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图 1是所提出的非互易光子自旋霍尔效应甲烷体积分数传感器的理论模型图。整体结构的拓扑排列形式为(ABAC)NA, 其中,电介质A层为甲烷敏感膜,B层为多孔硅porous Si, C层为金属银Ag,N为周期数,θ为入射角,xi和yi表示入射光的参考坐标系,xr和yr表示反射光的参考坐标系。
图 1 非互易PSHE甲烷体积分数传感器的理论模型图
Figure 1. Theoretical model diagram of a non-reciprocal PSHE methane volume fraction sensor
采用毛细管浸涂技术制备了一种紫外固化含氟硅(UV-curable fluoro-silicone, UVCFS)纳米膜[21]。温度和湿度对甲烷敏感膜的影响非常小。CH4气体的折射率接近空气,难以直接测量,且容易受到其它气体的干扰,因此检测精度较低。利用该敏感膜的特性,可将折射率变化现象扩大两个数量级,并可避免其它气体的干扰。甲烷敏感膜层响应时间低于60 s[22]。为了方便表示,甲烷体积分数用φCH4表示, 当φCH4为0~3%时,敏感膜的折射率呈线性变化, φCH4每增加1%,折射率降低0.0038,如果φCH4从0上升到3%,那么敏感膜的折射率neff从1.4478下降到1.4364[21]。
$ n_{\text {eff }}=1.4478-0.38 \varphi_{\mathrm{CH}_4} $
(1) 介质硅的折射率nSi可以描述为[23]:
$ n_{\mathrm{si}}=\sqrt{1+\frac{10.6684293 \lambda^2}{\lambda^2-0.301516485^2}+\frac{0.0030434748 \lambda^2}{\lambda^2-1.13475115^2}+\frac{1.54133408 \lambda^2}{\lambda^2-1104^2}} $
(2) 式中:λ为入射光波长。
利用Bruggeman的有效介质理论,多孔硅的介电常数np, Si可以被获得[23]:
$ n_{\mathrm{p}, \mathrm{Si}}=0.5 \sqrt{X+\sqrt{X^2+8 n_{\mathrm{Si}}{ }^2 n_{\text {gas }}{ }^2}} $
(3) $ X=3 P\left(n_{\text {gas }}{ }^2-{n_{\mathrm{Si}}}^2\right)+\left(2{n_{\mathrm{Si}}}^2-n_{\text {gas }}{ }^2\right) $
(4) 式中:np, Si、nSi和ngas分别是多孔硅、硅和孔隙内气体的折射率;P为气体孔隙率。
金属银Ag的介电常数εr采用德鲁德-洛伦兹模型表示[21]:
$ \varepsilon_{\mathrm{r}}=\varepsilon_{\infty}-\frac{\omega_{\mathrm{p}}^2}{\omega^2+\mathrm{i} \omega \gamma}-\frac{\mathit{\Omega}^2 \mathit{\Delta}}{\left(\omega^2-\mathit{\Omega}^2\right)+\mathrm{i} \mathit{\Gamma}} $
(5) 式中:ω=2πf为入射光的角频率;f=550 THz为频率;ε∞=2.4064为高频介电常数;ωp=2π×2214.6×1012 Hz为等离子体频率;γ=2π×4.8×1012 Hz为碰撞频率;Δ=1.6604是洛伦兹权重;Ω=2π×1330.1×1012 Hz为洛伦兹谐波强度;Γ=2π×620.7×1012为振动谱宽。
除了介电函数外,其它的参数可见表 1。表中, deff为甲烷敏感膜的厚度, dp, Si为多孔硅层的厚度, dAg为金属银层的厚度。
表 1 仿真所涉及的参数
Table 1. Parameters involved in the simulation
deff/nm dp, Si/nm dAg/nm N P 20 138 20 7 0.29 层与层之间的能量传播使用传输矩阵法进行计算。当一束线偏振光以折射率梯度反射到结构表面时,会分裂为左旋和右旋圆偏振光,反射光的水平偏振和垂直偏振的位移量可以表示为[20]:
$ \delta_{\mathrm{h}}^{+}=-\frac{\lambda}{2 {\rm{ \mathsf{ π}}}}\left[1+\frac{\left|r_s\right|}{\left|r_p\right|} \cos \left(\phi_s-\phi_p\right)\right] \cot \theta $
(6) $ \delta_{\mathrm{h}}^{-}=+\frac{\lambda}{2 {\rm{ \mathsf{ π}}}}\left[1+\frac{\left|r_s\right|}{\left|r_p\right|} \cos \left(\phi_s-\phi_p\right)\right] \cot \theta $
(7) $ \delta_{\mathrm{v}}{ }^{+}=-\frac{\lambda}{2 {\rm{ \mathsf{ π}}}}\left[1+\frac{\left|r_p\right|}{\left|r_s\right|} \cos \left(\phi_p-\phi_s\right)\right] \cot \theta $
