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microfiber hybrid waveguide
Sensors and Actuators B 194 (2014) 142–148
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
All-optical Mach–Zehnder interferometric NH3 gas sensor based on graphene/microfiber hybrid waveguide Baicheng Yao a , Yu Wu a,c,∗ , Yang Cheng a , Anqi Zhang a , Yuan Gong a , Yun-Jiang Rao a,∗∗ , Zegao Wang b , Yuanfu Chen b a Key Laboratory of Optical Fiber Sensing and Communications (Education Ministry of China), University of Electronic Science and Technology of China, Chengdu 610054, PR China b State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China c Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong, China
a r t i c l e
i n f o
Article history: Received 2 September 2013 Received in revised form 19 December 2013 Accepted 21 December 2013 Available online 30 December 2013 Keywords: Optical fiber sensors Graphene Microfiber NH3 gas sensor M–Z interferometer
a b s t r a c t In this paper, we report an all-optical NH3 gas sensor based on graphene/microfiber hybrid waveguide (GMHW). The study on the sensing mechanism shows that as the adsorption of NH3 modifies the conductivity of graphene and thus the effective refractive index of the GMHW, and the transmitting light along the GMHW is very sensitive to NH3 gas concentration. The wavelength shift induced by the NH3 absorption is spectrally demodulated by using a microfiber-based Mach–Zehnder interferometer (MZI). A high sensitivity of ∼6 pm/ppm is obtained for the NH3 adsorption measurement. The resolution of such a sensor is ∼0.3 ppm, mainly limited by the resolution of the optical spectrum analyzer used. The work of this paper may open a window for the development of novel GMHW-based gas sensors with high sensitivity, small footprint, easy fabrication and low cost. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a monolayer of carbon atoms, has attracted worldwide interests for its special electronic and photonic properties [1,2]. In recent years, researches indicate that graphene has great potential for sensing applications due to its high carrier mobility [3,4]. Currently, most of the graphene-based gas sensors employ the voltage or current effect of graphene [5,6], such as utilizing pristine graphene for sensing polar gases (NH3 , NO2 ), doping metal elements for sensing non-polar gases (H2 ) and optimizing the molecular structure of graphene for specific gas detection [7–11]. As all the electric gas sensors are active, a graphene-based passive optical gas sensor is needed for a number of practical applications where the immunity of the sensor to electro-magnetic interference is essential. Based on the optical intensity attenuation effect of graphene, the concentration of chemical gas can be measured [12]. However, the sensitivity and accuracy of the
∗ Corresponding author at: Key Laboratory of Optical Fiber Sensing and Communications (Education Ministry of China), University of Electronic Science and Technology of China, Chengdu 610054, PR China. Tel.: +86 18608039262 ∗∗ Corresponding author. E-mail addresses: [email protected] (Y. Wu), [email protected] (Y.-J. Rao). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.12.085
intensity-based sensors are limited by the sensing principle, while optical interferometric detection would offer much higher sensitivity [13,14]. In this paper, a highly sensitive optical interferometric NH3 sensor based on graphene/microfiber hybrid waveguide (GMHW) has been theoretically analyzed and experimentally demonstrated. For this hybrid structure, a microfiber [15] is used to launch and collect transmission light from the graphene waveguide [16], and light can propagate along the surface of graphene several millimeters. The GMHW integrated in a Mach–Zehnder interferometer (MZI) can detect the gas concentration variation via measuring the spectral fringe shift in the interference spectrum. 2. Structure and fabrication The GMHW-based gas sensor is shown in Fig. 1, schematically. In Fig. 1(a), broadband light is launched into and collected from a MZI with one arm where the GMHW is integrated. In front of the MZI, a polarization controller (PC) is adopted to optimize the polarization state of the lunched light. In the MZI, through a 3 dB coupler, the transmission light is divided into 2 interfering arms. In arm A, a single mode fiber (SMF) is drawn into a microfiber with the diameter of ∼1 m and the waist length of ∼15 cm. Part of the microfiber is attached onto the graphene film. As shown in
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Fig. 1. (a) Setup of the GMHW-MZI for NH3 sensing. (b) Schematic diagram of the GMHW.
Fig. 1(a), the output of the GMHW is also collected by a SMF. The GMHW is sealed in a gas chamber where NH3 gas can be injected. In arm B, light is guided by a SMF with a tunable attenuator for intensity matching. Finally, arms A and B are combined via
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another 3 dB coupler and the interference spectrum is observed through an optical spectrum analyzer (OSA, Model Si 720, Micron Optics, Inc., USA). The lengths of the arms A and B are LA = ∼80 cm and LB = ∼75 cm, respectively. The whole MZI is protected in a temperature-controlled environment and a polarization controller is adapted to pre-optimize the polarization in front of the MZI. The structure of the GMHW is shown schematically in Fig. 1(b). The microfiber is attached onto the graphene film tightly via the Van der Waals and electrostatic forces. The graphene is deposited on a MgF2 substrate (flat in y–z plane). The evanescent waves transmit along the GMHW from one taper of the microfiber to the other at the output port. Here, the length of the microfiber attaching onto the graphene film (LG ) is adjustable. Fig. 2(a) indicates the fabrication process of the GMHW sensor. The graphene film was grown on Cu foils (Alfa Aesar, No. 13382) by chemical vapor deposition (CVD) [17]. Then, PMMA was spin-coated on the surface of graphene/Cu foil and then the underlying Cu foil was etched with 1 M FeCl3 solution. Subsequently, the PMMA/graphene was washed in DI water several times and transferred onto the MgF2 substrate which had been ultrasonically cleaned in sequence by acetone, ethanol and DI water. Then it was allowed to dry at room temperature for 12 h and baked at 180 ◦ C for 10 min. Finally, the PMMA was removed by acetone. The graphene/MgF2 substrate was then fixed on a translation stage and the microfiber was attached onto the graphene. The Raman spectrum of graphene and the scanning electron micrograph (SEM) of the microfiber is shown in Fig. 2(b) and (c), respectively. The G-to2D intensity ratio was ∼0.23.
