基于耦合开口方环共振空腔的可控法诺共振研究

基于表面等离子激元理论与金属-介质-金属波导结构提出一个由开口方环共振空腔、挡板及MIM波导组成的波导结构,并使用有限元方法系统地研究了该结构的透射特性.仿真计算结果表明:该结构可以产生法诺共振现象,其共振波长可以通过改变开口方环空腔的长度及开口大小进行调节,该结构敏感度可达1 600nm/RIU,品质因数为1.31×10~5.此外,通过调整方环共振空腔上开口的位置,在波导中产生了双重法诺共振现象,其敏感度可达1 700nm/RIU,品质因数为8.3×10~4.该结构有望在光学集成回路,尤其是纳米生物传感器方面得到比较广泛的应用.     

Photonic crystal bends and power splitters based on ring resonators

In this paper, theoretical and numerical analysis of low-loss right angle waveguide bends and T-junctions based on ultra-compact photonic crystal ring resonators (PCRR) is presented. Desirable characteristics are obtained by designing the waveguide bends and T-junctions as low-Q resonant cavities with certain symmetries and small radiation loss. A simple analysis, based on coupled mode theory (CMT) in time, is used to explain the operation principles which agree qualitatively with the numerical results. These structures have high transmission efficiency over a large bandwidth in third communication window. Also effects of changing the ring size on power splitter transmission characteristics are discussed. The perfect transmission and zero reflection conditions are discussed by applying coupled mode theory. Results obtained by the finite difference time domain (FDTD) simulations are consistent with those from the coupled mode theory.     

MIM结构的SPP模式理论与仿真计算研究

为了进一步明确MIM （Metal-insulator-metal）波导结构的SPP （Surface plasmon polariton）模式特性,建立了MIM结构的SPP模式关系、激发系数和反射系数的理论模型。仿真数值计算结果表明：较大的介质厚度的TM基态模式衰减超过了振荡模式衰减,与传统的介质波导明显不同;TE模式表现为失真的介质光波导模式特性,其传播距离要远大于TM₀;MIM结构中腔的Q值随着长度增加而增大,表明了SPP反射受限;腔的品质因数改变与端面关系密切;MIM波导可以在具有更大Q值下确保光波更好地耦合成需要的SPP模式。     

Independently tunable dual resonant dip refractive index sensor based on metal—insulator—metal waveguide with Q-shaped resonant cavity

A plasmonic resonator system consisting of a metal—insulator—metal waveguide and a Q-shaped resonant cavity is proposed in this paper. The transmission properties of surface plasmon polaritons in this structure are investigated by using the finite difference in time domain (FDTD) method, and the simulation results contain two resonant dips. The physical mechanism is studied by the multimode interference coupled mode theory (MICMT), and the theoretical results are in highly consistent with the simulation results. Furthermore, the parameters of the Q-shaped cavity can be controlled to adjust the two dips, respectively. The refractive index sensor proposed in this paper, with a sensitivity of 1578 nm/RIU and figure of merit (FOM) of 175, performs better than most of the similar structures. Therefore, the results of the study are instructive for the design and application of high sensitivity nanoscale refractive index sensors.     

Loss measurement of plasmonic modes in planar metal-insulator-metal waveguides by an attenuated total reflection method

We report experimental excitation and characterization of surface plasmon modes in planar metal-insulator-metal (MIM) waveguides. Our approach is based on determining the width of the reflection angular spectrum in the attenuated total reflection (ATR) configuration. Owing to its transverse character, the ATR configuration provides a more straightforward and simpler way to determine the loss of plasmonic modes in MIM structures, compared to using tapered end couplers with multiple waveguide samples or scanning near-field optical microscopy. In this Letter, two waveguide structures with Au claddings and 50/200 nm SiO(2) cores are investigated. The propagation lengths measured at λ = 1.55 μm are 5.7 and 18 μm, respectively, in agreement with the theoretical predictions.     

Theoretical analysis of the characteristic impedance in metal-insulator-metal plasmonic transmission lines

We propose a closed form formulation for the impedance of the metal-insulator-metal (MIM) plasmonic transmission lines by solving the Maxwell's equations. We provide approximations for thin and thick insulator layers sandwiched between metallic layers. In the case of very thin dielectric layer, the surface waves on both interfaces are strongly coupled resulting in an almost linear dependence of the impedance of the plasmonic transmission line on the thickness of the insulator layer. On the other hand, for very thick insulator layer, the impedance does not vary with the insulator layer thickness due to the weak-coupling/decoupling of the surface waves on each metal-insulator interface. We demonstrate the effectiveness of our proposed formulation using two test scenarios, namely, almost zero reflection in T-junction and reflection from line discontinuity in the design of Bragg reflectors, where we compare our formulation against previously published results.     

