Thin films based one-dimensional photonic crystal for refractive index sensing

Optik 158 (2018) 1512–1518

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Original research article

Thin films based one-dimensional photonic crystal for refractive index sensing Jiankun Peng a,∗ , Dajuan Lyu a , Yapeng Qu a , Weijia Wang a , Tengpeng Sun a , Minghong Yang a,b a b

National Engineering Laboratory for Fiber Optic Sensing Technologies, Wuhan University of Technology, Wuhan, 430070, China Key Laboratory of Fiber Optic Sensing Technology and Information Processing, Ministry of Education, China

a r t i c l e

i n f o

Article history: Received 29 September 2017 Received in revised form 14 January 2018 Accepted 15 January 2018 Keywords: Optical fiber sensor Refractive index Porous thin films Physical vapor deposition

a b s t r a c t Thin films based one-dimensional photonic crystal for refractive index sensing has been theoretically and experimentally investigated in this article. The sensing elements are porous one-dimensional photonic crystal deposited on multimode fiber end face by e-beam evaporation. Theoretical simulation shows that the resonant mode of the photonic crystal shifts to longer wavelength since the thin film pores are filled with higher refractive index solutions. Experimental results on refractive index detection show that the wavelength shift is 10.66 nm when refractive index ranges from 1.3333 to 1.4584; especially the wavelength shift of the proposed sensor has linear dependence relation with the refractive index variation. © 2018 Elsevier GmbH. All rights reserved.

1. Introduction Refractive index (RI) is an important optical parameter of solutions especially in chemical analysis. Therefore, many kinds of RI sensor have been developed in various technology. In recent years, optical fiber RI sensor becomes a promising sensing technology due to its capacity of remote sensing, corrosion resistance, and anti-electromagnetic interference. Examples of optical fiber RI sensors include fiber Bragg gratings (FBG) [1–3], Long-Period gratings (LPG) [4], Fabry-Perot interferometer [5,6], tapered fiber [7–10], 2-D photonic crystal structure [11], and surface plasmon resonance (SPR) [12,13]. In FBG RI sensors, since the effective RI is not influenced by the external environment for standard FBG, it has to remove the FBG cladding by etching [1,2], or it has to use microfiber to fabricate FBG [3]. However, if fiber cladding is removed from the FBG, the sensor is very fragile. In LPG RI sensors, the transmission dips of the LPG are broad, resulting in a poor measurement accuracy [4]. In Fabry-Perot RI sensors, the Fabry-Perot interferometer is made by machining a micro-notch on a single-mode fiber using a femtosecond laser [5,6]. While the sensor is brittle on account of the fiber is eroded by laser. In tapered fiber RI sensors, the thin tapered fiber is more sensitive to index variations [7], but its thin waist probably results in fiber fracture. Otherwise, it has to combine with Michelson interferometer [8], Mach-Zehnder interferometer [9], or Long-Period Grating Pair [10] to construct an RI sensor. In 2-D photonic crystal RI sensor, the tiny holes of PCF have to be filled with the solution to be measured, the infiltration of solution typically needs a few minutes. Furthermore, the solution which lies in the micro holes have to be eliminated before the subsequent measurement [11]. In SPR RI sensor, the sensor is influenced by the surrounding temperature [12] and solution ions [13].

∗ Corresponding author. E-mail address: [email protected] (J. Peng). https://doi.org/10.1016/j.ijleo.2018.01.047 0030-4026/© 2018 Elsevier GmbH. All rights reserved.

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Fig. 1. Schematic diagram: (a) initial stages of film growth; (b) porous film.

Fig. 2. (a) Schematic diagram of the RI sensor; (b) Schematic diagram of the thin films based photonic crystal; (c) SEM image of the thin films based photonic crystal depositing on a silicon wafer (transversal section, EHT = 5.00 kV WD = 5.0 mm Mag = 30.00 K × Signal A = InLens).