Fig. 2. (a) Fabrication of the GMHW. (b) Raman spectrum of graphene. (c) SEM of the GMHW.
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Fig. 3. (a) A NH3 molecule is adsorbed by graphene, the cyan dots represent hydrogen atoms while the orange dot represents nitrogen atoms. (b) Fermi level of graphene rises after NH3 adsorption. (c) Percentile conductivity variations with the concentration of NH3 , recalculated from [7]. (d) Relationship between the imaginary part of conductivity and real part of refractive index in graphene. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
3. Sensing principles 3.1. Graphene’s RI change due to NH3 absorption Due to the ultrahigh carrier activity of graphene, it tends to absorb polar molecules effectively [4,18,19]. The schematic view for graphene to absorb a NH3 molecule is shown in Fig. 3(a), as the nitrogen atom could be pentavalent when it is adsorbed by graphene. This can be regarded that 2 “free electrons” of 2 carbon atoms in graphene are captured by the nitrogen atom, then the resistance of graphene would be increased [8,20]. For quantum physics, it means that the Fermi level of graphene increases over the Dirac point, and the interband transition is blocked and the value of the conductivity decreases [21,22]. Fig. 3(b) shows that the NH3 absorption alters the Fermi energy of graphene schematically, here EF,1 is the Fermi energy of graphene in air while EF,2 is the Fermi energy of NH3 adsorbed graphene. Fig. 3(c) recalculates the percentile conductivity changes of pristine graphene in NH3 [7]. It shows that the conductivity of graphene is very sensitive to NH3 . In this work, the NH3 molecules are detected through all-optical way. We calculate the graphene’s refractive index ng with different conductivities via εg = − g,i /ω + i g,r /ω and εg = (ng )2 in Eq. (1) [16]. Here εg and g are the permittivity and the conductivity of graphene, ε0 = 8.85 × 10−12 F/m, = 1 nm is the thickness of graphene. Fig. 3(d) simulates the relationship of Im() and Re(n), and it can be seen that when graphene is exposed in NH3 , its real part of refractive index decreases with the concentration of NH3 . ng =
1 2ω ε0
1/2
−i +
1/2
4r2 + i2
(1)
3.2. Optical measurement of graphene’s refractive index change The alteration of graphene’s refractive index would influence the effective index of the GMHW. By adopting the finite element method (FEM) to multilayer waveguides [23,24], the GMHW’s effective index RI (neff ) varying with ng can be numerically
calculated and represented in Fig. 4(a). In this simulation, the diameter of the microfiber is assumed to be 1 m, the RI of the microfiber, MgF2 and air are 1.45, 1.37 and 1.00, respectively. A larger ng can excite more evanescent fields distributing in the air around graphene (out of the microfiber), as a result the neff becomes smaller. The cross-sectional electric distributions of the fundamental TE mode are simulated with neff = 1.388 and 1.379, as shown in Fig. 4(a). It is clear that ng influences the spatial distribution of the transmitting light. In the sensing process, when the NH3 molecules are adsorbed, ng decreases, so that neff increases. It is known that the neff determines the phase velocity of transmission light. As Fig. 1 shows, in the gas chamber, when the GMHW is surrounding by air, its phase velocity v1 = c/neff,1 , for certain resonant dip of the MZI, its spectral location d is shown in Eq. (2). Here c is the light velocity in vacuum, LA is the length of arm A, LB is the length of arm B, LG is the length of the GMHW, vf is the light velocity in fibers, N is the order number. When NH3 interact with the GMHW, the phase velocity on it becomes v2 = c/neff,2 , thus for the same N, the location d changes. The value of the spectral shift is expressed in Eq. (3), here neff = neff,2 - neff,1 , and LA > LB . Fig. 4(b) illustrates the relationship between , LG and neff numerically. It is noteworthy that in the experiment, the spectral shift direction is determined both by the sign of LA –LB and neff . Fig. 4(c) provides a calculated correlation between the NH3 gas concentration and the spectral shift for the GMHW with LG = 5 mm. However, in experiment, the influence of the microfiber should be also considered. 2 c d =
LA − LG
vf
+
LG
v1
−
LB
vf
= (2N + 1)
−2LG neff 2N + 1
(2)
(3)
4. Experimental results and discussion To evaluate the ability of the attached graphene for enhancing the sensitivity of the microfiber for NH3 detection, Fig. 5(a) and
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Fig. 4. (a) Effective refractive index of the GMHW is influenced by the graphene’s refractive index. Inset: the field distribution in neff = 1.388 (upper) and neff = 1.379 (under). (b) The spectral shift is determined by the alteration of neff and LG . The inset is the top view. (c) The simulated correlation between the NH3 concentration and the spectral shift.
