Utilizing the Metallic Nano-Rods in Hexagonal Configuration to Enhance Sensitivity of the Plasmonic Racetrack Resonator in Sensing Application

A high sensitive plasmonic refractive index sensor based on metal-insulator-metal (MIM) waveguides with embedding metallic nano-rods in racetrack resonator has been proposed. The refractive index changes of the dielectric material inside the resonator together with temperature changes can be acquired from the detection of the resonance wavelength, based on their linear relationship. With optimum design and considering a tradeoff among detected power, structure size, and sensitivity, the finite difference time domain simulations show that the refractive index and temperature sensitivity values can be obtained as high as 2610 nm per refractive index unit (RIU) and 1.03 nm/°C, respectively. In addition, resonance wavelengths of resonator are obtained experimentally by using the resonant conditions. The effects of nano-rods radius and refractive index of racetrack resonator are studied on the sensing spectra, as well. The proposed structure with such high sensitivity will be useful in optical communications that can provide a new possibility for designing compact and high-performance plasmonic devices.     

Structure of Multiple Fano Resonances in Double-baffle MDM Waveguide Coupled Cascaded Square Cavity for Application of High Throughput Detection

A structure of double-baffle metal-dielectric-metal (MDM) waveguide coupled cascaded square cavity is designed based on the transmission characteristics of the surface plasmon polaritons. Combined with coupled mode theory (CMT), the mechanism of multiple Fano resonances generated by this structure is analyzed qualitatively. The wide-band spectrum mode generated by the F-P resonant cavity and the four narrow-band spectrum modes produced by the cascaded square resonant cavities interfere with each other. Moreover, an new scheme of introducing a semiconductor material InGaAsP into this structure is designed for improving the transmittance of the Fano peaks. Analyze the influence of refractive indexes of the test objects on sensing performance by finite element method (FEM) quantitatively, which shows the improved structure can achieve the independent tuning of multiple Fano resonances. Combining with 96-well microplate technology, the structure can achieve the detection of multiple different samples with high-performance simultaneously. It is believed that the proposed structure has a strong reference significance for the design of optical micro-nanostructures for high throughput detection.

     

Terahertz Microfluidic Sensing Using a Parallel-plate Waveguide Sensor

Refractive index (RI) sensing is a powerful noninvasive and label-free sensing technique for the identification, detection and monitoring of microfluidic samples with a wide range of possible sensor designs such as interferometers and resonators ^1,2. Most of the existing RI sensing applications focus on biological materials in aqueous solutions in visible and IR frequencies, such as DNA hybridization and genome sequencing. At terahertz frequencies, applications include quality control, monitoring of industrial processes and sensing and detection applications involving nonpolar materials.Several potential designs for refractive index sensors in the terahertz regime exist, including photonic crystal waveguides ³, asymmetric split-ring resonators ⁴, and photonic band gap structures integrated into parallel-plate waveguides ⁵. Many of these designs are based on optical resonators such as rings or cavities. The resonant frequencies of these structures are dependent on the refractive index of the material in or around the resonator. By monitoring the shifts in resonant frequency the refractive index of a sample can be accurately measured and this in turn can be used to identify a material, monitor contamination or dilution, etc.The sensor design we use here is based on a simple parallel-plate waveguide ^6,7. A rectangular groove machined into one face acts as a resonant cavity (Figures 1 and 2). When terahertz radiation is coupled into the waveguide and propagates in the lowest-order transverse-electric (TE₁) mode, the result is a single strong resonant feature with a tunable resonant frequency that is dependent on the geometry of the groove ^6,8. This groove can be filled with nonpolar liquid microfluidic samples which cause a shift in the observed resonant frequency that depends on the amount of liquid in the groove and its refractive index ⁹.Our technique has an advantage over other terahertz techniques in its simplicity, both in fabrication and implementation, since the procedure can be accomplished with standard laboratory equipment without the need for a clean room or any special fabrication or experimental techniques. It can also be easily expanded to multichannel operation by the incorporation of multiple grooves ¹⁰. In this video we will describe our complete experimental procedure, from the design of the sensor to the data analysis and determination of the sample refractive index.     

High-Resolution Plasmonic Filter and Refractive Index Sensor Based on Perturbed Square Cavity with Slits and Orthogonal Feeding Scheme

Plasmonics - We present a plasmonic bandpass filter and refractive index sensor based on perturbed square cavity resonator with slits, which is fed by orthogonally oriented feeding waveguides. The...     