In this article, we proposed a thin films based one-dimensional photonic crystal used as RI sensor. The sensor is fabricated with depositing one-dimensional photonic crystal on optical fiber end face. In the sensing probe, silicon oxide and titanium oxide are used as different dielectric materials to construct the photonic crystal structure. Since a defective layer is existing in the photonic crystal, a resonant mode can be generated in the reflective spectrum of the proposed sensor. Due to the thin films have porous structure which is intrinsically derived from the film generation process by control the deposition environment, the proposed sensor has the capacity to detect fluid refractive index. The paper is organized as follows. In Section 2, we introduced the theory and simulation of the proposed structure with RI sensing. In Section 3, we displayed the RI sensor manufacture process and the white light demodulation system. In Section 4, we presented the experimental results on refractive index detection and temperature cross sensing. Finally, in Section 5, we concluded our findings. 2. Theory and simulation Dielectric film deposited by e-beam physical vapor deposition is porous film, which is intrinsically derived from its generative pattern. In the initial stages of film growth (as shown in Fig. 1(a)), the monomer density rapidly deposits onto the substrate surface, leading to a rapid increase of dimers, trimers and multimers by monomer-monomer encounter. Therefore, there are plenty multimers spreading among the substrate while the other places are still empty. As the deposition continues, the multimers gradually grow bigger to become islands. When the islands occupy a large fraction of the substrate surface, they gather and coalesce upon contact to generate thin film on the substrate surface [14–16]. In the coalescing process, there are cracks and defects existing in the islands, which will remain within thin film even if the deposition process is over. During the thin film growth, its microstructure are essentially ruled by the shadowing effect [17,18], which leads to columnar growth and porous structure of the thin films. As thin film grows thicker, it comprises pores which are in the form of random gaps, ellipsoids and spheres within the film [19–21], which can be filled by liquids. Since the thin films are porous films, the effective refractive index of thin films is determined by the deposition materials and the substance which exists in the pores. When the thin film is immersed into solutions, the pores will be filled with liquid leading to the thin film effective RI changes. In other words, the influence of different solution infiltration in porous thin films can also be understood in terms of the effective RI changes. The effective RI of the porous thin films can be calculated by using the Bruggeman effective medium model [22]. The Bruggeman equation for the porous film and solution system is shown as follow: f (n2xo − n2eff )/(n2xo +2n2eff ) + p(n2S − n2eff )/(n2S +2n2eff ) = 0

(1)

In the above formula, p is the pores volume fraction, f = 1-p is the thin film material volume fraction. nXO and nS are the RI of the thin film material and the solution respectively. And neff is the effective RI of the thin film filled with solutions. It can be deduced that neff becomes larger with nS increasing by solving formula (1), which means that the effective RI of thin films changes with infiltrating different solutions. Schematic diagram of the thin films based photonic crystal depositing on a multi-mode fiber end face is shown in Fig. 2(a) and (b). Fig. 2(c) is a scanning electron microscope (SEM) image of the thin films based photonic crystal depositing on a silicon wafer which is cut by a diamond glass cutter on its backside. The photonic crystal is composed of two different dielectric slabs and contained a defective layer in its center. The existence of a defective layer leads to generate a resonant mode within the photonic crystal band gap. 2 × 2 Transfer Matrix Method is effective in simulating the optical properties of finite

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Table 1 Effective RI of the porous film filling with different substance. Filling substance

Air

Deionized water

Glycerol

SiO2 film Ti3 O5 film

1.429 2.285

1.497 2.347

1.526 2.375

Fig. 3. Simulative reflective spectrum of the proposed sensor.

one-dimensional photonic crystals [23]. Therefore, reflective spectrum of the RI sensor can be calculated with the standard 2 × 2 Transfer Matrix Method. The transfer matrix of a single layer is shown as follow:

     cos(k0 nd cos ) − i sin(k0 nd cos )   p M(d) =    −ip sin(k0 nd cos ) cos(k0 nd cos ) 

(2)

