A Novel Refractometric Sensor with High Sensitivity Using Nanofibers

Biqing Ye Department of Science

Zhijiang College of Zhejiang University of Technology Hangzhou, China

Pinghui Wu, Chenghua Sui Department of Applied Physics

Zhejiang University of Technology Hangzhou, China

Abstract—In a nanofiber a considerable amount of the energy of the guided light is in form of evanescent waves. These waves are very sensitive to changes occurring in the external environment which makes nanofibers ideal for the development of highly-sensitive sensors. Based on theoretical modelling, here a highly-sensitive refractometric sensor employing a nanofiber-assembled Mach-Zehnder structure is suggested and investigated. The sensor is used to measure the refractive indices of isopropyl alcohol (IPA) solutions of different concentrations. Phase change of the guided mode caused by index change is obtained by solving Maxwell’s equation. In addition, the important parameters, including sensitivity and detection limit, are also estimated. The results show that the refractometric sensor exhibits the capability of detecting an index variation of 10-6. Our simulations are helpful for studying and developing new miniaturized optical sensors with high sensitivity.

Keywords-silica nanofiber; refractive index; evanescent wave; Mach-Zehnder interferometer; sensitivity

I. INTRODUCTION In the past years, nanoscience and nanotechnology have

attracted intensive attention since their appearance. It has been believed that nanoscience and nanotechnology have great potentials of application in various fields; nanobiomedicine is one of the important and promising branches and being developed rapidly as problems related to health, safety, and the environment. For the development of biomedical application, it is essential to make compact and sensitive sensors to detect a very small volume of a specific chemical or biomedical species. Among them, accurate measurement of liquid refractive index has acquired great practical significance. For instance, in biomedical enterprises it is often necessary to monitor the concentration of medical solution and this can be done by checking the refractive index of the solution during preparation. Highly concentrated solution implies high refractive index of the medium. Similarly in other food processing industries it is required to monitor refractive index of various solutions as it conveys important information to the manufacturer.

Recently, there has been interest in the fabrication of optical fibers with micrometer- and nanometer-order diameters [1-6]. Amongst the proposed techniques, using a simple, high-temperature taper-drawing technique, these nanofibers show excellent diameter uniformity and surface smoothness for low-

loss optical wave guiding. It has been demonstrated that such fibers have interesting properties such as high power density at the fiber surface and cylindrical asymmetry in the field distribution [7], large penetration length of the evanescent wave [8]. Owing to their excellent physical and chemical properties, these nanofibers show great promise for nanophotonic, biomedical and mechanical components and tools [9-11].

However, to the best of our knowledge, so far the use of nanofiber sensor for detection of environment refractive index change has not been fully explored. In this paper, the possibility to exploit a nanofiber sensor for measuring liquid refractive index is investigated. A prototype of a nanofiber-guiding structure with a Mach-Zehnder interferometry detection system is modeled. Phase change of the guided mode caused by index change of aqueous solution is obtained. The system is used to measure the refractive index of isopropyl alcohol (IPA) solutions of different concentrations, exhibiting the capability of detecting very small index variations of ambient medium. Based on the results obtained here, we believe that the model presented here is promising for developing high-sensitivity refractometric sensors of significantly small sized.

II. PROPERTIES OF SINGLE MODE NANOFIBERS Unlike the case of optical fibers with diameter larger than

the wavelength, many properties of nanofibers have not been adequately investigated. In such a thin fiber, the original silica core is vanishing. Therefore, the original silica-clad acts like a core while the surrounding medium acts like a clad. Thus far, there are some theoretical analyses for silica nanofibers [1,12]. However, in practical applications, e.g. for biomedical and chemical sensing, the sensitive element is usually immersed in or exposed to a liquid such as aqueous solution, and single-mode operation is generally required in waveguide-based optical sensing when coherent (rather than intensity) detection is used to achieve high sensitivity. Also, only when the nanofiber works under single-mode condition can it leave a large fraction of guided field outside the nanofiber as evanescent waves. In our previous work [13], we have given detailed analyses on the properties of silica nanofibers in aqueous solutions.

The optical wave guiding behavior of these nanofibers can be obtained by numerically solving Maxwell’s equations. For

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reference, Fig. 1 shows the propagation constant ( β ) of the fundamental mode (HE11) in water-clad silica fibers at the wavelength of 1550 nm. To be specific, here we assume that the nanofibers used in our work are made from a commercial single-mode fiber (SMF-28, Corning). The corresponding refractive indices of the silica nanofiber and the surrounding medium (deionized water) are 1.4468 and 1.3325 respectively. Starting from the propagation constants, a variety of guiding properties of the nanofiber waveguides can be obtained.

Figure 1. Numerical solutions of propagation constant of water-clad silica fibers at 1550 nm wavelength. (Dashed line is critical diameter for single-

mode.)

It is important to know the profile of the evanescent fields and power distribution around the waveguide for evanescent-wave-based optical sensing. In order to obtain more straightforward information of the power distribution in the radial direction, we calculate the fractional power of the fundamental mode inside the fiber ( insideη ) and outside the fiber ( outsideη ) at the wavelength of 1550 nm, which is shown in Fig. 2. It shows that, depends on the diameter, a single-mode silica nanofibers can guide light with about 20 to almost 100 percent of energy as evanescent wave.

Figure 2. Power distribution of silica fibers in deionized water operating at 1550 nm wavelength.

