Optical sensors are widely used for refractive index measurement in biomedical, chemical, and food processing industries and offer high sensitivity to ambient refractive index variations due to the specific field distribution of the resonances. The sensor's sensitivity is greatly dependent on its material and structure. In this review, we focused on plasmonic refractive index sensors with no fluorescent labelling required. The latest developments on special types of plasmonic structures such as metal-insulator-metal waveguides and their application in refractive index sensors are presented. Moreover, numerous types of plasmonic waveguides, geometries, materials, fabrication processes, and losses related to plasmonic waveguides are explained to provide a better understanding of this topic. The highlight of this review is to discuss the spectral performance of recently reported refractive index sensors by emphasising their sensitivity and figure of merits.

Plasmonic sensors based on Metal-insulator-metal waveguides for refractive index sensing applications: A brief review

A plasmonic refractive index (RI) sensor based on metal-insulator-metal (MIM) waveguide coupled with concentric double rings resonator (CDRR) is proposed and investigated numerically. Utilizing the novel supermodes of the CDRR, the FWHM of the resonant wavelength can be modulated, and a sensitivity of 1060 nm/RIU with high figure of merit (FOM) 203.8 is realized in the near-infrared region. The unordinary modes, as well as the influence of structure parameters on the sensing performance, are also discussed. Such plasmonic sensor with simple framework and high optical resolution could be applied to on-chip sensing systems and integrated optical circuits. Besides, the special cases of bio-sensing and triple rings are also discussed.

https://www.mdpi.com/1424-8220/18/1/116/pdf

Plasmonic Refractive Index Sensor with High Figure of Merit Based on Concentric-Rings Resonator.

An enhanced structure for compact and high sensitivity plasmonic refractive index sensor by using the coupling of two metal-insulator-metal (MIM) waveguides with a silver nanorods array embedded into a square resonator is proposed and analyzed in this paper. We placed silver nanorods inside the resonator due to their unique optical properties in nanoscale confinement and low propagation losses. Based on obtained results, the resonance wavelengths of the sensor having an approximate linear relationship with the refractive index of the materials under detecting that are placed into the square resonator. With an 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 2320 nm per refractive index unit (RIU) and 0.84 nm/°C. The achieved sensor can be used in nanophotonic and plasmonic integrated circuits and in this research; we show its applicability for the detection of different human blood groups as well.

High sensitivity plasmonic refractive index sensing and its application for human blood group identification

High-Q Plasmonic Resonances: Fundamentals and Applications

Surface plasmon (SP) modes possesses an intriguing feature of confining light beyond diffraction limit, which makes it very attractive for sensing applications Here, we theoretically investigated an ultra compact SP sensor using metal–insulator–metal (MIM) waveguide geometry MIM waveguide is coupled to a pair of stub resonators and both the stub resonators are loaded with a metallic nanoslit of silver The stubs and the MIM waveguide are filled with liquid/gaseous material which is to be sensed The Fano resonance, which is very sensitive to any change in refractive index of the material, is excited in the structure by breaking marginal symmetry The structure is numerically simulated by the finite difference time-domain method (FDTD), and the result shows that the resonance wavelength has a linear relation with refractive index of the material under sensing In the optimum design of the proposed sensor, the maximum sensitivity is obtained as high as $S= 1060$ nm/refractive index unit with a large value of figure of merit (FOM = 1767) and an ultra narrow linewidth $\Delta \lambda =6$ nm Thus, the device is well suited for designing on-chip optical sensors

Enhanced Figure of Merit in Fano Resonance-Based Plasmonic Refractive Index Sensor

An ultra-compact plasmonics-based sensor is investigated which is excited by Fano resonance. The structure is numerically simulated by the finite-difference time-domain method. The sensor utilizes unique waveguide geometry named as metal–insulator–metal (MIM) waveguide geometry which has an intriguing feature to confine signal far beyond diffraction light. Thus, it is used to devise ultra-compact optical circuits. The MIM waveguide is coupled to a pair of elliptical ring resonators and the interaction between the resonators excites special mode which is known as Fano resonance mode. It is a unique phenomenon which exhibits asymmetrical resonance profile and supports ultra narrow line width. Because of its exciting feature, a large value of sensitivity = 1100 nm/RIU and figure of merit = 224RIU−1 is obtained for the proposed sensor. The sensing performance of the device can be further enhanced by tailoring the geometrical parameters. The applicability of the device is also tested to detect the concentration of hemoglobin in blood. Thus, the device is well suited to design on-chip optical sensors.

Plasmonics-Based Refractive Index Sensor for Detection of Hemoglobin Concentration

Wideband slow light achievement in MIM plasmonic waveguide by controlling Fano resonance

Photonics provides a key solution to limit the myriads of challenges offered by existing silicon technology. Here, we propose a simple 2-channel Plasmonic demultiplexer with Metal-Insulator-Metal (MIM) waveguide geometry. The demultiplexer is capable to separate two fundamental optical windows (i.e., 1310 nm and 1550 nm). An FDTD (finite difference time domain) method is used to investigate the structure numerically. The optical properties of the demultiplexer are also investigated. The performance of the proposed demultiplexer is measured using crosstalk and quality factor. The minimum value of crosstalk is − 30 dB and maximum quality factor is 60. The maximum transmission is limited to 45% due to large metallic losses. The proposed demultiplexer is ultra-compact in size and is capable to provide sub-wavelength confinement. It is suited for WDM applications and can pave a way to design the photonic integrated circuits.

Metallic Slit–Loaded Ring Resonator–Based Plasmonic Demultiplexer with Large Crosstalk

A Fano resonance has stimulated interests due to its asymmetrical resonance profile. Fano resonance generally emerges due to coupling and destructive interference between two resonating modes having disparate quality factors. This coupling between modes leads to an asymmetrical resonance profile which makes Fano resonance quite distinct from the traditional Lorentzian profile. Asymmetry is a key feature of Fano resonance. It is examined using Complex plane analysis and redefined using Reflection Argand diagram. The newly defined parameter named as Argand Asymmetric ratio (ρ) is in line with the previously defined asymmetrical factor of Fano resonance. This newly defined parameter (ρ) is also studied against coupling and frequency detuning between resonating modes. The parameter is also numerically investigated by exciting Fano resonance in a pair of resonators coupled MIM (Metal-Insulator-Metal) waveguide using the OptiFDTD tool. The newly defined parameter can find its way in analyzing different Fano resonance based applications like as optical buffers, sign reversal Fano tuning, switches, modulators, etc.

Analysis of asymmetry of Fano resonance in plasmonic metal-insulator-metal waveguide

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