Surface Plasmon Resonance (SPR)is a prominent optical phenomenon that arises as to the extent of energy transferring from photons to surface plasmon waves under appropriate conditions[
Acta Photonica Sinica, Volume. 51, Issue 9, 0906007(2022)
Design and Analysis of D-shaped Surface Plasmon Resonance Fiber Biosensor for Liquid Analytes
Surface Plasmon Resonance (SPR) is a prominent optical phenomenon that arises as the extent of energy transferring from photons to surface plasmon waves under appropriate conditions. In the past few years, this optical effect, owing to its high sensitivity, real-time detection, and anti-interference has already been extensively investigated and applied in medical treatment, environment monitoring, biomedical sensing and so on. Based on the principle of SPR, a novel D-shaped gold surface plasmon resonance photonic crystal fiber with one open-ring is proposed for detecting low refractive index materials has been investigated in detail. The proposed photonic crystal fiber of the simulation model is composed of three layers of air holes. The radii of air holes in the first-layer and third-layer are r1 and r3, respectively. While the second-layer air ring consists of air holes with two different radii, r2 and rs. The refractive index of air is fixed at nair = 1 and the radius of the cladding is R. A thin gold film with thickness tg is deposited on the inner surface of the micro-opening analyte channel on the upper side, the radius and the central location of the channel are rs and 2.5×Λ-1.25×rs, respectively. The fiber material is fused silica and the RI is determined by the Sellmeier equation, the relative dielectric constant of gold can be demonstrated by the Drude-Lorentz model. This paper uses the finite element method and sets the boundary conditions of the perfect matching layer for simulation. In order to investigate how the sensing performance of the proposed PCF-SPR sensor is affected by the parameters of the optical fiber, the effect of various parameters of the fiber such as air radii (r1, r2, r3, rs), air hole spacing (Λ) and the gold film (tg) on the SPR loss spectrum have been studied separately. The simulation results show that the confinement loss decreases as r1 increases. This can be attributed to the fact that more energy is confined to the core when r1increases, which affects the coupling between the core and plasmonic modes. At the same time, the confinement loss also decreases with the increase of r2, and the corresponding blue shift occurs with the resonance peaks moving toward a shorter wavelength over the process. The reason is that the increase of r2 will increase the refractive index difference between the plasmonic mode and core mode, which will affect the coupling between them. Therefore, with the increase of r2, the shorter wavelength can excite the plasmonic mode, resulting in the phenomenon of wavelength blue shift in the loss spectrum. Since the air holes of the third layer are located at the outermost part of the fiber, the change of r3 has little impact on the confinement loss, which can greatly reduce fabrication difficulty of the sensor. The pitches between the air holes are also an important factor in confinement loss, the change of Λ will influence the refractive index of core mode and plasmonic mode, which in turn affects the phase matching condition and energy coupling between them. The thickness of gold film plays a vital role in the sensing performance. If the gold film is too thick, the electric field can not penetrate the gold film, which will reduce the sensitivity of the proposed sensor. While if the gold film is too thin, the plasmonic wave will be strongly suppressed due to radiation damping. Therefore, the thickness of gold film can significantly affect the coupling between the core mode and the plasmonic mode. After optimizing the various parameters affecting the sensing performance of the sensor, we analyse the analytes with different refractive indices. Simulation results show that the sensor operates in the near-infrared and mid-infrared region with the wavelength range of 2 020~3 036 nm in the refractive index range of the analyte of 1.18~1.30. When the refractive index of the analyte is in the range of 1.23 to 1.30, the sensor operates in the band of 2 135~3 036 nm, and the average value of spectral sensitivity is up to 11 650 nm/RIU. When the refractive index of the analyte is between 1.29 and 1.30, the sensor operates in the mid-infrared band of 2 648~3 036 nm, and the maximum spectral sensitivity and resolution are 38 800 nm/RIU and 2.37×10-6 RIU, respectively. The proposed sensor shows great significance in detecting low refractive indexes in near- and mid-infrared waveband, and has potential applications in biomedical sensing, water environment and humidity detection and so on.
