The entire outer surface of the proposed sensor is covered with a thin gold film to introduce the well-known SPR phenomenon. Finite element method (FEM) based mode solver software (COMSOL Multiphysics version 5.3a) is used using Matlab interface to investigate the performance of the proposed sensor.
Introduction
Literature Review
Guo et al. proposed a single-mode, highly sensitive PCF-based SPR RI sensor Furthermore, an air-core PCF-based SPR biosensor was presented by Paul et al.
Problem Statement
The most attractive feature of the PCF-based SPR technique is the infinite single-mode behavior that is independent of the materials used. The main goal of my work is to maximize the sensitivity of the biosensor with the most simplified and realistic design that reduces manufacturing complexity as well as cost.
Objectives of the Thesis
To evaluate the sensitivity, sensor length, wavelength resolution and figure of merit of the proposed sensor including the inclination effect of the air holes. To investigate the effect of double layer plasmonic material instead of single layer on the performance of the sensor and to make a trade-off between the area of plasmonic material (PM) used and sensing parameters.
Possible Outcome
Methodology
Thesis Outline
SPR for the biosensing and detection of gas was demonstrated by Liedberg and Nylander in 1982 [50]. For the performance analysis of the above research works, mode conduction characteristics of the PCFs are needed.
Optical Sensor
By the end of the 1970s, the potential of SPR for the characterization of thin layers and the monitoring of processes at metal interfaces was realized. Since then, there has been an explosion in the development of SPR sensing techniques, configurations, and devices for the detection and measurement of biological and chemical substances [51].
Optical Biosensor Principles
Interferometry
Luminescence
Fundamental of SPR
The propagation constant for SPW is higher than the propagation constant of light in the dielectric medium. Moreover, the propagation constant of the SPW and light source must match at another.
Conventional SPR Mechanism
Prism Coupler
- Otto Configuration
- Kretschmann Configuration
SP needs the light with extra momentum or energy because it cannot be excited with the normal light that has the same polarization state as the SPW. The width of the air gap is 200 nm, and the challenge is to realize the air gap practically.
Grating Coupler
Fiber Based SPR Configuration
PCF
When a light source enters this crystal, it can localize the light and spontaneous emission of atoms occurs [61]. The motion of the photon is affected in a similar way as the motion of electrons in solids is affected by the ionic lattice. Light can pass into a certain frequency band for the proposed structure at a certain angle and cannot propagate into another depending on the wavelength.
The wavelengths that are allowed to pass are known as modes, and the other wavelengths that are not allowed are known as the photon band gap (PBG) – this phenomenon results in the spontaneous emission of photons. The Schrodinger equation expresses the behavior of an electronic crystal, and the behavior of PhC is given by the wave equation (two Maxwell curls). Hole assisted fiber –— a higher index core modified by the presence of an air hole in the core.
PCF Based SPR Sensing
Again, this not only increases the sensitivity of the sensor, but also provides a sharp resonant peak. Electrons begin to resonate when the frequencies of the free electrons on the surface of the PM and the EF are equal. At the resonance mode, the real part of the effective mode index (nef f) of the surface plasmon polariton (SPP) mode and the core mode overlap [18,39].
The resonance wavelength of the phase-matching mode changes with the variation of the RI of the analyte. With the variation of the RI of the sample or analyte, the corresponding wavelength of the phase-matching mode can be measured. Finally, after processing the output, the peak shift of the output signal can be observed and analyzed through a central processing unit.
Interrogation of Surface Plasmon Waves
- Amplitude Interrogation
- Wavelength Interrogation
- Phase Interrogation
- Angular Interrogation
- Polarization Interrogation
Changes in these properties can be related through changes in the SPW propagation constant [51]. This interrogation method involves measuring the change in intensity of the light wave after its interaction with the SPW at a fixed wavelength and angle of incidence. Coupling the EW to the SPW resonantly transfers energy to the SPW, thereby recording a change in the intensity of the light wave.
This form of interrogation provides a way to determine the component of the light wave vector parallel to the metal surface that excites the SPW. This interrogation method involves measuring the change in polarization of the light wave after its interaction with the SPW at a fixed wavelength and angle of incidence [63]. The connection of the EW to the SPW transmits energy resonantly to the SPW, which in turn registers the change in the polarization of the light wave.
SPR in Biosensing
The most significant phase shift indicates the wavelength and angle at which the strongest coupling occurred [66]. The coupling strength of SPW to EW is measured at a constant wavelength while the angle of incident light wave is varied until an angle that gives the strongest coupling. The most significant change in light polarization indicates the wavelength and angle at which the strongest coupling occurred.
Numerical Methods
FEM
In the case of propagation mode computation, the FEM creates a matrix that numerically approximates the partial differential operator of the problem and transforms it into a numerical eigenvalue problem.
Formation of FEM
Full vectorial FEM incorporates anisotropic perfectly matched layers (PML), which allows solving as many states as desired in a single run without iteration [70]. The component of [p] and [q] are given in Table 2.1 and nx, ny and nz represent RIs inx,yandzplane.
Optical Tunable Source
Optical Spectrum Analyzer
The OSA screen is visible and efficiently transmits data from the analyzed network.
Refractive index of some analytes
Summary
Design with Theoretical Modeling
- Design Approach
- Selected Material Properties
- Perfectly Matched Layer
- Proposed Block Diagram Specimen
The radius of the circle containing the gold layer is 5.03 µm, and the distance from the center of any air hole is 5.03 µm. All the parameters used to develop the structure of the proposed sensor are detailed in Table 3.1. The height of smaller air openings is h1, and for larger air openings it is indicated by h2.
