.

Ultrawideband bandstop filter based on Fano resonance and rectangular resonators

In this study, the effect of length of the stub on the formation of the Fano resonance in structures which possess a metal-insulator-metal (MIM) waveguide coupled to rectangular cavities by the stub is investigated theoretically and numerically. The resulting Fano resonance is used to design an ultrawideband bandstop filter that can filter all wavelengths between two telecommunication windows of $\lambda = 850\;{\rm nm}$ and $\lambda = 1310\;{\rm nm}$. The structure is based on two rectangular cavities coupled to the MIM waveguide by stubs that are located at an adjusted distance from each other; the interference superposition of reflected and transmitted waves from each other will make this filtering phenomenon. The center wavelength of the bandstop of the structure is highly adjustable by changing the dimensions of the structure. The theoretical and the numerical results are, respectively, based on the transmission line model and the finite-difference time-domain method. The theoretical results comply well with the numerical ones. To analyze the Fano resonance, temporal coupled mode theory is also exploited. The proposed structure has significant applications in highly integrated optical circuits.

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

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

Slow light using magnetic and electric Mie resonances

The ability to slow down light leads to strong light-matter interaction, which is important for a number of optical applications such as sensing, nonlinear optics, and optical pulse manipulation. Here, we show that a dramatic reduction in the speed of light can be realized through the interference of electric and magnetic dipole resonances in Mie-type resonators made of a dielectric material with a high refractive index. We present a general theory that links the maximal speed reduction of light to resonator radiation losses and then consider a specific realization based on silicon nanodisk arrays.

Plasmonic nanosensor based on multiple independently tunable Fano resonances

A novel refractive index nanosensor with compound structures is proposed in this paper. It consists of three different kinds of resonators and two stubs which are side-coupled to a metal-dielectric-metal (MDM) waveguide. By utilizing numerical investigation with the finite element method (FEM), the simulation results show that the transmission spectrum of the nanosensor has as many as five sharp Fano resonance peaks. Due to their different resonance mechanisms, each resonance peak can be independently tuned by adjusting the corresponding parameters of the structure. In addition, the sensitivity of the nanosensor is found to be up to 1900 nm/RIU. For practical application, a legitimate combination of various different components, such as T-shaped, ring, and split-ring cavities, has been proposed which dramatically reduces the nanosensor dimensions without sacrificing performance. These design concepts pave the way for the construction of compact on-chip plasmonic structures, which can be widely applied to nanosensors, optical splitters, filters, optical switches, nonlinear photonic and slow-light devices.

Plasmon-Induced Transparency in an Asymmetric Bowtie Structure

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.

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.

Dual-band unidirectional reflectionlessness at exceptional points in a plasmonic waveguide system based on near-field coupling between two resonators

Dual-band unidirectional reflectionlessness at exceptional points is investigated theoretically in a non-Hermitian plasmonic waveguide system, based on near-field coupling by using only two resonators. The system consists of a metal-insulator-metal waveguide end-coupled to two nanohole resonators. The reflectivity for the forward (backward) direction is ∼0 (∼0) at frequency 205.20 THz (194.56 THz), while for the backward (forward) direction it is ∼0.76 (∼0.78). Moreover, the quality factors of the dual-band unidirectional reflectionlessness for forward and backward directions can reach ∼132 and ∼137, respectively.

Plasmonic-induced absorption based on an end-coupled combined resonance system of a semiannular cavity and rectangular cavity

A semiannular rectangular composite cavity structure based on metal-insulator-metal waveguides is proposed. By adding a rectangular cavity at a suitable position around the semiannular cavity (SAC), a single analogous plasmonic-induced absorption (PIA) effect is achieved at the expected mode of the SAC structure. After adding two rectangular cavities together in the SAC system, dual analogous PIA effects for both modes can be realized simultaneously. In addition, the phase response is also studied, and abnormal dispersions are achieved in the PIA windows, which can be used in the integrated optics for fast/slow light. The performances of the proposed two-dimensional structure are analyzed and studied by the coupled-mode theory and the finite-difference time-domain method, respectively.