0 Introduction
Surface Plasmon Resonance (SPR)is a prominent optical phenomenon that arises as to the extent of energy transferring from photons to surface plasmon waves under appropriate conditions[
As is well known that the sensing performance of Photonic Crystal Fiber-surface Plasmon Resonance (PCF-SPR)sensors depends on the metallic materials and the structure of PCF. Among the various plasmonic materials,gold[
However,with the development of biomedicine and material chemistry,the detection for low refractive index analyte has been paid more attention. For example,the RI of sevoflurane in the drug is around 1.27[
Herein,a novel D-shaped PCF-SPR sensor for detecting low RI analytes is proposed and analyzed. The open-ring channel coated with gold film can not only reduce the coating area,but also simplify the fabrication process. Numerical results show that a maximum spectral sensitivity of 38 800 nm/RIU can be obtained when analyte RI is 1.30 with a high resolution of 2.37×10-6 RIU. The operating wavelength of this fiber sensor includes the near- and mid-infrared regions ranging from 2 020 nm to 3 036 nm. This proposed sensor is of significance in environmental engineering,biosensors and healthcare.
1 Design and analysis of the model
The cross-section of the proposed D-shaped SPR sensor is illustrated in
Figure 1.Schematic illustration of the proposed D-shaped PCF-sensor
The D-shaped sensors can be fabricated by stack-and-draw method[
where
where
In order to prevent the light reflects from the PCF boundary,an artificial boundary condition known as the Perfectly Matched Layer (PML)[
where
2 Results and discussion
Figure 2.Real part of effective refractive index (left axis)and confinement loss (right axis)of core mode and plasmonic mode
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Figure 3.The distribution of the energy of the core mode and the plasmonic mode at different wavelength
In order to optimize the sensing performance,the air holes radii,the pitch and gold film thickness were investigated separately. The refractive index of the analyte is fixed at 1.2 and the structural parameters of the PCF are calculated by keeping the rest of the structural parameters constant. The initial structural parameters r1, r2, r3, rs of the PCF are 0.5 µm,0.6 µm,0.7 µm and 1 µm,respectively. The innermost air hole pitch is 2.4 µm and the thickness of the gold film is 50 nm. It is worth noting that the variation of air hole diameter affects the coupling intensity and phase matching condition of core mode and plasmonic mode,thereby also affecting the energy conversion between them.
It can be seen from
Figure 4.The confinement loss affected by changing the radii of the air holes and the open-ring
Figure 5.The confinement loss of the proposed sensor varies with Λ and tg
The thickness of gold film plays a vital role in the sensing performance. If the gold film is too thick,the electric field can not penetrate the gold film,which will reduce the sensitivity of the proposed sensor. While if the gold film is too thin,the plasmonic wave will be strongly suppressed due to radiation damping. Therefore,the thickness of gold film can significantly affect the coupling between the core mode and the plasmonic mode.
The amplitude sensitivity is a critical parameter for evaluating the quality of the proposed sensor,and it is defined by the following equation[
where
The resonance wavelength is used to detect the refractive index of the analyte,which means the analyte has a significant effect on the coupling intensity between the core mode and the plasmonic mode. The wavelength dependence of the loss spectra and the amplitude sensitivity of the sensor for analytes
Figure 6.The increase of the confinement loss and amplitude sensitivity varies with wavelength for analyte
Figure 7.The resonance wavelength varies with analyte RI from 1.18 to 1.30
The spectral sensitivity has equal significance to the amplitude sensitivity at estimating the performance of the PCF sensor. The spectral sensitivity is defined by the following equation[
where
In addition to the spectral and amplitude sensitivities,the resolution is a critical parameter of the proposed sensor. The resolution is expressed as[
where
3 Conclusion
In this work,a novel D-shaped PCF sensor based on SPR effect is proposed and numerically investigated. Unlike conventional D-shaped structures,the SPR effect is excited by coating in the open-ring channel with a thin gold film,which can enhance the resonance effect and improve the spectral sensitivity of proposed sensor. In addition,the usage of this channel reduces the coating area of gold film. This makes the fabrication process more simplified and cost-effective. Simulation results show that the sensor can detect low refractive indexes ranging from 1.18 to 1.30 with the highest spectral sensitivity and resolution of 38 800 nm/RIU and 2.37×10-6 RIU. Due to the low refractive index sensing range and the simple compact fiber design,the proposed SPR sensor could be a competitive candidate in low refractive index detection.
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Qingmin LIU, Shanglin HOU, Jingli LEI. Design and Analysis of D-shaped Surface Plasmon Resonance Fiber Biosensor for Liquid Analytes[J]. Acta Photonica Sinica, 2022, 51(9): 0906007
Category: Fiber Optics and Optical Communications
Received: Feb. 8, 2022
Accepted: May. 26, 2022
Published Online: Oct. 26, 2022
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