亚波长之字形MIM等离子波导宽带滤波器

运用时域有限差分（FDTD）方法数值研究了一种亚波长之字形金属-绝缘体-金属（MIM）等离子波导结构的传输属性。之字形波导在连续两个拐角可向外延伸出1~4个短切口。每个切口独立构成一个谐振腔,谐振波长近似与切口深度成线性正比,而与切口方向无关。当任意一个切口满足谐振条件时,该波导结构在对应波长的透射率均趋近于0。随着同深度切口数目的增加,禁带波长区域逐渐展宽,形成一个良好的宽带滤波器。     

Resonant cavity based antireflection structures for surface plasmon waveguides

The concept of antireflection coating in the theory of multilayer films is introduced to the two-dimensional metal–insulator–metal (MIM) structures to realize total transmission of optical energy at the waveguide discontinuities. The antireflection structure consists of a resonant cavity which is constructed by changing the insulator width of the waveguide. A numerical method is used to achieve the optimal design directly. A T-splitter with zero reflection is proposed, utilizing a cavity structure in the input waveguide. A transformer with enhanced transmission between different waveguides is presented for further validating the efficiency and generality of these cavity based antireflection structures. The simulation results show that such a structure can realize a perfect antireflection function.     

Multimode interference effect in plasmonic subwavelength waveguides and an ultra-compact power splitter

The real part of the effective index is enhanced in a metal-insulator-metal (MIM) surface plasmon waveguide when the thickness of the insulator is quite small and this mechanism can be utilized to achieve some ultra-compact photonic devices. The mode properties of an index-guided multimode plasmonic waveguide of subwavelength size are analyzed and the multimode interference (MMI) effect is investigated. A novel ultra-compact MMI power splitter, with the transverse size less than 1/3 of the free space wavelength, is proposed.     

Wavelength demultiplexing structure based on arrayed plasmonic slot cavities

A compact wavelength demultiplexing structure based on arrayed metal-insulator-metal (MIM) slot cavities is proposed and demonstrated numerically. The structure consists of a bus waveguide perpendicularly coupled with a series of slot cavities, each of which captures SPPs at the resonance frequency from the bus waveguide and tunes the transmission wavelength by changing its geometrical parameters. A cavity theory model is used to design the operating wavelengths of the structure. Moreover, single band transmission of each channel and the adjustable transmission bandwidth can be obtained by altering the drop waveguide positions and the coupling distance. The proposed arrayed slot cavity-based structure could be utilized to develop ultracompact optical wavelength demultiplexing device for large-scale photonic integration.     

Induced transparency in nanoscale plasmonic resonator systems

An optical effect analogous to electromagnetically induced transparency (EIT) is observed in nanoscale plasmonic resonator systems. The system consists of a slot cavity as well as plasmonic bus and resonant waveguides, where the phase-matching condition of the resonant waveguide is tunable for the generation of an obvious EIT-like coupled resonator-induced transparency effect. A dynamic theory is utilized to exactly analyze the influence of physical parameters on transmission characteristics. The transparency effect induced by coupled resonance may have potential applications for nanoscale optical switching, nanolaser, and slow-light devices in highly integrated optical circuits.     

Multi-channel plasmonic waveguide filters with disk-shaped nanocavities

A kind of multi-channel plasmonic filters based on metal–insulator–metal waveguides with disk-shaped nanocavities is proposed and numerically investigated. By calculating the resonant mode of disk-shaped nanocavity, it is found that the radius and refractive index of the nanocavity effectively control the resonance wavelength, which is consistent with the results obtained by finite-difference time-domain method. The characteristics of resonance spectra are influenced by the gap width between the cavity and waveguide, as can be exactly analyzed by temporal coupled mode theory. In addition, multi-channel plasmonic filters are achieved by increasing the number of nanocavity.     

Systematical research on the characteristics of a vertical coupled Fabry–Perot plasmonic filter

A nanometric surface plasmon polariton (SPP) filter based on a vertical coupled metal–insulator–metal (MIM) Fabry–Perot resonator is proposed and analyzed. The transmission characteristics of the SPP filter are analyzed in detail by using the finite difference time domain method. And the resonance condition derived by the numerical method is consonant with the analytic model based on the Fabry–Perot model, which includes the metal loss and dispersion effects. And the simulation results show that multiple transmission dips can be formed and the resonance wavelengths of the transmission dips can be altered by the Fabry–Perot cavity length and width. Also the extinction ratio and the quality factor of the filter are affected by the barrier thickness of the coupling region. The proposed nanometric plasmonic filter is simple and very promising for the SPP waveguides platform.     