Wherein, k0 is the vector of the incident light, n is the RI, d is thickness, and  is the incident angle. Supposing the  dielectric ε/ cos . permittivity and magnetic permeability of the material are ε and , then the parameter p can be expressed as p = In our calculation, the refractive indices of film materials (i.e. SiO2 and Ti3 O5 ) are 1.54 and 2.61, the RI of substrate (i.e. the multimode fiber) is 1.477, and the porosity is 20%. From Eq. (1), effective RI of the SiO2 and Ti3 O5 films filling with air, deionized water (DI water) and glycerol are calculated and listed in Table 1. The thicknesses of Ti3 O5 , SiO2 and defective layer are 65, 90 and 180 nm respectively. The reflective spectrum of the photonic crystal is simulated through MATLAB with 2 × 2 Transfer Matrix Method. Fig. 3 shows the simulation results. From the simulation results, it can be confirmed that a resonant mode exists within the band gap and shifts to longer wavelength since the film pores are filled by higher RI substance. In other words, when a liquid is filled into the thin film pores, it will cause the increasing of thin film optical thickness which directly influence the wavelength location of the resonant mode. 3. Sensor fabrication and detection system The proposed sensor structure is consisted of 19 layer thin films which is deposited onto a multi-mode fiber (62.5/125 ␮m) end face. The thin films were deposited by e-bean evaporation physical vapor deposition system (ZZS1100-8/G, Rankuum Machinery Co. Ltd.). In the experiment, performance of the 19 layers structure is optimal since more layers will cause severe material absorption. In the preparation process of RI sensor, titanium oxide (Ti3 O5 : RI = 2.221 at 550 nm measured by ellipsometer, J.A. Woollam Co. Ltd.) and silicon oxide (SiO2 : RI = 1.449 at 550 nm measured by ellipsometer, J.A. Woollam Co. Ltd.) are selected as high and low RI materials to construct the one dimensional photonic crystal. In the vacuum chamber, multimode fibers were fixed on the rotary plate (shown in Fig. 4) with polished end face down toward the source material. During the deposition process, the baking temperature was 120 ◦ C and the vacuum degree was 3.6 × 10−2 Pa with oxygen (O2 ) filling velocity of 300 sccm. Deposition rates of Ti3 O5 and SiO2 were 2.5 Å/s and 6 Å/s, respectively. For the prepared sensor, thicknesses of Ti3 O5 , SiO2 and defective layer are 61, 86 and 172 nm respectively, which is monitored by Thin Film Deposition Controller (IC6, INFICON Co. Ltd.). Fig. 4 is the schematic diagram of the vacuum chamber of e-beam PVD technology. Due to the resonant mode of the thin films based sensor is laid within the visible light band, therefore the sensor reflective spectrums were detected by white light demodulation system with different RI matching solution. The RI detection system is schematically shown in Fig. 5. In general, the demodulation mechanism is detecting the shift of the resonant mode, since the resonant mode is a sharp dip which is suitable for the wavelength measurement. In the experiment, a white light source (HL2000, Ocean Optics) and an optic spectrometer (USB-2000+, Ocean Optics, resolution: 0.1 nm) were used, the RI sensor was fusion splicing to a 3 dB multimode fiber coupler and connected to the light source and the spectrometer by the fiber coupler. A water bath is used to keep stationary temperature of the RI solutions which is generated by using glycerol solutions with different mass concentrations. Finally, the real-time reflective spectrum of the proposed sensor is displayed on a computer.

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Fig. 4. Schematic diagram of the evaporation PVD.

Fig. 5. Configuration of the RI detection system.

Fig. 6. Reflective spectrum of the RI sensor within different glycerol solution.

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Table 2 Resonant mode wavelength of the RI sensor with different glycerol solution. w

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

RI wavelength

1.3333 571.82

1.3448 572.98

1.3574 574.15

1.3707 575.31

1.3841 576.48

1.3981 577.64

1.4130 578.80

1.4279 580.16

1.4429 581.32

1.4584 582.48

Fig. 7. Sensor response of the different RI solution.

