Side-coupled Cavities Research Articles

Tunable multiple plasmon-induced transparencies in multiple T-shaped cavities waveguide based on Dirac semimetals

We study the multiple plasmon-induced transparencies (PIT) in the waveguide structure based on bulk Dirac semimetal (BDS) containing multiple side-coupled T-shaped cavities. Using the finite element method, we mainly investigate the transmission spectra, the transmission phase shifts, the delay times, and the magnetic field distributions of the two T-shaped cavities waveguide structure. It is shown that their transmission characteristics strongly depend on the geometric parameters of the waveguide structure. In addition, we discuss the effect of the Fermi level of BDS and the refractive index of the medium on the transmission spectrum of the two T-shaped cavities structure. To get more PIT peaks, we try to simulate the transmission spectrum of three T-shaped cavities waveguide structure. All the results about the multiple PIT phenomena may have some possible applications in designing lenses, high-capacity storage devices, and new filters.

Analysis of the Dual Side-Coupled RF Cavities for the HPM Devices—An Equivalent Circuit Approach

An equivalent circuit analysis was carried out for the dual side-coupled cavities (DSCCs) suitable as RF interaction structure in the high power microwave (HPM) devices and linear accelerators. Considering lossless structure, its equivalent shunt capacitance per unit length and series inductance per unit length are determined for the TM₀₁ mode operation. These equivalent circuit parameters were used to calculate the phase velocity and characteristic impedance of the DSCC. This was utilized further to appreciate the dispersion behavior of the DSCC. The developed analysis was also validated through electromagnetic simulations.

Proposal for metal–insulator–metal plasmonic power splitter and demultiplexer suitable for implementation in optical switches

The authors propose a nanoplasmonic structure based on metal–insulator–metal (MIM) waveguides which consists of a 3 db power splitter and a side-coupled nanocavity placed between its two output ports. The proposed structure is analysed by the resonant theory and dispersion relation. Transmission spectra of the presented device are also extracted using the finite difference time domain method. Numerical simulations demonstrate that the structure operates as a band-stop filter in each output channel. Moreover, the resonance wavelengths can be tuned by adjusting the geometrical parameters of the nanocavity. Meanwhile, the authors developed the cavity and added an extra stub to it to append a new stopband to the output spectrum. Each resonance wavelength can be easily manipulated by changing the widths of the nanocavity and the added stub. In the end, by choosing two different lengths for the side-coupled cavities, the authors convert the power splitter into a wavelength division multiplexer. These introduced MIM, the authors proposed here, may have various meaningful applications in nanoscale high-density photonic circuits such as optical switches, logic plasmonic circuits and slow-light devices.

Anti-external interference sensor based on cascaded side-coupled photonic crystal nanobeam cavities for simultaneous sensing of the refractive index and temperature

We present a cascaded side-coupled silicon-on-insulator (SOI) photonic crystal nanobeam cavities (PCNCs) sensor for simultaneous sensing of the refractive index (RI) and temperature. Two types of PCNCs are designed for increasing the demodulation coefficient: the low-index mode cavity exposed to water directly ( c a v 1 ) and the high-index mode cavity covered with SU-8 cladding ( c a v 2 ). Moreover, the cascaded side-coupled system is modeled, and the transmission expression is derived based on temporal coupled-mode theory. Furthermore, the correlations between the structural parameters, the transmission, quality factor are discussed. By using three-dimensional finite-difference time-domain simulations, we further prove the ability to detect RI and temperature simultaneously, obtaining a RI sensitivity of 210 nm/RI unit (RIU) and a temperature sensitivity of 53 pm/K for c a v 1 , whereas there is a RI sensitivity of 0 nm/RIU and a temperature sensitivity of − 90 p m / K for c a v 2 . Moreover, the maximum of external interference ranges of the proposed sensor are 7.5 × 10 − 6 and 0.01 for the RI and temperature, respectively, and the deviation ratios are approximately 0.56% and 3.33% for the RI and temperature, respectively. Therefore, the proposed sensor has strong anti-interference ability and accuracy, which is a promising platform for future applications of dual-parameter sensing.