Integrated Tunable Optofluidics Optical Filter Based on MIM Side-Coupled-Cavity Waveguide

A tunable wavelength filter based on plasmonic metal?Cdielectric?Cmetal waveguide with optofluidics pump system has been proposed and numerically investigated. The finite difference time domain method with perfectly matched layer-absorbing boundary condition is adopted to simulate and study their properties. An analytical solution to the resonant condition of the structure is derived by means of the cavity theory. It is found that the resonant wavelength of the filter is easily tuned in a broadband by manipulating the fluid filled in the cavity. Both analytical and simulative results reveal that the resonant wavelengths are proportional to the volume and refractive index of liquid in the cavity and are related to the structure of the filter. The resonant wavelengths of this structure can be changed from 1,106 to around 1,800?nm in this paper. The waveguide filter may become a choice for the design of devices in highly integrated optical circuits.     

Ultra-High Sensitivity Nanosensor Based on Multiple Fano Resonance in the MIM Coupled Plasmonic Resonator

This paper proposes a compact plasmonic structure that is composed of a metal-insulator-metal (MIM) waveguide coupled with a groove and stub resonators, and then investigates it by utilizing the finite element method (FEM). Simulation results show that the interaction between the local discrete state caused by the stub resonator and the continuous spectrum caused by the groove resonator gives rise to one of the two Fano resonances, while the generation of the other resonance relies only on the groove. Meanwhile, the asymmetrical linear shape and the resonant wavelength can be easily tuned by changing the parameters of the structure. By adding stubs on the groove, we excited multiple Fano resonances. The proposed structure can serve as an excellent plasmonic sensor with a sensitivity of 2000 nm/RIU and a figure of merit of about 3.04?×?10³, which can find extensive applications for nanosensors.     

Self-Referencing Plasmonic Array Sensors

A self-referencing plasmonic platform is proposed and analyzed. By introducing a thin gold layer below a periodic two-dimensional nano-grating, the structure supports multiple modes including localized surface plasmon resonance (LSPR), surface plasmon resonance (SPR), and Fabry-Perot resonances. These modes get coupled to each other creating multiple Fano resonances. A coupled mode between the LSPR and SPR responses is spatially separated from the sensor surface and is not sensitive to refractive index changes in the surrounding materials or surface attachments. This mode can be used for self-referencing the measurements. In contrast, the LSPR dominant mode shifts in wavelength when the refractive index of the surrounding medium is changed. The proposed structure is easy to fabricate using conventional lithography and electron beam deposition methods. A bulk sensitivity of 429 nm/RIU is achieved. The sensor also has the ability to detect nanometer thick surface attachments on the top of the grating.

     

Electromagnetically Induced Transparency and Refractive Index Sensing for a Plasmonic Waveguide with a Stub Coupled Ring Resonator

A plasmonic refractive index sensor based on electromagnetically induced transparency (EIT) composed of a metal-insulator-metal (MIM) waveguide with stub resonators and a ring resonator is presented. The transmission properties and the refractive index sensitivity are numerically studied with the finite element method (FEM). The results revealed an EIT-like transmission spectrum with an asymmetric line profile and a refractive index sensitivity of 1057 nm/RIU are obtained. The coupled mode theory (CMT) based on transmission line theory is adopted to illustrate the EIT-like phenomenon. Multiple EIT-like peaks are observed in the transmission spectrum of the derived structures based on the MIM waveguide with stub resonator coupled ring resonator. To analyze the multiple EIT-like modes of the derived structures, the H _z field distribution is calculated. In addition, the effect of the structural parameters on the EIT-like effect is also studied. These results provide a new method for the dynamic control of light in the nanoscale.     

Characteristics of Plasmonic Filters with a Notch Located Along Rectangular Resonators

We propose a plasmonic filter with a notch located along a rectangular resonator. The finite difference time domain method is utilized to investigate and analyze the transmission characteristics of the filter. Results reveal that the introduction of the notch affects the first and second resonant modes of the resonator in different manners due to different magnetic field distributions inside the resonator. The evolution of the transmission-peak wavelengths as a function of the notch position with the same total resonator length is given. Effects of geometrical parameters of the notch on peak wavelengths are also studied. The corresponding theoretical model of our proposal is discussed, which agrees well with simulation results.     