We select the right material that affects the sensing performance and the detection capability of the proposed sensor. For the proposed geometric structure, fused silica (SiO2) is chosen as the background material, and the RI of the SiO2 is obtained from the Sellmeier equation. The cross-sectional view of the geometric structure of the proposed sensor is shown in figure 3.3 (a) the angular displacement of the air holes and figure 3.3 (b) to (d) show step-by-step development of the proposed sensor with an inclined approach using gold film on the outside of the SiO2 structure.
Numerical Analysis
- Confinement Loss
- Wavelength Sensitivity
- Amplitude Sensitivity and Sensor Length
- Sensor Resolution
- FOM and Detection limit
The second SMF is connected to the sensor via a coupling that helps the OSA measure the transmitted signal coming from the sensor. In this subsection, the variation of the resonance peak CL is measured to determine the variation of RI. The transmitted power is observed at a static wavelength under phase-matched conditions, and the core loss depends on the change in RI, and the core loss varies greatly with the change in RI,∆na.
Excluding AS and WS, there is another important parameter that is used to measure the smallest change that can be detected by the sensor. SR RI is the ability to accurately measure the smallest fraction of RI, and in the case of wavelength, resolution measures the peak shift of the resonance wavelength for a specified change in analyte RI with minimum spectral resolution. In addition, power measure (FOM) and detection accuracy (DA) are special characterization parameters and help to calculate sensor performance.
Fabrication Feasibility Study
A lower FWHM value and a higher WS value reduce the probability of false detection.
Summary
Introduction
Optimization of structural and performance reinforcement parameters . 34
- Variation of the gold layer thickness
- Variation of Air Hole Height
- Effect of Inclination
- Inclined Approach
- Inclination of Air holes
- Thickness variation of the gold layer
- Variation of Air Hole height
- Variation of Air Hole Width
- Addition and Effect of the Adhesive Layer
- Effect of Metallic Stripe
- Thickness Variation of Analyte Layer
- Thickness Variation of PML
To detect the effect of the air hole height, we manually changed the air hole height, leaving other parameters untouched. This subsection emphasizes the effect of angular displacement (Φ) of the air holes in the case of WS. 1.39, the performance of the proposed sensor will be optimized for different air gaps.
In this subsection, the effect of adhesive layer thickness on the performance of the proposed sensor is discussed considering other optimized parameters as constant for the inclined approach of the air holes. In this subsection, the effect of PML thickness variation on the performance of the proposed sensor is discussed considering other optimized parameters as constant for the inclined approach of the air holes. Changes in the performance parameters are negligible due to the thickness variation of the PML.
Performance Analysis
- Dispersion relationship
- Normal Approach
- Inclined Approach
- Confinement Loss
- Normal Approach
- Inclined Approach
- Wavelength Sensitivity
- Normal Approach
- Inclined Approach
- Linearity
- Amplitude Sensitivity and Sensor length
- Normal Approach
- Inclined Approach
- Sensor Resolution
- Normal Approach
- Inclined Approach
- Performance Analysis with Double Layer PM
- Application
- Concentration Detection of Sucrose Solution
- Summary
- Normal Approach
- Inclined Approach with Single PM
- Inclined Approach with Double PM
The lower depth of the CL is found at RI = 1.29, and the depth increases with the increase of RI. All necessary data related to SR for the oblique approximation of the air gaps from RI = 1.29 to 1.41 are given in Table 4.10 and via Eq. Finally, the oblique approximation of the air holes was studied and numerically investigated for the double-layer PM.
The conducting characteristics of the proposed sensor are analyzed based on the wavelength and amplitude interrogation method. To support SPR, the entire outer surface of the proposed sensor is covered by the thin gold film (25 nm). Considering the extra layer of TiO2, the sensing performance of the proposed sensor is given in Table 5.1.
Future Prospects of Our Work
Abbott, “Dually polarized surface plasmon resonance biosensor based on helical photonic crystal fibers,” IEEE Sensors Journal , vol. Zhao, “A surface plasmon resonance refractive index sensor based on a tapered structure of coreless optical fibers,” Journal of Lightwave Technology , vol. Houet al., “Surface plasmon resonance biosensor based on gold-plated side-polished photonic crystal fibers with hexagonal structure,” Optics express, vol.
Hong, “Surface plasmon resonance sensor based on side-polished d-shaped photonic crystal fiber with split cladding air holes,” IEEE Transactions on Instrumentation and Measurement , vol. Yee, “Surface plasmon resonance sensors based on diffraction gratings and prism couplers: sensitivity comparison,” Sensors and Actuators B: Chemical, vol. Chu, “Symmetric double d-form photonic crystal fibers for surface plasmon resonance sensing,” Optics Express , vol.
The propagation of SPW wave at metal and dielectric interface, adapted
Commonly used coupling mechanism a) Prism coupler, b) Grating cou-
Otto coupling mechanism, adapted from Ref [44]
Kretschmann coupling Configuration, adapted from Ref [44]
Fiber based SPR Configuration adapted from Ref [44]
Basic graphical setup of practical sensing using a proposed sensor with
Biosensing using the SPR principle adapted from Ref [50]
Finite element method
A cross-section view of the proposed sensor’s geometrical structure
Cross-sectional view of the geometrical structure of the proposed sensor
Confinement loss with different thicknesses of Gold layer (t g )
Effect of air holes height on confinement loss with RI = 1.38 and 1.39
Effect of the height of air holes on AS with RI = 1.38 and 1.39
Effect of inclination on confinement loss with RI = 1.38
Effect of angular rotation of the air holes on (a) CL and (b) AS with RI