Plasmonic refractive index sensor with multi-channel Fano resonances based on MIM waveguides

A multi-channel Fano resonant structure is proposed and analyzed based on subwavelength metal–insulator–metal (MIM) waveguides. First, two MIM output ports associated with specific side-coupled cavities are designed to locate at the center and quarter positions of an end-coupled cavity, respectively. Since the interference between the dark and bright modes, dual-channel Fano resonances with asymmetrical lines shapes are obtained at both ports, respectively. High sensitivity and figure of merits are investigated. Besides, phase shifts are also investigated leading to positive and negative group delays available at the Fano peaks and dips, respectively. Likewise, two extra output ports with identical resonant cavities are placed on the other side of the end-coupled cavity. In this case, four-channel Fano resonances with considerable performances are obtained. The proposed structure is analyzed by the coupled mode theory and the finite difference time domain method. It is believed this device can be used as a chip-scale refractive index sensor and optical filter.

Optimization of the S-band side-coupled cavities for proton acceleration

The proton beam with energy around 100 MeV has seen wide applications in modern scientific research and in various fields. However, proton sources in China fall short for meeting experimental needs owing to the vast size and expensive traditional proton accelerators. The Institute of Nuclear Science and Technology of Sichuan University proposed to build a 3 GHz side-coupled cavity linac (SCL) for re-accelerating a 26 MeV proton beam extracted from a CS-30 cyclotron to 120 MeV. We carried out investigations into several vital factors of S-band SCL for proton acceleration, such as optimization of SCL cavity geometry, end cell tuning, and bridge coupler design. Results demonstrated that the effective shunt impedance per unit length ranged from 22.5 to 59.8 MΩ/m throughout the acceleration process, and the acceleration gradient changed from 11.5 to 15.7 MV/m when the maximum surface electric field was equivalent to Kilpatrick electric field. We obtained equivalent circuit parameters of the biperiodic structures and applied them to the end cell tuning; results of the theoretical analysis agreed well with the 3D simulation. We designed and optimized a bridge coupler based on the previously obtained biperiodic structure parameters, and the field distribution un-uniformness was < 1.5% for a two-tank module. The radio frequency power distribution system of the linac was obtained based on the preliminary beam dynamics design.

Refractive index sensor based on multiple Fano resonances in a plasmonic MIM structure.

A chip-scale refractive index sensor based on multiple Fano resonances is proposed by using a metal-insulator-metal (MIM) structure, which is constructed by two side-coupled semi-ring cavities and a vertical cavity. The finite-difference time-domain method and multimode interference coupled-mode theory are employed to simulate and analyze the transmission spectra of this structure, respectively. First, dual Fano resonances are generated in the MIM structure with a baffle and a semi-ring cavity. By arranging two additional cavities, the mode interferences successfully induce up to six ultra-sharp and asymmetrical Fano peaks. The calculated sensing performances are available with ultra-high sensitivity of 1405 nm/RIU and figure of merit of 3.62×105. This chip-scale refractive index sensor may have great applications in highly integrated photonic circuits.

High Sensitivity Plasmonic Metal-Dielectric-Metal Device With Two Side-Coupled Fano Cavities

In this paper, we propose a compact plasmonic sensor structure comprised of a metal-dielectric-metal (MDM) waveguide, and a baffle plate in waveguide core and two side-coupled rectangular cavities. In this structure, two Fano resonances are achieved and can be tuned independently by changing the structural parameters of the cavities. Especially, when the resonant wavelengths of the two Fano resonances are the same, the sensing sensitivity can be enhanced by coupling between two Fano resonances. By investigating the transmission spectrum, the effect of structural parameters on Fano resonances and the refractive index sensitivity of the sensor structure are analyzed in detail. The numerical simulations demonstrate a sensitivity as high as 1295nm/RIU and a figure of merit of 1647.

Tunable multimode plasmonic filter based on asymmetric dual side-coupled U-shape cavities resonators

In this paper, the plasmonic system composed of a Metal-Insulator-Metal (MIM) waveguide and dual asymmetric side-coupled U-shaped cavities is reported. It demonstrates that the strong coupling effect can lead to the opening of new transmission window. One of new peaks originates from the interference between the dual U-shaped cavities and bus waveguide, and the other is induced by the asymmetry of the structure, and each peak can be well tuned independently by changing the structural parameters of the U-shaped cavity. The effects of dielectric parameters on the transmission characteristics of the coupled system are also analysis, we find that the sensor sensitivity can reach 496 nm/RIU and a new sharp peak will exist in the short-wave region when dielectric parameter n = 1.2. By combining the magnetic field distributions, the internal physical mechanism is discussed. The proposed compact and simple plasmonic structure may have potential applications in nano-scale filter and other high density plasmonic integration circuits.