Resonant Mode Analysis of the Nanoscale Surface Plasmon Polariton Waveguide Filter with Rectangle Cavity

The resonant mode characteristics of the nanoscale surface plasmon polaritons (SPP) waveguide filter with rectangle cavity are studied theoretically. By using the finite difference time domain method, both the band-stop- and band-pass-type rectangle SPP filters are analyzed. The results show that the whispering gallery mode (WGM) and the Fabry–Perot (FP) mode can be supported by the rectangle SPP resonator. Furthermore, both traveling-wave mode and standing-wave mode can be realized by the WGM, while only standing-wave mode can be introduced by the FP mode. The traveling-wave mode can only be realized by the square-shaped SPP resonator, and the traveling-wave mode is splitted into two standing-wave modes by transforming the cavity shape from square to rectangle. Also, the effects of the cavity shape, cavity size, and coupling gap size on the transmission spectra of the SPP resonators are analyzed in detail. This simple SPP waveguide filter is very promising for the high-density SPP waveguide integrations.     

Tunable Multiple Fano Resonances and Stable Plasmonic Band-Stop Filter Based on a Metal-Insulator-Metal Waveguide

Plasmonics - A metal-insulator-metal (MIM) waveguide consisting of two stub resonators and a ring resonator is proposed, which can be used as refractive index sensor and stop-band filter at the...     

High Sensitivity Nanoplasmonic Sensor Based on Plasmon-Induced Transparency in a Graphene Nanoribbon Waveguide Coupled with Detuned Graphene Square-Nanoring Resonators

A novel nanoscale structure for high sensitivity sensing which consists of a graphene nanoribbon waveguide coupled with detuned graphene square-nanoring resonators (GSNR) based on edge mode is investigated in detail. By altering the Fermi energy level of the graphene, the plasmon-induced transparency (PIT) window from the destructive interference between a radiative square-nanoring resonator and a dark square-nanoring resonator can be easily tailored. The coupled mode theory (CMT) is used to show that the theoretical results agree well with the finite difference time domain (FDTD) simulations. This nanosensor yields a ultrahigh sensitivity of ∼2600 nm/refractive index unit (RIU) and a figure of merit (FOM) of ∼54 in the mid-infrared (MIR) spectrum. The revealed results indicate that the Fermi energy level of the graphene and the coupling distance play important roles in optimizing the sensing properties. Our proposed structure exerts a peculiar fascination on the realization of ultra-compact graphene plasmonic nanosensor in the future.

     

Coupled Mode Theory for Surface Plasmon Polariton Waveguides

In allusion to special modes supported by surface plasmon polariton (SPP) waveguides, explicit expression for mode coupling coefficient which plays a central role in coupled mode theory is firstly redefined by adding the longitudinal electric field component. The mode coupling coefficients calculated by the proposed formula improve greatly compared with the coupled mode theory suited to conventional optical waveguides, and reasonable explanations from the point of view of physics and mathematics have been given. Afterwards, the coupling lengths, the transmission lengths, the normalized power exchanges, and the cross talk performances of adjacent parallel SPP waveguides with varying waveguide separation distances D and waveguide lengths L are investigated at telecom wavelength. The results are encouraging as they indicate that the coupled mode theory is developed in a self-consistent manner by retaining the longitudinal electric field component in the derivation and neglecting it only when the waveguides structure satisfies the weakly guiding situations. As a result, the new mode coupling coefficient formula for SPP waveguides considered in this paper is an important complement in the theory of SPP waveguides.     

Design of Optical Filters with Flat-on-Top Transmission Lineshapes Based on Two Side-Coupled Metallic Grooves

We investigate theoretically and numerically the resonant transmission through side-coupled metallic grooves. In the framework of coupled mode theory (CMT), a single metallic groove can be considered as a lossy optical resonator and two metallic grooves coupled via tunneling effect can be treated as a second-order cascade resonator. The relationship between the transmission lineshape of the coupled grooves and the cross-coupling between the grooves is analyzed by CMT. It is found that a flat-on-top lineshape can be obtained when the cross-coupling is equal to the total decay rate of the groove mode. Predictions based on the CMT analysis are in good agreement with the simulation results based on the finite-difference time-domain technique.     