4. Experimental results and analysis Reflective spectrums of the RI sensor are measured in different RI solutions (glycerol solutions with mass concentrations of 0%, 30%, 60% and 90%) and shown in Fig. 6, which is demonstrated in the simulation of Section 2. From Fig. 6, it can be confirmed that the resonant mode within the band gap shows red shift since the RI sensor is immersed in higher mass concentration glycerol solutions. Afterwards, the proposed sensor is used to detect the RI of glycerol/water solutions. The different RI solutions were prepared by mixing deionized water and glycerol at different glycerol mass concentrations from 0% to 90%. The measurement results of the resonant mode wavelength shift and the RI of different glycerol solutions (with temperature of 20 ◦ C) are listed in Table 2. The data listed in Table 2 demonstrate that the resonant mode shows red shift with increasing the solution RI. The data is also plotted and fitted in Fig. 7 which shows the resonant mode wavelength changing with the solution RI variation from 1.3333 to 1.4584. Experimental results reveal the resonant mode wavelength shifting from 571.82 nm to 582.48 nm. The prepared RI sensor has a sensitivity of 85 nm/RIU with good linearity of 0.99875. Sensing response time and repeatability of the RI sensor was also evaluated. The sensor reflective spectrums were recorded with a rate of 1 frame per second, and the resonant mode wavelength value is plotted and shown in Fig. 8. The prepared sensor was placed in glycerol solution with the mass concentration of 10%, 30% and 50% for about 1 min respectively and consistently. The experiment contained three testing cycles to investigate the repeatability and hysteresis/recover time which is 3 s between different mass concentration glycerol solutions. The liquids absorption is just needed a few seconds due to the sensing structure of the sensor is a very small cylinder with diameter of 125 ␮m and thickness of 1.47 ␮m, and the liquids absorption process is similar to the process that a sponge absorbs water. Furthermore, the pores within the thin films are in the size of 2–50 nm while the solution molecular diameter is around 0.5 nm, therefore the sample solution molecules can be diffused into the porous thin films freely. When the mass concentration of glycerol solution is increased, the sensor response time has not significantly increased. The reason is that in the situation of high glycerol mass concentration, the water molecules can be considered as the solute so the re-equilibrium process is still very fast when there is a small fluctuation of the solution. Similarly, in the situation that the sensor head is placed from a solution into the other solution, there exist residual solution in the thin films pores, but the residual solution will be replaced by the new solution through the re-equilibrium process and the residual solution within the thin films is too little to influence the new solution’s glycerol concentration. During the experiment, given the sensor sensitivity is 85 nm/RIU and the spectrum wavelength fluctuation is ± 0.1 nm, the sensor resolution is calculated to be 2.4 × 10−3 RIU. The sensor resolution is close to that achieved with an Abbe refractometer, while the proposed sensor has the capacity of remote sensing and real-time monitor. Besides, the proposed sensor is suitable for batch production i.e. the sensor structure can be fabricated for thousands during one deposition process. Therefore, when there is a need for mass manufacturing, the provided manufacture method is better than the others refractive index sensors’ fabrication. For further improvement of the sensor resolution, it is need to enhance the thin film porosity or using more accurate optic spectrometer. Temperature cross sensing of the RI sensor is also investigated in the following experiment. RI sensor reflective spectrums with temperature at 20 ◦ C, 25 ◦ C, 30 ◦ C, 35 ◦ C, 40 ◦ C, 45 ◦ C, 50 ◦ C, 55 ◦ C and 60 ◦ C are recorded and shown in Fig. 9(a). The 30% mass concentration glycerol solution is used as RI solution and placed in the water bath. During the experiment, the sensor reflective spectrums were recorded every 30 min to ensure the glycerol solution was evenly heated. The recorded different

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Fig. 8. Response time and cycling test of the RI sensor.