(8) $ \delta_{\mathrm{v}}{ }^{-}=+\frac{\lambda}{2 {\rm{ \mathsf{ π}}}}\left[1+\frac{\left|r_p\right|}{\left|r_s\right|} \cos \left(\phi_p-\phi_s\right)\right] \cot \theta $
(9) 式中: 上标+和-分别表示左旋和右旋偏振分量; rs和rp为s波和p波的菲涅耳反射系数; ϕs和ϕp为反射相位; θ为入射角。
灵敏度S、品质因素Q、质量因素fm和检测下限D是衡量传感器的重要参数,性能突出的传感器对应着高S、高Q、高fm和低D。对应的定义如下:
$ S=\frac{{\Delta} \theta}{{\Delta} n_{\text {eff }}} $
(10) 式中: Δθ和Δneff为角度和折射率变化。
灵敏度S表征了传感器的谐振角度偏移量与待检测参量的变化量之间的对应关系[21, 23-24]。因此,灵敏度和角度的变化成正比,和有效折射率的变化成反比。当参数变化量相同时,检测到的谐振角度偏移量越大,传感器的灵敏度越高,也就越能够检测出越小的待测参量变化。
$ Q=\frac{\theta}{F} $
(11) 式中: F为半峰全宽[21]。品质因素Q是一个无量纲数值,表征微腔在时间上局域光的能力,与谐振腔内的损耗相关联,损耗越大,品质因素越低。该值的大小可以直观地从透射光谱中反映出来,透射峰越尖锐,微腔的品质因素越高,那么在读取谐振角度的大小时会更加准确。从定义上来看[21, 23],品质因素和谐振角度的位置的变化成正比,和峰值半峰全宽的变化成反比。
$ f_{\mathrm{m}}=\frac{S}{F} $
(12) 质量因素fm被认为是指示传感器性能的重要参数。品质因数被定义为角度灵敏度与峰值半峰全宽的比值[21, 23]。质量因素和灵敏度的变化成正比,和峰值半峰全宽的变化成反比。通常,为了实现高质量因数性能传感器,灵敏度应尽可能高,而峰值半峰全宽应尽可能低。
$ D=\frac{\theta}{20 S Q} $
(13) 检测下限D反映的是传感器在检测分析物时能够检测到的分析物最小变化的能力。它表现为谐振角度与20倍的灵敏度和品质因素的乘积的比值[21, 23],因此从公式上来看,检测下限和谐振角度的位置的变化成正比,和灵敏度以及品质因素的变化成反比。
基于光子自旋霍尔效应的甲烷检测理论研究
Theoretical research of methane detection based on photonic spin Hall effect
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摘要: 为了实现对甲烷体积分数的高精度检测, 采用多层结构激发了具有高品质因素的光子自旋霍尔效应现象。利用非对称的排列方式, 将甲烷敏感膜引入结构中, 通过敏感膜的折射率变化, 实现对甲烷体积分数的检测; 研究了气体孔隙率、周期数、金属厚度和敏感膜厚度对光子自旋霍尔效应的影响, 并采用传输矩阵法进行了数值分析。结果表明, 该传感器可对体积分数为0~3%、折射率变化为1.4364~1.4478的甲烷气体进行检测, 灵敏度为29.6°, 最高质量因素和最低检测下限分别为395和0.00012。该传感器结构简单、检测能力强, 为光学传感器的研究提供了新思路。Abstract: In order to realize the high precision detection of the volume fraction of methane, the photonic spin Hall effect phenomenon with high-quality factor is excited by a multilayer structure. In an asymmetric arrangement, a methane-sensitive film was introduced into the structure, and the volume fraction of methane could be detected by the change in the refractive index of the sensitive film. The effects of gas porosity, period number, metal thickness, and sensitive film thickness on the photon spin Hall effect were studied, and the transfer matrix method was used for numerical analysis. The results show that the sensor can detect methane gas with volume fraction of 0~3% (refractive index change is 1.4364~1.4478) with a sensitivity of 29.6°. The highest figure of merit and the lowest detection limit are 395 and 0.00012, respectively. The sensor has a simple structure and strong detection ability, which provides a new idea for the research of optical sensors.