(b) displays the spectral shifts of the microfiber on the MgF2 substrate and GMHW in NH3 with concentrations ranging from 0 ppm to 360 ppm, respectively. Here, the microfiber length is 20 cm and the contacting length on graphene is 5 mm, and the MgF2 substrate does not influence the evanescent light in NH3 [12]. According to Fig. 5, it is clear that NH3 causes both additional loss and phase modulation along the microfiber, and graphene can enhance these effects obviously. Fig. 6(a) shows the correlation between spectral shift and concentration change of NH3 . For the microfiber solely, as NH3 alters the surrounding RI, the interfering spectrum shifts 20 pm, 70 pm, 180 pm and 300 pm. When the concentration is below ∼40 ppm, it is very hard to detect due to the limit of the OSA’s resolution (∼1 pm) in the experimental setup. However, as NH3 adsorption on graphene enhances the sensitivity significantly,
the spectral shifts of the GMHW in cycles are 140 pm, 360 pm, 550 pm and 670 pm. By using the same setup, even the concentration is as low as ∼0.3 ppm, it is still detectable by using the GMHW. As the slope of the red curve is much larger than that of the blue curve when NH3 concentration is very low, the GMHW shows much more sensitive than the microfiber for gas sensing. Comparing with the experimental results between the microfiber (5 mm attached on MgF2 ) and the GMHW, Fig. 6(b) demonstrates the time-dependent spectral responses of the microfiber and GMHW to NH3 gas cycled with concentrations from 40 to 360 ppm. Experimental results indicate that good reversibility of the GMHW sensor is obtained. The recovery ratio in each cycle is ∼90%, as the remaining NH3 in the chamber could not be expelled completely within a short time.
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Initially, IA and IB are tuned almost equal to acquire the largest ER (∼12 dB in experiment). When IA is attenuated by NH3 , the ER becomes smaller, and when NH3 is removed, the ER is recovered. In Fig. 6(c), the response time is only ∼0.4 s (baseline to 90% signal recovery), due to the fact that the evanescent waves around the microfiber can response without delay to the alteration of neff [25]. Hence, the physical interaction between the pristine graphene and NH3 is fast. ER =
Fig. 5. (a) Spectra of the microfiber on MgF2 and (b) spectra of GMHW with LG = 5 mm, with different NH3 concentrations. Blue: 0 ppm (in dry air), red: 40 ppm, green: 120 ppm, purple: 240 ppm and cyan: 360 ppm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 6(c) reveals the response time of the GMHW sensor by observing the time dependent extinction ratio (ER). As Fig. 5 shows, the gas adsorption not only changes the spectrum of the MZI, but also decreases the ER of the MZI, here ER is written as Eq. (4) [15].
Imax IA + IB = Imin IA − IB
(4)
Furthermore, the sensing range of the GMHW sensor has been investigated in this experiment, as shown in Fig. 6(d). After 360 ppm NH3 interacted, the ER of the MZI based on the microfiber solely and the GMHW decreases from 8 dB to 1.7 dB, and from 12 dB to 1.8 dB, respectively. The attenuations of the microfiber and the GMHW are found to be ∼5.4 dB and ∼6.1 dB, respectively. When the NH3 concentration increases to above 240 ppm, the ER attenuates to be almost constant ∼2 dB, this GMHW-based sensor keep a high sensitivity when the gas concentration <240 ppm. Moreover, we also investigated how the performance of the GMHW sensor is influenced by LG . Eq. (3) shows that the sensitivity increase linearly with LG in theory, in fact, graphene induced optical attenuation for evanescent light is very serious. When LG is too long, the evanescent light modulated by NH3 is too weak to be detected, so that the sensitivity of the GMHW sensor would decrease. In this experiment, the whole spectral shift of the GMHW can be described in Eq. (5) with considering the sensitivity of the microfiber. Here, ˛ is the attenuation coefficient of transmission light along the GMHW, depending on the concentration of NH3 . LMF = 15 cm is the length of microfiber. Fig. 7 shows LG – correlation in NH3 with concentration of 40 ppm and 240 ppm experimentally. The blue and red dashed curves present the numerical fitting based on Eq. (5), while the blue cubes and the red circles present the experimental results. It is observed that when LG is larger than 12 mm in 240 ppm NH3 or 16 mm in 40 ppm NH3 , it is hard to acquire any resonance
Fig. 6. Sensing performances of the GMHW (red) and the microfiber on MgF2 without graphene attached (blue). (a) The correlation between spectral shifts and concentrations. (b) The spectral responses to NH3 gas cycled with concentrations from 40 ppm to 360 ppm. (c) Time dependent extinction ratios when the concentration of NH3 changes from 360 ppm to 0 ppm: the response time is ∼0.5 s. (d) The extinction ratio of the MZI is modulated by the concentration. (For interpretation of the references to color in the text, the reader is referred to the web version of the article.)