Fig. 9. (a) Reflective spectrums of the sensor with different temperature; (b) Refractive index of 36.6% and 30% mass concentration glycerol solution with different temperature. Table 3 Glycerol solution RI variation with the increased temperature [24]. temperature (◦ C)

20◦ C

30◦ C

40◦ C

50◦ C

60◦ C

RI (w = 36.6%)

1.3792

1.3777

1.3764

1.3742

1.3731

spectrums is almost remained stable at a temperature variation of 40 ◦ C. The resonant mode wavelength with temperature of 20 ◦ C and 60 ◦ C are 574.98 and 574.40 nm. The wavelength change is 0.58 nm corresponding to an RI variation of 0.0068 since the sensor sensitivity is 85 nm/RIU. In the reference [24], it has been demonstrated that the 36.6% mass concentration glycerol solution has RI changes of 0.0061 when temperature varies from 20 ◦ C to 60 ◦ C.The refractive indices of 36.6% mass concentration glycerol solution are listed in Table 3, which is extracted from reference [24]. Fig. 9(b) shows the RI changes of 30% and 36.6% mass concentration glycerol solution when temperature varies from 20 ◦ C to 60 ◦ C. In the Fig. 9(b), the black solid line represent the RI change of glycerol solution while the red dash line is the experimental results of the proposed sensor. Therefore, the resonant wavelength change is conformed to the variation of glycerol solution with the temperature changing from 20 ◦ C to 60 ◦ C. From the experimental results, it can be concluded that the sensor spectrum is hard to be affected by ambient temperature. The reason is that the thin films have small thermal expansion coefficient and its