Control of Multiple Fano Resonances Based on a Subwavelength MIM Coupled Cavities System

In this paper, we investigate and analyze numerically and theoretically a coupled plasmonic resonant system by using finite-difference time-domain (FDTD) method and multimode interference coupled-mode theory (MICMT), respectively. First, by employing an end-coupled cavity associated with two side-coupled cavities, three ultra-sharp and asymmetrical Fano resonant peaks are observed in the transmission spectrum. High refractive index sensitivities and high figure of merits are achieved for all the peaks. In the interest of highly integrated optical devices, up to seven Fano peaks with asymmetrical line shapes are achieved when the composite structure is successfully extended by two additional cavities, whereas most of the reported results are about 1-4 Fano resonances in a single structure. Preferred performances are also obtained for this chip-scale structure. This proposed structure may have great potential applications for on-chip optical sensing and optical communication in highly integrated photonic circuits.

A Method for Fast and Reliable Analysis and Optimization of Side-Coupled Cavity Filters and Multiplexers

This paper presents an efficient segmentation method of a filter made up of side-coupled cavities that use a reduced complexity model. The model is built by using the reactance matrix of only three parallel waveguides, either coaxial or hollow, coupled via apertures among them. This allows building the databases necessary for fast and reliable analysis and optimization of very complex filters and multiplexers. The method was used to design a low-cost eight-channel coaxial-cavity input multiplexer.

Fano resonance and Rabi splitting in MDM side-coupled cavities systems

We theoretically investigate the Fano resonance and Rabi splitting in the metal–dielectric–metal plasmonic waveguides with stub resonators. The proposed nanostructure waveguide consists of bulb-like resonators and a thin Metamaterial (MM) baffle. The bulb-like resonator with low Q factor can work as a continuum state, and the plasmonic waveguide inserted with a MM baffle can work as a discrete state. Fano resonances are obtained because of the interaction between the two states in the structure and the Fano resonance can be tuned by changing the separation between the resonator and the MM baffle. In the plasmonic waveguide system with two resonators and a MM baffle, Rabi splitting occurs because of the strong coupling between the narrow spectral response of MM baffle and the Fabry–Perot resonance caused by the two bulb-like resonators The pronounced anticrossing behavior of the splitting peaks can be modified by varying the resonant frequency of MM baffle, the bulb-like resonator’s radius and the stub height. Furthermore, it is also found that the Rabi splitting can also be tuned by the separation between resonator and the MM baffle. Such plasmonic waveguide system may have potential applications in providing a stimulating insight to explore new fundamental physics in analogous atomic systems.

Design of a high-forward-transmission all-optical diode based on cascaded side-coupled photonic crystal cavities

A high-forward-transmission all-optical diode based on cascaded photonic crystal cavities is proposed. To obtain a high forward transmission and a large nonreciprocity, we generate a box-like spectrum by cascading two side-coupled photonic crystal cavities. We find that by appropriately adjusting the distance between two cavities, a high contrast of the transmission and a flat spectral lineshape with almost 100% transmission can be obtained. These characteristics are preferable for achieving a high forward transmission and a large nonreciprocity. Numerical results show that the designed all-optical diode performs perfect transmission with a large nonreciprocal transmission ratio of >50 dB.

Simultaneous measurement of refractive index and temperature using cascaded side-coupled photonic crystal nanobeam cavities

We present the design, fabrication, and characterization of cascaded side-coupled Silicon-on-Insulator (SOI) photonic crystal (PhC) nanobeam cavities for simultaneous measurement of refractive index (RI) and temperature. Due to the different mode distribution in air-mode cavity and dielectric-mode cavity, the two types of PhC nanobeam cavities have quite different sensitivities towards the changes of ambient RI and temperature. We demonstrated the feasibility to obtain RI and temperature simultaneously with a single measurement, obtaining a RI sensitivity of 254.6 nm/RIU (refractive index unit) and a temperature sensitivity of 30.1 pm/°C for air-mode cavity, while a RI sensitivity of 105.5 nm/RIU and a temperature sensitivity of 56.4 pm/°C for dielectric-mode cavity.