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Fig. 7. LG – correlation in NH3 with concentrations of 40 ppm (blue cubes) and 240 ppm (red circles), the dashed curves are the numerical fittings for the experimental results. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
in the output spectrum due to the attenuation. In our experiment, the best resolution of ∼0.3 ppm is obtained by setting LG = ∼3 mm. In fact, the microfiber with a larger diameter may offer a longer LG to get a larger ||. However, while the diameter of the microfiber increases, there is small proportion of evanescent wave transmitting outside the fiber core, the sensitivity to NH3 would be lower. Thus, LG and microfiber’s diameter should be optimized simultaneously to improve the performance of the GMHW sensor. || =
2LMF − 2LG 2LG neff,MF + exp(−˛LG ) neff,GNHW 2N + 1 2N + 1
(5)
5. Conclusions In conclusion, a highly sensitive all-optical interferometric NH3 sensor based on the graphene/microfiber hybrid waveguide is proposed and demonstrated. The NH3 adsorption on graphene has a strong influence on the effective refractive index of the GMHW, resulting in the spectral changes of the interferometric phase signal of an all-optical microfiber-based MZI. Highly sensitive detection of the NH3 concentration with a resolution of 0.3 ppm is achieved. Moreover, the sensor sensitivity is tunable by adjusting the contacting length between the microfiber and graphene film. Such a graphene-based optical chemical sensor with high sensitivity, fast response, small footprint, tunable ability, easy fabrication and chemical stability would have great potential to create a number of novel optical fiber sensors for gas sensing. Acknowledgements This work was supported by National Natural Science Foundation of China (NSFC) under Grant 61290312, 61107072, 61107073 and the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT), as well as the Fundamental Research Funds for the Central Universities of China (ZYGX2010J005). References [1] A.K. Geim, K.S. Noveselov, The rise of graphene, Nat. Mater. 6 (2007) 183–191. [2] F. Bonaccorso, Z. Sun, T. Hasan, A.C. Ferrari, Graphene photonics and optoelectronics, Nat. Photonics 4 (2010) 611–622. [3] N.M. Gabor, J.C.W. Song, Q. Ma, N.L. Nair, T. Taychatanapat, K. Watanabe, T. Taniguchi, L.S. Levitov, P.J. Herrero, Hot carrier-assisted intrinsic photoresponse in graphene, Science 334 (2011) 648–652. [4] O. Leenaerts, B. Partoens, F.M. Peeters, Adsorption of H2 O, NH3 , CO, NO2 , and NO on graphene: a first-principles study, Phys. Rev. B 77 (2008) 125416. [5] F. Yavari, N. Koratkar, Graphene-based chemical sensors, J. Phys. Chem. Lett. 3 (2012) 1746–1753. [6] S. Basua, P. Bhattacharyya, Recent developments on graphene and graphene oxide based solid state gas sensors, Sens. Actuators B 173 (2012) 1–21.
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Biographies Baicheng Yao is currently working toward the PhD degree for optical engineering at the Key Lab of Optical Fiber Sensing & Communications (Education Ministry of China), University of Electronic Science & Technology of China, Chengdu, PR China. He received bachelor degree in telecommunication engineering at 2011, University of Electronic Science & Technology of China, Chengdu 610054, PR China. His current research interests are graphene-based optical fiber devices, including graphenemicrofiber structures and graphene waveguide sensors. Yu Wu received his BA from the University of Electronic Science and Technology of China, Sichuan, China, in 2003. He received his PhD degree in measurement technology and instruments from the Zhejiang University, in 2008. His major research interests are photonics devices and microfiber sensors. He is currently an associate professor in the Key Lab of Optical Fiber Sensing & Communications (Education Ministry of China), University of Electronic Science and Technology of China, Sichuan, China. Now, he is also the research associate in the Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong, China. Yang Cheng is now working toward the master’s degree for optical engineering at the Key Lab of Optical Fiber Sensing & Communications (Education Ministry of China), University of Electronic Science & Technology of China, Chengdu, PR China. He received bachelor degree in Information Engineering at 2011, Huazhong Normal University, Wuhan, PR China. His research interests are graphene based micro structural optical sensing system. Anqi Zhang is pursuing her master’s degree for optical engineering at the Key Lab of Optical Fiber Sensing & Communications (Education Ministry of China), University of Electronic Science & Technology of China, Chengdu, PR China. She received her BA in UESTC at 2012. Yuan Gong received his PhD degree from the Institute of Optics And Electronics, The Chinese Academy of Sciences, Sichuan, China, in 2008. His major research interests
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are optical fiber sensors and fiber sensing system. He is currently the associate professor in the Key Lab of Optical Fiber Sensing & Communications (Education Ministry of China), University of Electronic Science and Technology of China, Sichuan, China.
Journal of Lightwave Technology and Optics & Laser Technology (Elsevier). He is also the founder and editor-in-chief of Springer journal – Photonic Sensors. His major research interests are optical fiber sensing and communication.
Yun-Jiang Rao was a research fellow/senior research fellow from 1992 to 1999 at the University of Kent at Canterbury, UK. From 1999 to 2004, he was the ChangJiang Chair Professor in optical engineering and head of the Optical Fiber Technology Group at Chongqing University, China. Since 2005, he has been the dean of School of Communication and Information Engineering, and director of the Key Lab of Optical Fiber Sensing & Communications (Ministry of Education), University of Electronic Science and Technology of China, as well as Chang-Jiang Chair Professor in Optical Engineering. Due to his outstanding contributions to Fiber Optics, he won the “Wang-Da-Heng Optics Award” of the Optical Society of China and the National Achievement Award for Returned Overseas Scientists in China. He has published more than 200 papers in international journals and conferences as well as several book chapters and two books (in Chinese). His H-index is 25. He is a senior member of OSA and SPIE. He serves as a TPC member of International Conferences on Optical Fiber Sensors (OFS) and the TPC Co-Chair of OFS-22. He is the founder of Asia-Pacific Optical Sensors Conferences (APOS). He is an associate editor of IEEE/OSA