Plasmonic slow light device using superfocusing on a bow-tied metallic waveguide

We demonstrate a plasmonic slow light device using super focusing on a bow-tied metallic waveguide that can be fabricated using complementary metal-oxide semiconductor compatible processes. By solving the characteristic equation of a bow-tied metallic waveguide, we confirmed that the group indices increased as the waveguide width decreased and that they could attain over 11.0 in the telecommunication wavelength band. Additionally, we experimentally confirmed using an autocorrelation measurement system in which the pulse width of the bow-tied metallic waveguide was 8.0 fs longer than that of a ridged metallic waveguide. Therefore, the proposed device will contribute to the realization of all-plasmonic memories and amplifiers.

Flexibly tunable high-quality-factor induced transparency in plasmonic systems

The quality (Q) factor and tunability of electromagnetically induced transparency (EIT)-like effect in plasmonic systems are restrained by the intrinsic loss and weak adjustability of metals, limiting the performance of the devices including optical sensor and storage. Exploring new schemes to realize the high Q-factor and tunable EIT-like effect is particularly significant in plasmonic systems. Here, we present an ultrahigh Q-factor and flexibly tunable EIT-like response in a novel plasmonic system. The results illustrate that the induced transparency distinctly appears when surface plasmon polaritons excited on the metal satisfy the wavevector matching condition with the guided mode in the high-refractive index (HRI) layer. The Q factor of the EIT-like spectrum can exceed 2000, which is remarkable compared to that of other plasmonic systems such as plasmonic metamaterials and waveguides. The position and lineshape of EIT-like spectrum are strongly dependent on the geometrical parameters. An EIT pair is generated in the splitting absorption spectra, which can be easily controlled by adjusting the incident angle of light. Especially, we achieve the dynamical tunability of EIT-like spectrum by changing the Fermi level of graphene inserted in the system. Our results will open a new avenue toward the plasmonic sensing, spectral shaping and switching.

Tunable plasmon-induced absorption in an integrated graphene nanoribbon side-coupled waveguide

By designing a novel graphene plasmonic band-pass filter with two gold ribbons, we have numerically and analytically investigated the transmission properties of plasmon-induced absorption (PIA) in a compact graphene nanoribbon side-coupled waveguide. The formation and evolution of the PIA window are dependent on the superposition of super resonances and the near-field coupling intensity between the designed two resonators. Interestingly, the induced absorption window not only can be engineered longitudinally in intensity, but also dynamically tuned horizontally in the resonant wavelength by changing the Fermi energy of the graphene layers. Optical time delay near 1.0 ps can be realized in the PIA window, which exhibits excellent slow light features. Double PIA resonance is also discussed. This result may have potential applications in graphene plasmonic switching and buffering.

Peak modulation in multicavity-coupled graphene-based waveguide system

Plasmonically induced transparency (PIT) in a multicavity-coupled graphene-based waveguide system is investigated theoretically and numerically. By using the finite element method (FEM), the multiple mode effect can be achieved, and blue shift is exhibited by tunable altering the chemical potential of the monolayer graphene. We find that the increasing number of the graphene rectangle cavity (GRC) achieves the multiple PIT peaks. In addition, we find that the PIT peaks reduce to just one when the distance between the third cavity and the second one is 100 nm. Easily to be experimentally fabricated, this graphene-based waveguide system has many potential applications for the advancement of 3D ultra-compact, high-performance, and dynamical modulation plasmonic devices.

Dynamically Tunable Plasmon-Induced Transparency in On-chip Graphene-Based Asymmetrical Nanocavity-Coupled Waveguide System

A graphene-based on-chip plasmonic nanostructure composed of a plasmonic bus waveguide side-coupled with a U-shaped and a rectangular nanocavities has been proposed and modeled by using the finite element method in this paper. The dynamic tunability of the plasmon-induced transparency (PIT) windows has been investigated. The results reveal that the PIT effects can be tuned via modifying the chemical potential of the nanocavities and plasmonic bus waveguide or by varying the geometrical parameters including the location and width of the rectangular nanocavity. Further, the proposed plasmonic nanostructure can be used as a plasmonic refractive index sensor with a sensing sensibility of 333.3 nm/refractive index unit (RIU) at the the PIT transmission peak. Slow light effect is also realized in the PIT system. The proposed nanostructure may pave a new way towards the realization of graphene-based on-chip integrated nanophotonic devices.