基于十字连通形环形谐振腔金属-介质-金属波导的折射率传感器和滤波器

提出了由十字连通形环形谐振腔耦合两个金属-介质-金属(metal-insulator-metal, MIM)波导的结构,并用有限元法数值研究了表面等离极化激元在结构中的传输特性.通过对透射谱的研究,系统地分析了MIM结构的传感特性.结果表明,在透射光谱中有三个共振峰,即存在三种共振模式,其中透射峰与材料的折射率呈线性关系.通过对结构参数的优化,得到了折射率灵敏度(S)高达1500 nm/RIU的理论值,相应的传感分辨率为1.33×10~(-4)RIU.更重要的是,灵敏度不受结构参数变化的影响,这意味着传感器的灵敏度不受制造偏差的影响.此外,谐振波长与环形腔中心半径成线性关系,该器件在较大波长范围内实现可调谐带通滤波.透射强度随着波导与环形腔间距的增大而减小,透射带宽同时减小,因此,可以通过控制环形腔与波导的耦合距离来调谐透射强度及透射带宽.研究结果对高灵敏度纳米级折射率传感器和带通滤波器的设计以及在生物传感器方面的应用都具有一定的指导意义.     

Theory of gyrotropic ring resonators with counterpropagating modes coupling

The Faraday effect in ring resonators leads to the resonance splitting of clockwise and counter-clockwise propagating modes. In this work we investigate how this resonance splitting influences the ring resonator's behavior if both modes are coupled by a sidewall corrugation of the resonator. Coupled mode theory is used to evaluate the transmission and reflection characteristics of a ring resonator coupled to a waveguide. Based on these evaluations, we characterize a novel ring resonator switch. The resonance of the corrugated gyrotropic resonator coupled to a waveguide can be switched off by the application of the external magnetic field. In this case, the resonance is not shifted, as what typically occurs at small perturbations, but suppressed.     

Ultracompact plasmonic racetrack resonators in metal-insulator-metal waveguides

Among various plasmonic waveguides, the metal-insulator-metal (MIM) type is the most promising for true subwavelength photonic integration. To date, many photonic devices based on MIM waveguides have been investigated, including resonators. However, most of the reported MIM ring resonators suffer from low extinction ratios and the reasons are unexplored in the literature. In this paper, we present a comprehensive analysis of the underlying causes of the low performance of MIM ring resonators, and give the analytical transmission relation for a universal all-pass ring resonator with coupling loss. Based on the analysis we propose plasmonic racetrack resonators in MIM waveguides and show that the performance can be greatly improved.     

Tunable plasmonic filter with circular metal–insulator– metal ring resonator containing double narrow gaps

Tunable filter based on two metal–insulator–metal (MIM) waveguides coupled to each other by a ring resonator with double narrow gaps is designed and numerically investigated by finite-difference time-domain (FDTD) simulations. The propagating modes of surface plasmon polaritons (SPPs) are studied. By introducing narrow gaps in ring resonators, the transmission in different resonance modes can be effectively adjusted by changing the gap width (g), and the transmitted peak wavelength has a nonlinear relationship with g. Another structure consisting two cascading ring resonators and regular MIM waveguide have also been proposed. The mechanism based on circular ring resonators with narrow gaps may provide a novel method for designing all-optical integrated components in optical communication and computing.     

基于耦合T型空腔的多重法诺共振现象

提出由T型空腔和挡板组成的两种金属-电介质-金属(MIM)波导结构,分别为:正T型空腔结构和倒T型空腔结构,并应用有限元法系统地研究了该结构的透射特性.对于正T型空腔结构,仿真结果出现了双重法诺共振现象,并且共振波长可以通过改变T型空腔长度和高度进行调节.该结构有助于设计成敏感度达到1 620nm/RIU、品质因数为5.4×10~4的纳米传感器.对于倒置T型空腔,在波导中产生了多重法诺共振现象,其敏感度可达1 560nm/RIU,品质因数为9.37×104.该结构有望在光学集成回路,特别是纳米传感器、光束分路器方面具有广泛应用.     

Design and analysis of two-dimensional photonic crystals channel filter

A novel three-port channel add/drop filter consisting of two waveguides and two cavities is proposed. One is used for a resonant tunneling-based channel add/drop operation from the bus waveguide to the add/drop waveguide, while the other is used to realize the wavelength-selective reflection feedback in the bus waveguide. By means of coupled mode theory in time, the conditions to achieve 100% add efficiency are derived thoroughly. Based on these theoretical analysis, the channel add filter and some other multi-channel filters are designed in two-dimensional photonic crystals (2D PCs) with square lattice of dielectric rods in air. The numerical results by using the finite-difference time-domain (FDTD) method demonstrate almost complete channel add/drop tunneling at resonance via the three-port systems.     

Single-Photon Scattering Grating in a Waveguide-Cavity System

We investigate single-photon scattering grating in a one-dimensional waveguide coupled to a cavity embedded with a driven Λ-type three-level atom. The single-photon reflection amplitude and transmission amplitude in the waveguide are obtained via a real-space approach, respectively. By spatially modulating a classical control field to drive the three-level emitter, alternating regions of high reflection and absorption as well as high transmission and absorption of the single photon are generated in both directions of the waveguide, which acts as a kind of scattering grating. The proposed scheme may have the potential for the design of chip-integrated grating.