Zegao Wang is currently working toward the PhD degree for materials science and engineering. He received his bachelor’s degree in solid state electronics technology, University of Electronic Science & Technology of China, Chengdu, PR China. His research interests are the chemical/electrical properties about graphene and graphene fabrications. Yuanfu Chen received his PhD degree in physics at Sichuan University, Chengdu, PR China, and work as post doctor in Institute of Physics, Chinese Academy of Sciences (CAS) and Tsing Hua University, Taiwan from July 2001 to February 2005. He worked as a researcher in Leibniz Institute of Solid Materials, University of Leipzig, Germany and Erlangen-Nuremberg University, Germany from March 2005 to September 2009. He has been a professor in the State Key Lab of electronic thin films and integrated devices, University of Electronic Science & Technology of China, PR China since 2005. His research interests are on Solid film materials, including the fabrication and fundamental researches about graphene.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
All-optical Mach–Zehnder interferometric NH3 gas sensor based on graphene/microfiber hybrid waveguide Baicheng Yao a , Yu Wu a,c,∗ , Yang Cheng a , Anqi Zhang a , Yuan Gong a , Yun-Jiang Rao a,∗∗ , Zegao Wang b , Yuanfu Chen b a Key Laboratory of Optical Fiber Sensing and Communications (Education Ministry of China), University of Electronic Science and Technology of China, Chengdu 610054, PR China b State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China c Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong, China
a r t i c l e
i n f o
Article history: Received 2 September 2013 Received in revised form 19 December 2013 Accepted 21 December 2013 Available online 30 December 2013 Keywords: Optical fiber sensors Graphene Microfiber NH3 gas sensor M–Z interferometer
a b s t r a c t In this paper, we report an all-optical NH3 gas sensor based on graphene/microfiber hybrid waveguide (GMHW). The study on the sensing mechanism shows that as the adsorption of NH3 modifies the conductivity of graphene and thus the effective refractive index of the GMHW, and the transmitting light along the GMHW is very sensitive to NH3 gas concentration. The wavelength shift induced by the NH3 absorption is spectrally demodulated by using a microfiber-based Mach–Zehnder interferometer (MZI). A high sensitivity of ∼6 pm/ppm is obtained for the NH3 adsorption measurement. The resolution of such a sensor is ∼0.3 ppm, mainly limited by the resolution of the optical spectrum analyzer used. The work of this paper may open a window for the development of novel GMHW-based gas sensors with high sensitivity, small footprint, easy fabrication and low cost. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a monolayer of carbon atoms, has attracted worldwide interests for its special electronic and photonic properties [1,2]. In recent years, researches indicate that graphene has great potential for sensing applications due to its high carrier mobility [3,4]. Currently, most of the graphene-based gas sensors employ the voltage or current effect of graphene [5,6], such as utilizing pristine graphene for sensing polar gases (NH3 , NO2 ), doping metal elements for sensing non-polar gases (H2 ) and optimizing the molecular structure of graphene for specific gas detection [7–11]. As all the electric gas sensors are active, a graphene-based passive optical gas sensor is needed for a number of practical applications where the immunity of the sensor to electro-magnetic interference is essential. Based on the optical intensity attenuation effect of graphene, the concentration of chemical gas can be measured [12]. However, the sensitivity and accuracy of the
∗ Corresponding author at: Key Laboratory of Optical Fiber Sensing and Communications (Education Ministry of China), University of Electronic Science and Technology of China, Chengdu 610054, PR China. Tel.: +86 18608039262 ∗∗ Corresponding author. E-mail addresses: [email protected] (Y. Wu), [email protected] (Y.-J. Rao). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.12.085
intensity-based sensors are limited by the sensing principle, while optical interferometric detection would offer much higher sensitivity [13,14]. In this paper, a highly sensitive optical interferometric NH3 sensor based on graphene/microfiber hybrid waveguide (GMHW) has been theoretically analyzed and experimentally demonstrated. For this hybrid structure, a microfiber [15] is used to launch and collect transmission light from the graphene waveguide [16], and light can propagate along the surface of graphene several millimeters. The GMHW integrated in a Mach–Zehnder interferometer (MZI) can detect the gas concentration variation via measuring the spectral fringe shift in the interference spectrum. 2. Structure and fabrication The GMHW-based gas sensor is shown in Fig. 1, schematically. In Fig. 1(a), broadband light is launched into and collected from a MZI with one arm where the GMHW is integrated. In front of the MZI, a polarization controller (PC) is adopted to optimize the polarization state of the lunched light. In the MZI, through a 3 dB coupler, the transmission light is divided into 2 interfering arms. In arm A, a single mode fiber (SMF) is drawn into a microfiber with the diameter of ∼1 m and the waist length of ∼15 cm. Part of the microfiber is attached onto the graphene film. As shown in
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Fig. 1. (a) Setup of the GMHW-MZI for NH3 sensing. (b) Schematic diagram of the GMHW.
Fig. 1(a), the output of the GMHW is also collected by a SMF. The GMHW is sealed in a gas chamber where NH3 gas can be injected. In arm B, light is guided by a SMF with a tunable attenuator for intensity matching. Finally, arms A and B are combined via
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another 3 dB coupler and the interference spectrum is observed through an optical spectrum analyzer (OSA, Model Si 720, Micron Optics, Inc., USA). The lengths of the arms A and B are LA = ∼80 cm and LB = ∼75 cm, respectively. The whole MZI is protected in a temperature-controlled environment and a polarization controller is adapted to pre-optimize the polarization in front of the MZI. The structure of the GMHW is shown schematically in Fig. 1(b). The microfiber is attached onto the graphene film tightly via the Van der Waals and electrostatic forces. The graphene is deposited on a MgF2 substrate (flat in y–z plane). The evanescent waves transmit along the GMHW from one taper of the microfiber to the other at the output port. Here, the length of the microfiber attaching onto the graphene film (LG ) is adjustable. Fig. 2(a) indicates the fabrication process of the GMHW sensor. The graphene film was grown on Cu foils (Alfa Aesar, No. 13382) by chemical vapor deposition (CVD) [17]. Then, PMMA was spin-coated on the surface of graphene/Cu foil and then the underlying Cu foil was etched with 1 M FeCl3 solution. Subsequently, the PMMA/graphene was washed in DI water several times and transferred onto the MgF2 substrate which had been ultrasonically cleaned in sequence by acetone, ethanol and DI water. Then it was allowed to dry at room temperature for 12 h and baked at 180 ◦ C for 10 min. Finally, the PMMA was removed by acetone. The graphene/MgF2 substrate was then fixed on a translation stage and the microfiber was attached onto the graphene. The Raman spectrum of graphene and the scanning electron micrograph (SEM) of the microfiber is shown in Fig. 2(b) and (c), respectively. The G-to2D intensity ratio was ∼0.23.