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thickness is very thin, which causes the change of optical path difference derived from external temperature change is much smaller than that derived from fluid refractive index change, therefore the temperature variation cannot affect the fluid refractive index detection. Hence the proposed RI sensor has good resistant performance on temperature cross sensitivity. 5. Conclusions In this paper, we have proposed a thin films based one-dimensional photonic crystal used for refractive index sensing. The reflective spectrum of the proposed sensor has been simulated through MATLAB with 2 × 2 transfer matrix method. The simulation results revealed that the sensor reflective spectrum shifts toward longer wavelength since the thin film pores are filled with higher refractive index solutions. The prepared sensor probe is porous structure which is formed by controlling thin films generation environment during the films deposition process. The refractive index sensor sensitivity and temperature cross sensitivity of the prepared sensor have been investigated. Experimental investigation on different mass concentration glycerol solution shows that the wavelength shift is 10.66 nm when refractive index ranges from 1.3333 to 1.4584, and the prepared sensor has a sensitivity of 85 nm/RIU with good linearity of 0.99875. The proposed sensor is robust and has good resistant performance on temperature cross sensitivity. Therefore, the proposed fiber optic refractive index sensor has the potential value for application. Acknowledgments This work was financially supported by the National Natural Science Foundation of China, NSFC (Project Number: 61575151, 51402228), the Creative Group Project of Hubei Provincial Natural Science Foundation (Project Number: 2015CFA016), the Fundamental Research Funds for the Central Universities (Project Number: WUT-2017-YB-026). References [1] A. Iadicicco, A. Cusano, A. Cutolo, R. Bernini, M. Giordano, Thinned fiber bragg gratings as high sensitivity refractive index sensor, IEEE Photonics Technol. Lett. 16 (2004) 1149–1151. [2] W. Liang, Y. Huang, Y. Xu, R.K. Lee, A. Yariv, Highly sensitive fiber bragg grating refractive index sensors, Appl. Phys. Lett. 86 (2005), 151122-1-3. [3] X. Fang, C.R. Liao, D.N. Wang, Femtosecond laser fabricated fiber bragg grating in microfiber for refractive index sensing, Opt. Lett. 35 (2010) 1007–1009. [4] H. Tsuda, K. Urabe, Characterization of long-period grating refractive index sensors and their applications, Sensors 9 (2009) 4559–4571. [5] Z.L. Ran, Y.J. Rao, W.J. Liu, X. Liao, K.S. Chiang, Laser-micromachined fabry-perot optical fiber tip sensor for high-resolution temperature-independent measurement of refractive index, Opt. Express 16 (2008) 2252–2263. [6] T. Wei, Y. Han, Y. Li, H. Tsai, H. Xiao, Temperature-insensitive miniaturized fiber inline fabry-perot interferometer for highly sensitive refractive index measurement, Opt. Express 16 (2008) 5764–5769. [7] P. Polynkin, A. Polynkin, N. Peyghambarian, M. Mansuripur, Evanescent field-based optical fiber sensing device for measuring the refractive index of liquids in microfluidic channels, Opt. Lett. 30 (2005) 1273–1275. [8] Z. Tian, S.S.-H. Yam, H. Loock, Refractive index sensor based on an abrupt taper Michelson interferometer in a single-mode fiber, Opt. Lett. 33 (2008) 1105–1107. [9] P. Lu, L. Men, K. Sooley, Q. Chen, Tapered fiber Mach-Zehnder interferometer for simultaneous measurement of refractive index and temperature, Appl. Phys. Lett. 94 (2009), 131110-1-3. [10] J. Ding, A.P. Zhang, L. Shao, J. Yan, S. He, Fiber-taper seeded long-period grating pair as a highly sensitive refractive-index sensor, IEEE Photonics Tech. Lett. 17 (2005) 1247–1249. [11] D.K.C. Wu, B.T. Kuhlmey, B.J. Eggleton, Ultrasensitive photonic crystal fiber refractive index sensor, Opt. Lett. 34 (2009) 322–324. [12] J.S. Velazquez-Gonzalez, D. Monzon-Hernandez, D. Moreno-Hernandez, F. Martinez-Pinon, I. Hernandez-Romano, Simultaneous measurement of refractive index and temperature using a SPR-based fiber optic sensor, Sens. Actuators B Chem. 242 (2017) 912–920. [13] S.K. Srivastava, B.D. Gupta, Influence of ions on the surface plasmon resonance spectrum of a fiber optic refractive index sensor, Sens. Actuators B Chem. 156 (2011) 559–562. [14] H. Brune, Microscopic view of epitaxial metal growth: nucleation and aggregation, Surf. Sci. Rep. 31 (1998) 121–229. [15] P. Jensen, Growth of nanostructures by cluster deposition: experiments and simple models, Rev. Mod. Phys. 71 (1999) 1695–1735. [16] C. Ratsch, J.A. Venables, Nucleation theory and the early stages of thin film growth, J. Vac.Sci. Technol. A 21 (2003) S96–S109. [17] T. Karabacak, J.P. Singh, Y.P. Zhao, G.C. Wang, T.M. Lu, Scaling during shadowing growth of isolated nanocolumns, Phys. Rev. B 68 (2003), 125408-1-5. [18] Y. He, Y. Zhao, Advanced multi-component nanostructures designed by dynamic shadowing growth, Nanoscale 3 (2011) 2361–2375. [19] T.J. Lu, C.G. Levi, H.N.G. Wadley, A.G. Evans, Distributed porosity as a control parameter for oxide thermal barriers made by physical vapor deposition, J. Am. Ceram. Soc. 84 (2001) 2937–2946. [20] U. Schulz, S.G. Terry, C.G. Levi, Microstructure and texture of EB-PVD TBCs grown under different rotation modes, Mat. Sci. Eng. A 360 (2003) 319–329. [21] B.K. Jang, H. Matsubara, Microstructure of nanoporous yttria-stabilized zirconia films fabricated by EB-PVD, J. Eur. Ceram. Soc. 26 (2006) 1585–1590. [22] E.V. Astrova, V.A. Tolmachev, Effective refractive index and composition of oxidized porous silicon films, Mater. Sci. Eng. B 69 (2000) 142–148. [23] C.J. Chen, A. Lien, M.I. Nathan, 4 × 4 and 2 × 2 matrix formulations for the optics in stratified and biaxial media, J. Opt. Soc. Am. A 14 (1997) 3125–3134. [24] Z. Kolska, M. Cernousek, M. Staszek, J. Leitner, V. Svorcik, Study of binary system glycerine-water and its colloidal samples of silver nanoparticles, J. Mol. Liq. 218 (2016) 363–372.