Multiple-band directional excitation of surface plasmons based on electromagnetically induced transparency

We propose a structure of two metallic slits, one of which is accompanied by two non-equal length cavities. Our simulation results, conducted by finite difference time-domain method, show that there are four bands that can achieve launching surface plasmons (SPs) unidirectionally in the communication region, and each band is very narrow (between 10 nm and 30 nm). Our design method is based on SPs interference, and the phase-shift variation of SPs in the electromagnetically induced transparency wavelength region. Our design method provides a new way to manipulate SPs and control SPs' propagating direction in photonic circuits.

Analogue of double-Λ-type atomic medium and vector plasmonic dromions in a metamaterial

We consider an array of the meta-atom consisting of two cut-wires and a split-ring resonator coupled with an electromagnetic field with two polarization components. We show that the system can be taken as a classical analogue of the atomic medium of a double-Λ-type four-level configuration coupled with four laser fields and working under the condition of electromagnetically induced transparency, exhibits an effect of plasmon induced transparency (PIT), and displays a similar behavior of atomic four-wave mixing (FWM). We show also that with the PIT and FWM effects the system can support vector plasmonic dromions when a nonlinear varactor is mounted onto the each gap of the split-ring resonator. Our work not only gives a plasmonic analogue of the FWM in coherent atomic systems but also provides the possibility for obtaining new type of plasmonic excitations in metamaterials.

Dual-band unidirectional reflectionless phenomena in an ultracompact non-Hermitian plasmonic waveguide system based on near-field coupling

Dual-band unidirectional reflectionlessness and coherent perfect absorption (CPA) are demonstrated in a non-Hermitian plasmonic waveguide system based on near-field coupling between a single resonator and the resonant modes of two resonators showing an electromagnetically induced-transparency-like (EIT-like) effect. The non-Hermitian plasmonic system consists of three metal-insulator-metal (MIM) resonators coupled to a MIM plasmonic waveguide.

Effect of Surface Plasmon Coupling to Optical Cavity Modes on the Field Enhancement and Spectral Response of Dimer-Based sensors

We present a theoretical approach to narrow the plasmon linewidth and enhance the near-field intensity at a plasmonic dimer gap (hot spot) through coupling the electric localized surface plasmon (LSP) resonance of a silver hemispherical dimer with the resonant modes of a Fabry-Perot (FP) cavity. The strong coupling is demonstrated by the large anticrossing in the reflection spectra and a Rabi splitting of 76 meV. Up to 2-fold enhancement increase can be achieved compared to that without using the cavity. Such high field enhancement has potential applications in optics, including sensors and high resolution imaging devices. In addition, the resonance splitting allows for greater flexibility in using the same array at different wavelengths. We then further propose a practical design to realize such a device and include dimers of different shapes and materials.

Design of a broadband reciprocal optical diode in a silicon waveguide assisted by silver surface plasmonic splitter

In this paper, we propose a new scheme for a reciprocal optical diode integrated in a multimode silicon waveguide. The compact 4μm long functional region consists of a tapered coupler, a narrow single-mode waveguide, and a half-elliptical silver surface plasmonic splitter with refractive index modification of silicon. This spatial asymmetric design achieves even-to-odd mode conversion in the forward direction and blocks propagation of the even mode in the backward direction. The maximum contrast ratio and forward transmission efficiency reach approximately 0.99 and 87% while the values respectively keep higher than 0.96 and 80% within a 100nm operational bandwidth in a two-dimensional design. Both freestanding and SOI-based three-dimensional devices are simulated and at least a 0.94 contrast ratio is observed. Moreover, the robustness is demonstrated by introducing deviations to the surface plasmonic splitter. The proposed scheme brings together advantages including a high contrast ratio (>0.94), a large operational bandwidth (100nm) and a small footprint (4μm long).