Fig. 2. (a) Fabrication of the GMHW. (b) Raman spectrum of graphene. (c) SEM of the GMHW.
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Fig. 3. (a) A NH3 molecule is adsorbed by graphene, the cyan dots represent hydrogen atoms while the orange dot represents nitrogen atoms. (b) Fermi level of graphene rises after NH3 adsorption. (c) Percentile conductivity variations with the concentration of NH3 , recalculated from [7]. (d) Relationship between the imaginary part of conductivity and real part of refractive index in graphene. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
3. Sensing principles 3.1. Graphene’s RI change due to NH3 absorption Due to the ultrahigh carrier activity of graphene, it tends to absorb polar molecules effectively [4,18,19]. The schematic view for graphene to absorb a NH3 molecule is shown in Fig. 3(a), as the nitrogen atom could be pentavalent when it is adsorbed by graphene. This can be regarded that 2 “free electrons” of 2 carbon atoms in graphene are captured by the nitrogen atom, then the resistance of graphene would be increased [8,20]. For quantum physics, it means that the Fermi level of graphene increases over the Dirac point, and the interband transition is blocked and the value of the conductivity decreases [21,22]. Fig. 3(b) shows that the NH3 absorption alters the Fermi energy of graphene schematically, here EF,1 is the Fermi energy of graphene in air while EF,2 is the Fermi energy of NH3 adsorbed graphene. Fig. 3(c) recalculates the percentile conductivity changes of pristine graphene in NH3 [7]. It shows that the conductivity of graphene is very sensitive to NH3 . In this work, the NH3 molecules are detected through all-optical way. We calculate the graphene’s refractive index ng with different conductivities via εg = − g,i /ω + i g,r /ω and εg = (ng )2 in Eq. (1) [16]. Here εg and g are the permittivity and the conductivity of graphene, ε0 = 8.85 × 10−12 F/m, = 1 nm is the thickness of graphene. Fig. 3(d) simulates the relationship of Im() and Re(n), and it can be seen that when graphene is exposed in NH3 , its real part of refractive index decreases with the concentration of NH3 . ng =
1 2ω ε0
1/2
−i +
1/2
4r2 + i2
(1)
3.2. Optical measurement of graphene’s refractive index change The alteration of graphene’s refractive index would influence the effective index of the GMHW. By adopting the finite element method (FEM) to multilayer waveguides [23,24], the GMHW’s effective index RI (neff ) varying with ng can be numerically
calculated and represented in Fig. 4(a). In this simulation, the diameter of the microfiber is assumed to be 1 m, the RI of the microfiber, MgF2 and air are 1.45, 1.37 and 1.00, respectively. A larger ng can excite more evanescent fields distributing in the air around graphene (out of the microfiber), as a result the neff becomes smaller. The cross-sectional electric distributions of the fundamental TE mode are simulated with neff = 1.388 and 1.379, as shown in Fig. 4(a). It is clear that ng influences the spatial distribution of the transmitting light. In the sensing process, when the NH3 molecules are adsorbed, ng decreases, so that neff increases. It is known that the neff determines the phase velocity of transmission light. As Fig. 1 shows, in the gas chamber, when the GMHW is surrounding by air, its phase velocity v1 = c/neff,1 , for certain resonant dip of the MZI, its spectral location d is shown in Eq. (2). Here c is the light velocity in vacuum, LA is the length of arm A, LB is the length of arm B, LG is the length of the GMHW, vf is the light velocity in fibers, N is the order number. When NH3 interact with the GMHW, the phase velocity on it becomes v2 = c/neff,2 , thus for the same N, the location d changes. The value of the spectral shift is expressed in Eq. (3), here neff = neff,2 - neff,1 , and LA > LB . Fig. 4(b) illustrates the relationship between , LG and neff numerically. It is noteworthy that in the experiment, the spectral shift direction is determined both by the sign of LA –LB and neff . Fig. 4(c) provides a calculated correlation between the NH3 gas concentration and the spectral shift for the GMHW with LG = 5 mm. However, in experiment, the influence of the microfiber should be also considered. 2 c d =
LA − LG
vf
+
LG
v1
−
LB
vf
= (2N + 1)
−2LG neff 2N + 1
(2)
(3)
4. Experimental results and discussion To evaluate the ability of the attached graphene for enhancing the sensitivity of the microfiber for NH3 detection, Fig. 5(a) and
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Fig. 4. (a) Effective refractive index of the GMHW is influenced by the graphene’s refractive index. Inset: the field distribution in neff = 1.388 (upper) and neff = 1.379 (under). (b) The spectral shift is determined by the alteration of neff and LG . The inset is the top view. (c) The simulated correlation between the NH3 concentration and the spectral shift.