Transmission characteristics of a Y-shaped MIM plasmonic waveguide with side-coupled cavities

A plasmonic Y-shaped metal-insulator-metal (MIM) plasmonic waveguide with two side-coupled Fabry–Perot (FP) resonant cavities is proposed. Simulation results show that there is a stopband existing in the transmission spectrum of each output port. When the lengths of the two cavities are equal, the transmission spectra of the two output ports are almost coincident. But if they are not equal, the two stopbands are not in the same place. Meanwhile, the transmission dip of one output port corresponds to the transmission peak of another port. By ad-justing the length, width and refractive index of the two FP cavities, one can control the resonant wavelength of each cavity to further achieve functions of filtering, power splitting, switching and refractive index sensing. Results show that the sensing sensitivity is up to 1280 nm/RIU with its figure of merit above 200 when the waveguide is used as a refractive sensor. The proposed waveguide structure has potential applications in the fields of integrated optical filter, nano-optic switch, power splitter and refractive sensor in subwavelength scale.

Tunable Plasmon-Induced Transparency Effect in MIM Side-Coupled Isosceles Trapezoid Cavities System

We propose a plasmonic structure based on the metal-insulator-metal waveguide with the side-coupled isosceles trapezoid cavities. Both of the structures based on the side-coupled trapezoid cavities separated or connected with waveguides can realize the plasmon-induced transparency (PIT). By adjusting the structure parameters, the off-to-on PIT response can be tunably achieved. The coupled mode theory (CMT) method is used to study the PIT phenomenon and explain the transmission characteristics. This work may provide a potential way for designing highly integrated photonic devices.

基于表面等离子双矩形腔结构的可调微流控类EIT系统的研究

基于表面等离子双矩形腔结构的可调微流控类EIT系统的研究

Plasmonic-induced transparency in a MIM waveguide with two side-coupled cavities

We investigate numerically the classical analogue of electromagnetically induced transparency (EIT) in a nanoplasmonic structure constituted by two side-coupled cavities. Two configurations are considered: (1) two cavities connected symmetrically on each side of the waveguide; (2) two cavities situated on the same side. In the first case, the EIT resonance occurs as a consequence of the destructive interference between the two cavities (playing the role of two coupled radiative oscillators), whereas in the second situation, the phenomenon arises due to a coupling between dark and radiative resonators. By detuning the sizes of the two cavities (i.e., the length difference ΔL, keeping their width w similar), we show that the position, width and quality factor of the EIT resonance depend strongly on ΔL. The effect of the metal gap separating the two cavities from the waveguide is also discussed. These results may have important applications for designing integrated devices such as narrow-frequency optical filters, novel sensors and high-speed switches.

Experimental demonstration of a band-notched line-defect waveguide in a surface-wave photonic crystal

We propose and experimentally demonstrate a band-notched line-defect waveguide in a surface-wave photonic crystal, which consists of a straight line-defect waveguide and side-coupled defect cavities. A narrow stop band can be observed in the broadband transmission spectra. We also demonstrate that both the filtering levels and filtering frequencies of the band-notched line-defect waveguide can be adjusted through changing the number and the height of metallic pillars of the side-coupled defect cavities. The band-notch function is based on the idea that the propagating surface modes with the resonance frequencies of the side-coupled defect cavities will be tightly localized around the defect sites, being filtered from the waveguide output. Transmission spectra measurements and direct near-field profile imaging are performed at microwave frequencies to verify our design. These results may enable various band-notched devices applications and provide routes for the realization of surface-wave filters on a single metal surface.

Ultra-high Sensitivity Plasmonic Nanosensor Based on Multiple Fano Resonance in the MDM Side-Coupled Cavities

We propose a compact plasmonic structure comprising a metal-dielectric-metal (MDM) waveguide coupled with a side cavity and groove resonators. The proposed system is investigated by the finite element method. Simulation results show that the side-coupled cavity supports a local discrete state and the groove provides a continuous spectrum, the interaction between them, gives rise to the Fano resonance. The asymmetrical line shape and the resonant wavelength can be easily tuned by changing the geometrical parameters of the structure. Moreover, we can extend this plasmonic structure by the double side-coupled cavities to gain the multiple Fano resonances. The proposed structure can serve as an excellent plasmonic sensor with a sensitivity of ∼1900 nm/RIU and a figure of merit of about ∼3.8 × 104, which can find wide applications for nanosensors.