Role of Resonance Modes on Terahertz Metamaterials based Thin Film Sensors

We investigate thin film sensing capabilities of a terahertz (THz) metamaterial, which comprises of an array of single split gap ring resonators (SRRs). The top surface of the proposed metamaterial is covered with a thin layer of analyte in order to examine various sensing parameters. The sensitivity and corresponding figure of merit (FoM) of the odd and even resonant modes are analyzed with respect to different thicknesses of the coated analyte film. The sensing parameters of different resonance modes are elaborated and explained with appropriate physical explanations. We have also employed a semi-analytical transmission line model in order to validate our numerically simulated observations. Such study should be very useful for the development of metamaterials based sensing devices, bio-sensors etc in near future.

Analogue of electromagnetically induced absorption with double absorption windows in a plasmonic system

We report the observation of an analog of double electromagnetically induced absorption (EIA) in a plasmonic system consisting of two disk resonators side-coupled to a discrete metal-insulator-metal (MIM) waveguide. The finite-difference time-domain (FDTD) simulation calculations show that two absorption windows are obtained and can be easily tuned by adjusting the parameters of the two resonance cavities. The consistence between the coupled-model theory and FDTD simulation results verify the feasibility of the proposed system. Since the scheme is easy to be fabricated, our proposed configuration may thus be applied to narrow-band filtering, absorptive switching, and absorber applications.

Method proposing a slow light ring resonator structure coupled with a metal-dielectric-metal waveguide system based on plasmonic induced transparency

We demonstrate the analogue of electromagnetically induced transparency (EIT) in a metal-dielectric-metal (MDM) plasmonic waveguide. Plasmonic induced transparency is a method similar to EIT. In this paper, a plasmonic MDM waveguide is proposed by using an ellipse shaped side-coupled ring resonator and simulated by finite difference time domain. Plasmonics as a new field of chip-scale technology is an interesting substrate, which is used to propose and numerically investigate a novel MDM structure. The aforementioned framework is a 2×2 plasmonic ring resonator, employing gold as a metal and polymethyl methacrylate as a dielectric. Simulations show that a transparent window is located at 1550 nm and signal wavelength is assumed to be 860 nm, which is the phenomenon of plasmonic induced transparency. The velocity of the plasmonic mode can be considerably slowed while propagating along the MDM bends. Our proposed configuration may thus be applied to storing and stopping light in plasmonic waveguide bends. This plasmonic waveguide system may find important applications for multichannel plasmonic filters, nano-scale optical switching, delay time devices, and slow-light devices in highly integrated optical circuits and networks. In comparison with our previous theoretical work based on circular shaped ring resonators, it is shown that ellipse shaped ring resonators demonstrate better specifications with a slow down factor estimated to be more than 30.

High Quality Plasmonic Sensors Based on Fano Resonances Created through Cascading Double Asymmetric Cavities

In this paper, a type of compact nanosensor based on a metal-insulator-metal structure is proposed and investigated through cascading double asymmetric cavities, in which their metal cores shift along different axis directions. The cascaded asymmetric structure exhibits high transmission and sharp Fano resonance peaks via strengthening the mutual coupling of the cavities. The research results show that with the increase of the symmetry breaking in the structure, the number of Fano resonances increase accordingly. Furthermore, by modulating the geometrical parameters appropriately, Fano resonances with high sensitivities to the changes in refractive index can be realized. A maximum figure of merit (FoM) value of 74.3 is obtained. Considerable applications for this work can be found in bio/chemical sensors with excellent performance and other nanophotonic integrated circuit devices such as optical filters, switches and modulators.

Dynamics of single photon transport in a one-dimensional waveguide two-point coupled with a Jaynes-Cummings system

We study the dynamics of an ultrafast single photon pulse in a one-dimensional waveguide two-point coupled with a Jaynes-Cummings system. We find that for any single photon input the transmissivity depends periodically on the separation between the two coupling points. For a pulse containing many plane wave components it is almost impossible to suppress transmission, especially when the width of the pulse is less than 20 times the period. In contrast to plane wave input, the waveform of the pulse can be modified by controlling the coupling between the waveguide and Jaynes-Cummings system. Tailoring of the waveform is important for single photon manipulation in quantum informatics.