(b) displays the spectral shifts of the microfiber on the MgF2 substrate and GMHW in NH3 with concentrations ranging from 0 ppm to 360 ppm, respectively. Here, the microfiber length is 20 cm and the contacting length on graphene is 5 mm, and the MgF2 substrate does not influence the evanescent light in NH3 [12]. According to Fig. 5, it is clear that NH3 causes both additional loss and phase modulation along the microfiber, and graphene can enhance these effects obviously. Fig. 6(a) shows the correlation between spectral shift and concentration change of NH3 . For the microfiber solely, as NH3 alters the surrounding RI, the interfering spectrum shifts 20 pm, 70 pm, 180 pm and 300 pm. When the concentration is below ∼40 ppm, it is very hard to detect due to the limit of the OSA’s resolution (∼1 pm) in the experimental setup. However, as NH3 adsorption on graphene enhances the sensitivity significantly,
the spectral shifts of the GMHW in cycles are 140 pm, 360 pm, 550 pm and 670 pm. By using the same setup, even the concentration is as low as ∼0.3 ppm, it is still detectable by using the GMHW. As the slope of the red curve is much larger than that of the blue curve when NH3 concentration is very low, the GMHW shows much more sensitive than the microfiber for gas sensing. Comparing with the experimental results between the microfiber (5 mm attached on MgF2 ) and the GMHW, Fig. 6(b) demonstrates the time-dependent spectral responses of the microfiber and GMHW to NH3 gas cycled with concentrations from 40 to 360 ppm. Experimental results indicate that good reversibility of the GMHW sensor is obtained. The recovery ratio in each cycle is ∼90%, as the remaining NH3 in the chamber could not be expelled completely within a short time.
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Initially, IA and IB are tuned almost equal to acquire the largest ER (∼12 dB in experiment). When IA is attenuated by NH3 , the ER becomes smaller, and when NH3 is removed, the ER is recovered. In Fig. 6(c), the response time is only ∼0.4 s (baseline to 90% signal recovery), due to the fact that the evanescent waves around the microfiber can response without delay to the alteration of neff [25]. Hence, the physical interaction between the pristine graphene and NH3 is fast. ER =
Fig. 5. (a) Spectra of the microfiber on MgF2 and (b) spectra of GMHW with LG = 5 mm, with different NH3 concentrations. Blue: 0 ppm (in dry air), red: 40 ppm, green: 120 ppm, purple: 240 ppm and cyan: 360 ppm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 6(c) reveals the response time of the GMHW sensor by observing the time dependent extinction ratio (ER). As Fig. 5 shows, the gas adsorption not only changes the spectrum of the MZI, but also decreases the ER of the MZI, here ER is written as Eq. (4) [15].
Imax IA + IB = Imin IA − IB
(4)
Furthermore, the sensing range of the GMHW sensor has been investigated in this experiment, as shown in Fig. 6(d). After 360 ppm NH3 interacted, the ER of the MZI based on the microfiber solely and the GMHW decreases from 8 dB to 1.7 dB, and from 12 dB to 1.8 dB, respectively. The attenuations of the microfiber and the GMHW are found to be ∼5.4 dB and ∼6.1 dB, respectively. When the NH3 concentration increases to above 240 ppm, the ER attenuates to be almost constant ∼2 dB, this GMHW-based sensor keep a high sensitivity when the gas concentration <240 ppm. Moreover, we also investigated how the performance of the GMHW sensor is influenced by LG . Eq. (3) shows that the sensitivity increase linearly with LG in theory, in fact, graphene induced optical attenuation for evanescent light is very serious. When LG is too long, the evanescent light modulated by NH3 is too weak to be detected, so that the sensitivity of the GMHW sensor would decrease. In this experiment, the whole spectral shift of the GMHW can be described in Eq. (5) with considering the sensitivity of the microfiber. Here, ˛ is the attenuation coefficient of transmission light along the GMHW, depending on the concentration of NH3 . LMF = 15 cm is the length of microfiber. Fig. 7 shows LG – correlation in NH3 with concentration of 40 ppm and 240 ppm experimentally. The blue and red dashed curves present the numerical fitting based on Eq. (5), while the blue cubes and the red circles present the experimental results. It is observed that when LG is larger than 12 mm in 240 ppm NH3 or 16 mm in 40 ppm NH3 , it is hard to acquire any resonance
Fig. 6. Sensing performances of the GMHW (red) and the microfiber on MgF2 without graphene attached (blue). (a) The correlation between spectral shifts and concentrations. (b) The spectral responses to NH3 gas cycled with concentrations from 40 ppm to 360 ppm. (c) Time dependent extinction ratios when the concentration of NH3 changes from 360 ppm to 0 ppm: the response time is ∼0.5 s. (d) The extinction ratio of the MZI is modulated by the concentration. (For interpretation of the references to color in the text, the reader is referred to the web version of the article.)