Tunable Multimode Plasmonic Filter Based on Side-Coupled Ring-Groove Joint Resonator

Plasmonics - A tunable multimode plasmonic filter is proposed by using a side-coupled ring-groove joint resonator. In addition to the integer resonance modes of the perfect ring resonator (RR),...     

Design of Plasmonic Comb-Like Filters Using Loop-Based Resonators

A subwavelength plasmonic comb-like filter is proposed by using dual symmetric slot cavities which are placed between two parallel metal–insulator–metal (MIM) structure waveguides. The structure can be considered as a resonance loop which consists of slot cavity resonators and MIM waveguide resonators. The reflective wavelength range and channel spacing are determined by the lengths of slot cavities and MIM waveguides, respectively. Three, four, and five reflective channels with high reflection are achieved in a small wavelength range. Higher channel count can be available by increasing the length or the real part of effective index of MIM waveguides. Such a device can find applications in various optical systems such as wavelength demultiplexing components.     

Electromagnetically Induced Transparency and Sharp Asymmetric Fano Line Shapes in All-Dielectric Nanodimer

Low-loss electromagnetically induced transparency (EIT) and asymmetric Fano line shapes are investigated in a simple planar silicon dimer resonator. The EIT and Fano effects emerge due to near-field coupling of the modes supported by both the nanoparticles in a dimer structure. Different configurations of the dimer nanostructure are analyzed, which provide distinct EIT and Fano resonances. Furthermore, the tunability of EIT and Fano resonant modes are incorporated by changing the structural parameters. It is also found that the dimer resonator exhibits high Q factor and large electromagnetic field enhancement at Fano resonance and EIT window due to extremely low absorption loss. Such values and narrow resonances are supposed to be useful highly sensitive sensors and slow-light applications.     

Multidepth screening of living cells using optical waveguides

The use of planar optical waveguides as substrata for label-free, non-invasive monitoring of cells growing on them is demonstrated. Different submicrometre depths (measured from and perpendicular to the substratum surface) can be selected for monitoring. The so-called symmetry waveguide configuration with a low refractive index waveguide support (nanoporous silica with refractive index approximately 1.2) and a polystyrene waveguiding film with a heat-embossed grating coupler is exploited to obtain practically useful differences between the penetration depths of different waveguide modes. Robust data processing techniques are developed to obtain quantitative information about the cell refractive index profile perpendicular to the substratum from the measured effective refractive indices of the modes. In particular, a method is introduced with which cell refractive index variations above and below a predefined and tunable depth can be separated using two modes. The technique can be extended to more modes to gain even more comprehensive information from predefined submicrometre slices of the cell layer. The introduced methods are also suitable for monitoring the kinetics of changes in cell refractive index profiles.     

Plasmonic Waveguide Filters Based on Tunneling and Cavity Effects

This work presents a bandstop plasmonic filter that comprises a metal–insulator–metal (MIM) waveguide and a few pairs of strip cavities that are embedded in the metal. The strip cavity acts as both a near-field antenna and an MIM resonator. The central frequency and the bandwidth of the forbidden band are inversely related to the cavity length and the cavity-to-waveguide distance, respectively. These results correlate with the predictions of the ring resonator model but only under the resonant condition that double the effective length of cavity is an integer multiple of the guiding wavelength in the cavity.     

Wide Bandpass and Narrow Bandstop Microstrip Filters Based on Hilbert Fractal Geometry: Design and Simulation Results

This paper presents new Wide Bandpass Filter (WBPF) and Narrow Bandstop Filter (NBSF) incorporating two microstrip resonators, each resonator is based on 2^nd iteration of Hilbert fractal geometry. The type of filter as pass or reject band has been adjusted by coupling gap parameter (d) between Hilbert resonators using a substrate with a dielectric constant of 10.8 and a thickness of 1.27 mm. Numerical simulation results as well as a parametric study of d parameter on filter type and frequency responses are presented and studied. WBPF has designed at resonant frequencies of 2 and 2.2 GHz with a bandwidth of 0.52 GHz, −28 dB return loss and −0.125 dB insertion loss while NBSF has designed for electrical specifications of 2.37 GHz center frequency, 20 MHz rejection bandwidth, −0.1873 dB return loss and 13.746 dB insertion loss. The proposed technique offers a new alternative to construct low-cost high-performance filter devices, suitable for a wide range of wireless communication systems.