Transmission characteristics and transmission line model of a metal-insulator-metal waveguide with a stub modified by cuts

We propose a structure of a metal-insulator-metal (MIM) waveguide with a stub modified by cuts. Our simulation results, conducted by the finite element method, show that the wavelengths of transmission dip vary with the position of the cuts and form the zigzag lines. A transmission line model is also presented, and it agrees with simulation results well. It is believed that our findings provide a smart way to design a plasmonic waveguide filter at the communication region based on MIM structures.

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.

Detuned Plasmonic Bragg Grating Sensor Based on a Defect Metal-Insulator-Metal Waveguide

A nanoscale Bragg grating reflector based on the defect metal-insulator-metal (MIM) waveguide is developed and numerically simulated by using the finite element method (FEM). The MIM-based structure promises a highly tunable broad stop-band in transmission spectra. The narrow transmission window is shown to appear in the previous stop-band by changing the certain geometrical parameters. The central wavelengths can be controlled easily by altering the geographical parameters. The development of surface plasmon polarition (SPP) technology in metallic waveguide structures leads to more possibilities of controlling light at deep sub-wavelengths. Its attractive ability of breaking the diffraction limit contributes to the design of optical sensors.

Tuning Fano resonances with a nano-chamber of air

By designing a polymer-film-coated asymmetric metallic slit structure that only contains one nanocavity side-coupled with a subwavelength plasmonic waveguide, the Fano resonance is realized in the experiment. The Fano resonance originates from the interference between the narrow resonant spectra of the radiative light from the nanocavity and the broad nonresonant spectra of the directly transmitted light from the slit. The lateral dimension of the asymmetric slit is only 825 nm. Due to the presence of the soft polymer film, a nano-chamber of air is constructed. Based on the opto-thermal effect, the air volume in the nano-chamber is expanded by a laser beam, which blueshifts the Fano resonance. This tunable Fano resonance in such a submicron slit structure with a nano-chamber is of importance in the highly integrated plasmonic circuits.

Theoretical analysis and applications in inverse T-shape structure

An inverse T-shape structure, consisting of a bus waveguide coupled with two perpendicular rectangular cavities, has been investigated numerically and theoretically. The position of the transparency window can be manipulated by adjusting the lateral displacement between the two perpendicular cavities. The effects of changing different structural parameters on the transmission features are investigated in detail. The results indicate that the length of two cavities play important roles in optimizing optical response. Finally, two simple applications based on the inverse T-shape structure are briefly discussed. The findings demonstrate that the first- and second-order modes can be separated without interference, and the sensitivity of the inverse T-shape is as high as 1750 nm per refractive index unit (RIU); the corresponding figure of merit (FOM) reaches up to 77.1  RIU^-1, which is higher than in previous reports. The plasmonic configuration possesses the advantages of easy fabrication, compactness, and higher sensitivity as well as higher FOM, which will greatly benefit the compact plasmonic filter and high-sensitivity nanosensor in highly integrated optical devices.

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.

Multi-layer topological transmissions of spoof surface plasmon polaritons

Spoof surface plasmon polaritons (SPPs) in microwave frequency provide a high field confinement in subwavelength scale and low-loss and flexible transmissions, which have been widely used in novel transmission waveguides and functional devices. To play more important roles in modern integrated circuits and systems, it is necessary and helpful for the SPP modes to propagate among different layers of devices and chips. Owing to the highly confined property and organized near-field distribution, we show that the spoof SPPs could be easily transmitted from one layer into another layer via metallic holes and arc-shaped transitions. Such designs are suitable for both the ultrathin and flexible single-strip SPP waveguide and double-strip SPP waveguide for active SPP devices. Numerical simulations and experimental results demonstrate the broadband and high-efficiency multi-layer topological transmissions with controllable absorption that is related to the superposition area of corrugated metallic strips. The transmission coefficient of single-strip SPP waveguide is no worse than -0.8 dB within frequency band from 2.67 GHz to 10.2 GHz while the transmission of double-strip SPP waveguide keeps above -1 dB within frequency band from 2.26 GHz to 11.8 GHz. The proposed method will enhance the realizations of highly complicated plasmonic integrated circuits.