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Fig. 7. LG – correlation in NH3 with concentrations of 40 ppm (blue cubes) and 240 ppm (red circles), the dashed curves are the numerical fittings for the experimental results. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
in the output spectrum due to the attenuation. In our experiment, the best resolution of ∼0.3 ppm is obtained by setting LG = ∼3 mm. In fact, the microfiber with a larger diameter may offer a longer LG to get a larger ||. However, while the diameter of the microfiber increases, there is small proportion of evanescent wave transmitting outside the fiber core, the sensitivity to NH3 would be lower. Thus, LG and microfiber’s diameter should be optimized simultaneously to improve the performance of the GMHW sensor. || =
2LMF − 2LG 2LG neff,MF + exp(−˛LG ) neff,GNHW 2N + 1 2N + 1
(5)
5. Conclusions In conclusion, a highly sensitive all-optical interferometric NH3 sensor based on the graphene/microfiber hybrid waveguide is proposed and demonstrated. The NH3 adsorption on graphene has a strong influence on the effective refractive index of the GMHW, resulting in the spectral changes of the interferometric phase signal of an all-optical microfiber-based MZI. Highly sensitive detection of the NH3 concentration with a resolution of 0.3 ppm is achieved. Moreover, the sensor sensitivity is tunable by adjusting the contacting length between the microfiber and graphene film. Such a graphene-based optical chemical sensor with high sensitivity, fast response, small footprint, tunable ability, easy fabrication and chemical stability would have great potential to create a number of novel optical fiber sensors for gas sensing. Acknowledgements This work was supported by National Natural Science Foundation of China (NSFC) under Grant 61290312, 61107072, 61107073 and the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT), as well as the Fundamental Research Funds for the Central Universities of China (ZYGX2010J005). References [1] A.K. Geim, K.S. Noveselov, The rise of graphene, Nat. Mater. 6 (2007) 183–191. [2] F. Bonaccorso, Z. Sun, T. Hasan, A.C. Ferrari, Graphene photonics and optoelectronics, Nat. Photonics 4 (2010) 611–622. [3] N.M. Gabor, J.C.W. Song, Q. Ma, N.L. Nair, T. Taychatanapat, K. Watanabe, T. Taniguchi, L.S. Levitov, P.J. Herrero, Hot carrier-assisted intrinsic photoresponse in graphene, Science 334 (2011) 648–652. [4] O. Leenaerts, B. Partoens, F.M. Peeters, Adsorption of H2 O, NH3 , CO, NO2 , and NO on graphene: a first-principles study, Phys. Rev. B 77 (2008) 125416. [5] F. Yavari, N. Koratkar, Graphene-based chemical sensors, J. Phys. Chem. Lett. 3 (2012) 1746–1753. [6] S. Basua, P. Bhattacharyya, Recent developments on graphene and graphene oxide based solid state gas sensors, Sens. Actuators B 173 (2012) 1–21.
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Biographies Baicheng Yao is currently working toward the PhD degree for optical engineering at the Key Lab of Optical Fiber Sensing & Communications (Education Ministry of China), University of Electronic Science & Technology of China, Chengdu, PR China. He received bachelor degree in telecommunication engineering at 2011, University of Electronic Science & Technology of China, Chengdu 610054, PR China. His current research interests are graphene-based optical fiber devices, including graphenemicrofiber structures and graphene waveguide sensors. Yu Wu received his BA from the University of Electronic Science and Technology of China, Sichuan, China, in 2003. He received his PhD degree in measurement technology and instruments from the Zhejiang University, in 2008. His major research interests are photonics devices and microfiber sensors. He is currently an associate professor in the Key Lab of Optical Fiber Sensing & Communications (Education Ministry of China), University of Electronic Science and Technology of China, Sichuan, China. Now, he is also the research associate in the Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong, China. Yang Cheng is now working toward the master’s degree for optical engineering at the Key Lab of Optical Fiber Sensing & Communications (Education Ministry of China), University of Electronic Science & Technology of China, Chengdu, PR China. He received bachelor degree in Information Engineering at 2011, Huazhong Normal University, Wuhan, PR China. His research interests are graphene based micro structural optical sensing system. Anqi Zhang is pursuing her master’s degree for optical engineering at the Key Lab of Optical Fiber Sensing & Communications (Education Ministry of China), University of Electronic Science & Technology of China, Chengdu, PR China. She received her BA in UESTC at 2012. Yuan Gong received his PhD degree from the Institute of Optics And Electronics, The Chinese Academy of Sciences, Sichuan, China, in 2008. His major research interests
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are optical fiber sensors and fiber sensing system. He is currently the associate professor in the Key Lab of Optical Fiber Sensing & Communications (Education Ministry of China), University of Electronic Science and Technology of China, Sichuan, China.
Journal of Lightwave Technology and Optics & Laser Technology (Elsevier). He is also the founder and editor-in-chief of Springer journal – Photonic Sensors. His major research interests are optical fiber sensing and communication.
Yun-Jiang Rao was a research fellow/senior research fellow from 1992 to 1999 at the University of Kent at Canterbury, UK. From 1999 to 2004, he was the ChangJiang Chair Professor in optical engineering and head of the Optical Fiber Technology Group at Chongqing University, China. Since 2005, he has been the dean of School of Communication and Information Engineering, and director of the Key Lab of Optical Fiber Sensing & Communications (Ministry of Education), University of Electronic Science and Technology of China, as well as Chang-Jiang Chair Professor in Optical Engineering. Due to his outstanding contributions to Fiber Optics, he won the “Wang-Da-Heng Optics Award” of the Optical Society of China and the National Achievement Award for Returned Overseas Scientists in China. He has published more than 200 papers in international journals and conferences as well as several book chapters and two books (in Chinese). His H-index is 25. He is a senior member of OSA and SPIE. He serves as a TPC member of International Conferences on Optical Fiber Sensors (OFS) and the TPC Co-Chair of OFS-22. He is the founder of Asia-Pacific Optical Sensors Conferences (APOS). He is an associate editor of IEEE/OSA
Zegao Wang is currently working toward the PhD degree for materials science and engineering. He received his bachelor’s degree in solid state electronics technology, University of Electronic Science & Technology of China, Chengdu, PR China. His research interests are the chemical/electrical properties about graphene and graphene fabrications. Yuanfu Chen received his PhD degree in physics at Sichuan University, Chengdu, PR China, and work as post doctor in Institute of Physics, Chinese Academy of Sciences (CAS) and Tsing Hua University, Taiwan from July 2001 to February 2005. He worked as a researcher in Leibniz Institute of Solid Materials, University of Leipzig, Germany and Erlangen-Nuremberg University, Germany from March 2005 to September 2009. He has been a professor in the State Key Lab of electronic thin films and integrated devices, University of Electronic Science & Technology of China, PR China since 2005. His research interests are on Solid film materials, including the fabrication and fundamental researches about graphene.