For the analyte ri of 139 at this point maximum

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for the analyte RI of 1.39. At this point, maximum energy transfers from the core-guided mode to theplasmonic mode.Sensors2019,19, x FOR PEER REVIEW23 of 33core-guided mode. Mathematically, resonance occurs when the real part of the effective modeindex (neff) of the core-guided mode and the SPP mode coincide[5]. At the resonancewavelength, a sharp loss peak was observed, and unknown samples could be effectivelydetermined by shifting this peak to a longer or shorter wavelength for different analyte refractiveindexes. Imaginary (confinement loss) and real effective mode indexes for the core-guided modeare shown inFigure 3a by red and blue lines. Additionally, the black line shows the SPP modefor the interaction of the evanescent wave on the golddielectric surface. The real part of the SPPand core-guided modes intersected at a wavelength of 0.85 μm, and a peak was observed at thepoint of intersection, which is the phase-matching condition for the analyte RI of 1.39. At thispoint, maximum energy transfers from the core-guided mode to the plasmonic mode.Figure 3.Complex refractive index (RI) of the sensor and optical field distribution (a) (inset (i) SPPmode, (ii) core mode, and (iii) resonance condition) for analyte RI of 1.39, (bd) representationof coupling strength for different analyte RIs from 1.381.40 with using the optimized sensorparameters.3.2. Influence of Analyte RI (na) Variations on Sensing Characteristics(b)(c)(d)1.431.4321.4341.4361.4381.441.4421.4441.4461.4481.4500.20.40.60.811.21.41.61.820.690.740.790.840.890.940.991.04Real (neff)Imag (neff)Wavelength (μm)Core mode imaginaryCore mode RealSPP mode(i)(ii)(iii)(a)Figure 3.Complex refractive index (RI) of the sensor and optical field distribution (a) (inset (i) SPPmode, (ii) core mode, and (iii) resonance condition) for analyte RI of 1.39, (b–d) representation ofcoupling strength for different analyte RIs from 1.38–1.40 with using the optimized sensor parameters.
Sensors2019,19, 37946 of 123.2. Influence of Analyte RI (na) Variations on Sensing CharacteristicsThe analyte’s RI variation has a significant impact on PCF-SPR-sensing characteristics.The SPR-sensing mechanism with PCF is susceptible to the surrounding environment. In contrast withthe other sensing technique, the proposed mechanism showed a comparatively larger resonance peakshift when small RI variations in the targeted dielectric occurred. For this analysis, they-polarized modewas considered as it exhibited a comparatively larger evanescent field, resulting in the propagationof maximum free-surface electrons, compared to thex-polarization mode. From Figure4a, it can beseen that the resonance peak of the confinement loss curve became sharper and gradually broader(redshifted) to the longer wavelength, with a varying RI of analyte (na) from 1.32 to 1.40.Thisphenomenon can be described as follows: with the increase of analyte RIs, the effective mode index ofthe sensing medium was reduced significantly, and the RI contrasts also decreased between the SPPmode and the core-guided mode. Owing to the small RI contrast, maximum light penetrated through

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Term
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Tags
refractive index, Photonic crystal, Photonic crystal fiber, Surface plasmon resonance, Sensing Performance

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