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图 5 a—不同甲烷气体体积分数的位移分布情况现象 b—甲烷气体体积分数连续变化时的PSHE变化 c—甲烷体积分数和共振角度的线性拟合 d—特定的体积分数情况下,fm和D的分布情况
Figure 5. a—displacement distribution of different methane gas volume fraction b—PSHE phenomenon when methane gas volume fraction continuously changes c—linear fitting of methane volume fraction and resonance angle d—distribution of fm and D at specific volume fractions
表 1 仿真所涉及的参数
Table 1. Parameters involved in the simulation
deff/nm dp, Si/nm dAg/nm N P 20 138 20 7 0.29 -
[1] XIA Y, WAN J, XU L, et al. A room-temperature methane sensor based on Pd-decorated ZnO/rGO hybrids enhanced by visible light photocatalysis[J]. Sensors and Actuators, 2019, B304: 127334. [2] MULLER S A, DEGLER D, FELDMANN C, et al. Exploiting synergies in catalysis and gas sensing using noble metal-loaded oxide composites[J]. ChemCatChem, 2018, 10(5): 864-880. doi: 10.1002/cctc.201701545 [3] LIU L, LI G. A remote sensor for detecting methane based on palladium-decorated single walled carbon nanotubes[J]. Sensors, 2013, 13(7): 8814-8826. doi: 10.3390/s130708814 [4] DHIVYA P, PRASAD A K, SRIDHARAN M. Effect of sputtering power on the methane sensing properties of nanostructured cadmium oxide films[J]. Journal of Alloys and Compounds, 2015, 620: 109-115. doi: 10.1016/j.jallcom.2014.09.107 [5] LIU H, WANG M, WANG Q, et al. Simultaneous measurement of hydrogen and methane based on PCF-SPR structure with compound film-coated side-holes[J]. Optical Fiber Technology, 2018, 45: 1-7. doi: 10.1016/j.yofte.2018.05.007 [6] YANG J Ch, XIN C, RUI S, et al. High-sensitivity photonic crystal fiber long-period grating methane sensor with cryptophane-A-6Me absorbed on a PAA-CNTs/PAH nanofilm[J]. Optics Express, 2017, 25(17): 20258-20267. doi: 10.1364/OE.25.020258 [7] LIU H, WANG H R, ZHANG W, et al. High sensitive methane sensor with temperature compensation based on selectively liquid-infiltrated photonic crystal fibers[J]. Photonic Sensors, 2019, 9: 213-222. doi: 10.1007/s13320-019-0536-y [8] ZHENG K Y, ZHENG C T, LI J H, et al. Near-infrared methane sensor system using off-axis integrated cavity output spectroscopy with novel dual-input dual-output coupling scheme for mode noise suppression[J]. Sensors and Actuators, 2020, B308: 127674. [9] VUONG N M, HIEU N M, HIEU H N, et al. Ni2O3-decorated SnO2 particulate films for methane gas sensors[J]. Sensors and Actuators, 2013, B192: 327-333. [10] ZHOU X X, SHENG L J, LING X H. Photonic spin Hall effect enabled refractive index sensor using weak measurements[J]. Scientific Reports, 2018, 8: 1221. doi: 10.1038/s41598-018-19713-3 [11] KAVOKIN A, MALPUECH G, GLAZOV M. Optical spin hall effect[J]. Physical Review Letters, 2005, 95: 13660. [12] BLIOKH K Y, SMIRNOVA D, NORI F. Quantum spin Hall effect of light[J]. Science, 2015, 