Besides, the curves show that the power outside the fiber increases with the decreasing of the fiber diameter, indicating that thinner nanofibers provide higher sensitivity. In other words, the single mode nanofibers in water have properties such as weak confinement ability, enhanced evanescent fields that are suitable for ideal sensing elements, which can be used to implement a fiber optic evanescent wave sensor with high sensitivity.

III. SENSOR MODELING WITH SINGLE MODE NANOFIBERS Based on waveguiding properties of optical nanofibers

discussed above, we propose to functionalize the nanofiber waveguide as sensing elements for detecting refractive index in aqueous solution. Mach-Zehnder interferometers are straightforward to design and fabricate and have long been demonstrated due to its phase sensitivity to the refractive index change of the waveguide and/or its surrounding medium. Therefore, we use Mach-Zehnder interferometer to coherently measuring the phase change of the probing light. As shown in Fig. 3, two silica nanofibers with same diameters are assembled to a Mach-Zehnder structure. One nanofiber (the upper one in the figure) is used as a sensing arm, which is exposed to the environment to be detected, and the other (the lower one) is used as reference arm that is kept in constant condition and is isolated from the measurand. We send the probing light into the left end of nanofiber, it propagates along the nanofiber and is bifurcated by the first 3-dB coupler. After traveling through the sensing arm where measured has access to the evanescent field at the sensitive area, the signal meets with the reference by the second 3-dB coupler, where highly sensitive interferometry technique is used to measure the phase change of the probing light, and the information of specimens can thus be retrieved.

Figure 3. Schematic of a proposed sensor with a Mach-Zehnder interferometer.

The phase change ( ΔΦ ) of the sensing arm can be obtained as

0( ) ,L Lβ β βΔΦ = − × = Δ × (1)

where L is the effective length of sensitive area, 0β and β are the initial and instant propagate constants of the light in the sensitive area, respectively. For given indices of silica nanofiber and surrounding medium, 0β and β can be obtained by numerically solving Maxwell’s equations, and ΔΦ can then be obtained with (1).

The sensitivity is a very important parameter to evaluate the device performance. To show the convincible and realizable sensing ability, we use the sensor to detect the variation of the refractive indices of IPA-water solutions with different concentrations, which allow arbitrarily small change of index. The refractive indices of the IPA solution are estimated based on the mole ratio of each component [14]. When there is an extreme small index change around the nanofiber, the guided light is changed in optical phase. For quantitative estimation, as a typical case, we use a laser (1550 nm wavelength) as probing light. Assuming the diameter of the nanofiber and the effective length of sensitive area are 800 nm and 5 mm respectively. We also assume the ambient refractive index varies from 1.3332 to

1.3350 with an interval of 42 10−× . Using them and (1), Fig. 4 shows the phase change of the sensor versus the refractive index variation. From the results in Fig. 4, the sensitivity of sensors investigated here can be calculated as 1 ( ) 3.767 (rad/μm)dL dn

ΔΦ× ≈ .

Figure 4. Phase change of the sensor versus the refractive index variation.

For comparison, sensitivity of conventional Mach-Zehnder sensors based on integrated planar waveguides is much lower [15], showing that much higher sensitivity, or equivalently much smaller size can be achieved when sensing with silica nanofibers. It also means that a smaller amount of analytes is need, which might be valuable in many applications.

In addition, the sensitivity will be improved with the decreasing of the nanofiber diameter. Fig. 5 shows the sensitivity of the sensor presented here at a wavelength of 1550 nm as a function of the silica fiber diameter.

Figure 5. The sensitivity of the sensor as a function of the silica fiber diameter.

Analysis and comparison for Fig. 5 and Fig. 2, as we can see, the key factor determining the sensitivity of the sensor is the power distribution outside the silica nanofiber, which, for a given silica nanofiber and a given surrounding medium, depends on the fiber diameter. When the diameter of silica nanofiber is decreased beyond a critical value (e.g. about 500 nm in our example), the sensitivity reaches a plateau, corresponding to the fractional power of the fundamental mode outside the silica nanofiber, outsideη , approaching 100%. For reference, when the nanofiber diameter is 600 nm, the sensitivity can up to ∼ 4.04 (rad/μm) . This sensitivity will be much higher than other biomedical sensors reported to date. Besides, in many applications a value of the sensitivity only a

fraction of the maximum achievable value may be high enough.

Furthermore, the sensitivity of detecting optical phase change in an integrated waveguide Mach-Zehnder interferometer can go down to 32 10 π−× [16], i.e., the sensor presented here can measure a refractive index change as small as ∼10-6 in theoretical. As the improvement of fabrication technology of nanofibers, we believe the theoretical detection limit of ∼10-6 refractive index unit will achieve.

IV. CONCLUSION In conclusion, a novel sensor capable of small volume

refractive index measurements in liquids has been suggested and investigated. Sensitivity of the sensor is also estimated. On the basis of the simulation, the nanofiber sensor displays the capability of detecting an index variation of 10-6. Compared to other sensors, the nanofiber sensors have a number of advantages such as high sensitivity, small size, immunity to electromagnetic interference, and compatibility to microfluidic or electronic devices. Our results show that the compact and flexible sensing scheme shown here may be applied to many diverse fields, including biomedical sensing, environmental monitoring, and process control.

ACKNOWLEDGMENT This work has been supported by Zhejiang Province

Natural Science Foundation (No. Y107547) and the National Physics Experimental Teaching Demonstration Center of Zhejiang University of Technology. The authors would like to thank Pan Zhao and Professor Gaoyao Wei for meaningful comments and useful discussions. Additionally, they would like to express their gratitude to Professor Limin Tong for his insightful technical suggestions.

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