Mingfei Zheng

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.

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 ultra-wide band-stop filter based on single-stub plasmonic-waveguide system

A nanoscale plasmonic filter based on a single-stub coupled metal–dielectric–metal waveguide system is investigated theoretically and numerically. A tunable wide band-stop can be achieved by loading a metal bar into the stub. The band-stop originates from the direct coupling between the resonance modes. The bandwidth and the center wavelength of the band-stop can be tuned by changing the parameters of the metal bar. Compared with previously reported filters, the plasmonic system has the advantages of easy fabrication and compactness. Our results indicate that the proposed system has potential to be utilized in integrated optical circuits and tunable filters.

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.

Aspect ratio control and sensing applications for a slot waveguide with a multimode stub

We report the aspect ratio control and sensing applications for metal–dielectric–metal (MDM) slot waveguides with a multimode stub. By adjusting the aspect ratio r = h/w of the stub, five types of optical evolutions in various aspect ratio r ranges are defined. We can realize a single or double plasmon-induced transparency (PIT) as well as Fano resonances. In addition, the figure of merit (FOM), which can describe the sensing performance of structures, is discussed in our paper. We can find up to three FOM peaks in our proposed structure. These findings provide guidance for the fundamental research of integrated plasmonic sensors.

Dual tunable plasmon-induced transparency based on silicon–air grating coupled graphene structure in terahertz metamaterial

A graphene plasmonic structure consists of three graphene layers mingled with a silicon-air grating is proposed. We theoretically predict and numerically simulate the plasmon-induced transparency effect in this system at terahertz wavelengths, and a dual plasmon-induced transparency peaks can be successfully tuned by virtually shifting the desired Fermi energy on graphene layers. We investigate the surface plasmon dispersion relation by means of analytic calculations, and we can achieve the numerical solution of propagation constant got by the dispersion relation. A suitable theoretical model is established to study spectral features in the plasmonic graphene system, and the theoretical results agree well with the simulations. The proposed model and findings may provide guidance for fundamental research of highly tunable optoelectronic devices.

Keeping Good Sensing Performance of Metal–Dielectric–Metal Waveguides After Coating Treatment

Good sensing performance is observed in metal–dielectric–metal (MDM) waveguides in recent research. However, Ag will be oxidized in air. The oxidization layer makes the sensing performance not as high as before. Plating an SiO2 film on the surface of the Ag waveguide not only can avoid oxidization but can keep good sensing performance through our numerical discussion as well. The decaying rate of electric field E at the interface and inside is also defined to illustrate the phenomenon mentioned above. We hope our findings will provide guidance for the integrated plasmonic devices.

Filtering Property Based on Ultra-Wide Stopband in Double Sector/Sectorial-Ring Stub Resonator Coupled to Plasmonic Waveguide

A double sector or sectorial-ring stub resonator coupled to a plasmonic waveguide is proposed and investigated. This resonator is built with two same stubs that are symmetrically arranged together, which has the advantages of realizing asymmetrical single stub and forming no-distance double stub. The characteristic spectral responses of the two novel systems are simulated by using the finite-difference time-domain method. The results show that an ultra-wide stopband is achieved, and a multiple double stub is realized by altering the structure size of the double stub that plays important role in the stopband phenomenon. A tunable stopband, a specific filtering waveband and an optimum structural parameter are obtained by adjusting the inner radius (r), (outer) radius (R), or central angle (θ) of the double stub. The wavelength and bandwidth of the stopband have various variations with the changing of r, R, or θ, and the stopbands in the two systems have similar changes and different features. This paper provides a promising application for band-stop nanofilters and plasmonic integrated optical circuits.

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 γc and dielectric damping factor γd. 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.

Multiple plasmon-induced transparency effects in a multimode-cavity-coupled metal–dielectric–metal waveguide

We numerically and theoretically investigate multiple plasmon-induced transparency (PIT) effects in a multimode-cavity-coupled metal–dielectric–metal (MDM) waveguide system. The introduced multimode coupled-radiating oscillator theory (MC-ROT) gives a clear understanding of multiple PIT effects in the proposed system. Two and three PIT peaks appear in the transmission spectra corresponding to the symmetrical and asymmetrical structures, respectively. Evolution of the PIT peaks can be effectively tuned by adjusting the geometric dimensions and asymmetry of the structure. The ultra-compact plasmonic waveguide structure may have important applications for multichannel filters, optical switches, and other devices in integrated optical circuits.