348(6242): 1448-1451. doi: 10.1126/science.aaa9519 [13] LUO H, ZHOU X, SHU W, et al. Enhanced and switchable spin Hall effect of light near the Brewster angle on refection[J]. Physical Review, 2011, A84: 043806. [14] WU Y D, SHENG L J, XIE L G, et al. Actively manipulating asymmetric photonic spin Hall effect with graphene[J]. Carbon, 2020, 166: 396-404. doi: 10.1016/j.carbon.2020.05.065 [15] SHALTOUT A, LIU J, KILDISHEV A, et al. Photonic spin hall effect in gap-plasmon metasurfaces for on-chip chiroptical spectroscopy[J]. Optica, 2015, 2(10): 860-863. doi: 10.1364/OPTICA.2.000860 [16] SRIVASTAVA A, SHARMA A K, PRAJAPATI Y K. On the sensitivity-enhancement in plasmonic biosensor with photonic spin Hall effect at visible wavelength[J]. Chemical Physics Letters, 2021, 774: 138613. doi: 10.1016/j.cplett.2021.138613 [17] LI N X, TANG T T, LI J, et al. Highly sensitive biosensor with graphene-MoS2 heterostructure based on photonic spin Hall effect[J]. Journal of Magnetism and Magnetic Materials, 2019, 484: 445-450. doi: 10.1016/j.jmmm.2019.04.003 [18] WANG Y H, JIN R C, LI J Q, et al. Photonic spin Hall effect by the spin-orbit interaction in a metasurface with elliptical nano-structures[J]. Applied Physics Letters, 2017, 110(10): 10190. [19] SHENG L J, XIE L G, LUO H L, et al. Sensitivity enhanced refractive index sensor by reducing the inflfluence of in-plane wavevector in photonic spin Hall effect[J]. IEEE Photonics Journal, 2018, 10(5): 6501209. [20] ZHANG P, TANG T, LUO L, et al. Magneto-optical spin Hall effect of light and its application in refractive index detection[J]. Optics Communications, 2020, 475: 126175. doi: 10.1016/j.optcom.2020.126175 [21] WAN B F, WANG Q Y, PENG H M, et al. A late-model optical biochemical sensor based on OTS for methane gas and glucose solution concentration detection[J]. IEEE Sensors Journal, 2021, 21: 21465-21472. doi: 10.1109/JSEN.2021.3103548 [22] LIU H, CHEN C, ZHANG Y Z, et al. A high-sensitivity methane sensor with localized surface plasmon resonance behavior in an improved hexagonal gold nanoring array[J]. Sensors, 2019, 19(21): 4803. doi: 10.3390/s19214803 [23] ZAKY Z A, AHMED A M, SHALABY A S, et al. Refractive index gas sensor based on the Tamm state in a one-dimensional photonic crystal: Theoretical optimization[J]. Scientific Reports, 2020, 10: 9736. doi: 10.1038/s41598-020-66427-6 [24] 陈强华, 丁锦红, 韩文远, 等. 光纤SPR传感器参数对折射率测量灵敏度的影响[J]. 激光技术, 2023, 47(3): 329-334. CHEN Q H, DING J H, HAN W Y, et al. Effect of optical fiber SPR sensor parameters on the sensitivity of refractive index measurement[J]. Laser Technology, 2023, 47(3): 329-334(in Chinese). [25] RENILKUMAR M, NAIR P. Properties of defect modes in geometrically chirped one-dimensional photonic crystals[J]. Optical Materials, 2011, 33(6): 853-858. doi: 10.1016/j.optmat.2011.01.008