All optic-fiber coupled plasmon waveguide resonance sensor using ZrS<sub>2</sub> based dielectric layer

Jinying Ma; Jinying Ma; Jinying Ma; Kun Liu; Kun Liu; Kun Liu; Kun Liu; Junfeng Jiang; Junfeng Jiang; Junfeng Jiang; Junfeng Jiang; Tianhua Xu; Tianhua Xu; Tianhua Xu; Shuang Wang; Shuang Wang; Shuang Wang; Pengxiang Chang; Pengxiang Chang; Pengxiang Chang; Zhao Zhang; Zhao Zhang; Zhao Zhang; Jiahang Zhang; Jiahang Zhang; Jiahang Zhang; Tiegen Liu; Tiegen Liu; Tiegen Liu

doi:10.1364/OE.389279

1. Introduction

Compared to the glass prism-based Kretschmann and Otto configuration surface plasmon resonance (SPR) scheme, the optical fiber based SPR sensor has advantages such as compact, flexible design, low cost, label free, and capability of remote sensing [1]. These advantages enable the optical fiber based SPR sensor to perform good characteristics as biosensors and chemosensors [2]. The coupled plasmon waveguide resonance (CPWR) sensors usually offer a narrower full width half-maximum (FWHM) of the resonance spectrum, leading to a relatively higher signal to noise ratio (SNR) and figure of merit (FOM) [3].

The sensitivity is a key challenge for the optical fiber based SPR sensors [4]. The two approaches can effectively enhance the sensitivity: one is to optimize the internal structure of the optical fiber [5], the other is to involve various sensitivity-improved materials [6]. For the optical fiber structure optimization, PCF [7], D-shape fiber [8,9], tapered fiber [10], cladding-off fiber [11], fiber grating [12,13] and their combinations have been employed. Meanwhile, some new materials such as transition-metal dichalcogenides (TMDCs) can be employed to further improve the sensitivity, due to their distinctive optical and electrical characteristics [14]. The graphene [15] and the molybdenum disulfide (MoS₂) [16] were investigated mostly, but major reported work were carried out only via theoretical simulations [6,16,17]. The theoretical dispersion relation models have been developed for graphene [18] and MoX₂ (X = S or Se) [14] only, but no generic model was developed for TMDCs based sensitivity-improved materials.

In this paper, we proposed and implemented an all optic-fiber CPWR sensor using zirconium disulfide (ZrS₂) based dielectric layer. To establish a generic model for the TMDCs, the dielectric constants of ZrS₂ have been obtained using the first-principles calculations, based on the density functional theory. Using the transfer matrix method, the theoretical model of the proposed sensor system was established, and the optimization of parameters in the CPWR sensor has been achieved via numerical simulations and theoretical analysis. The sensor was fabricated by depositing a gold layer on the fiber core and immobilizing the ZrS₂ layer on the gold layer. Based on the numerical simulation of the CPWR sensor, an experimental setup was implemented for measuring the refractive index of liquid solution. The experimental results were discussed and analyzed in detail.

2. Theoretical simulation

2.1 Structure of the CPWR sensor

The structure of the all optic-fiber CPWR sensor is shown in Fig. 1, which was designed based on a plastic-cladding silica-core multimode fiber. The sensing components include the optical fiber core, the metal layer and the dielectric layer, as depicted in Fig. 1(a). The optical fiber core was supposed to be made of silica, while the metal layer can be gold, silver, copper and aluminum. Gold was selected as the material of the metal layer due to its low loss and strong resonance response. The dielectric layer was made of new 2D materials such as TMDCs. Here we took the ZrS₂ as an example. The layer structure of the sensor is shown in Fig. 1(b).

Fig. 1. The structure of the CPWR sensor. (a) The profile view of the whole sensor; (b) The example of the layer constituents.

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2.2 Dielectric constants calculation of ZrS₂

The first-principles calculations can be used for estimating the properties of TMDCs [19], based on the density functional theory in the local density approximation [20], implemented the CASTEP module of Materials Studio 7.0 [21]. The structure of ZrS₂ layer belongs to the space group 164 (P-3m1) and contains one monolayer per primitive unit cell [22]. A single primitive unit cell is illustrated in Fig. 2(a), with one zirconium atom (blue) and two sulphur atoms (yellow) in it. The ZrS₂ layer was formed by cascaded nanolayers as shown in Fig. 2(b).

Fig. 2. The structure of the ZrS₂ layer. (a) One primitive unit cell; (b) The side view of the nanolayers.

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According to the local density approximation, the equilibrium structural parameters of the ZrS₂ were a = b = 3.598Å, c = 5.660Å, α=β=90°, γ=120°, u = 0.257Å [20], where a and c were the lattice constants, u represented the internal coordinate, and α, β and γ denoted the axial angles for primitive unit cell, respectively. Using the first-principles calculations with the CASTEP module, the dielectric constants of ZrS₂ ɛ_ZrS2 can be numerically simulated as a function of wavelength, as shown in Fig. 3. The real and image parts of the dielectric constants were marked in blue and red, respectively. The refractive index of the ZrS₂ can be calculated using n_ZrS2(λ)=[ɛ_ZrS2(λ)]^1/2, which was also a complex function of the wavelength λ. This calculation process can be also employed for any other TMDCs materials.

Fig. 3. The dielectric constants of ZrS₂ as the function of wavelength.

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2.3 Basic model of the CPWR sensor

The refractive index of the optical fiber core for different wavelengths can be described according to the Sellmeier dispersion relation as

(1)$${n_{co}}(\lambda )\textrm{ = }\sqrt {{\varepsilon _{co}}(\lambda )} = \sqrt {1 + \frac{{{a_1}{\lambda ^2}}}{{{\lambda ^2} - b_1^2}} + \frac{{{a_2}{\lambda ^2}}}{{{\lambda ^2} - b_2^2}} + \frac{{{a_3}{\lambda ^2}}}{{{\lambda ^2} - b_3^2}}} ,$$

where λ is the wavelength in µm and a₁=0.6961663, a₂=0.4079426, a₃=0.8974794, b₁=0.0684043 µm, b₂=0.1162414 µm, b₃=9.896161 µm are Sellmeier constants [23]. The dielectric constant of a metal can be written according to the Drude model as

(2)$${\varepsilon _m}(\lambda )= 1 - \frac{{{\lambda ^2}{\lambda _c}}}{{\lambda _p^2({{\lambda_c} + i\lambda } )}},$$

where λ_p=0.16826 µm and λ_c=8.9342 µm are the plasma and the collision wavelengths of the metal, respectively, which are constant for gold [23].

Using the dielectric constants of the optical fiber core, the gold and the ZrS₂, the sensing characteristic of the all optic-fiber CPWR sensor were theoretically analyzed, as shown in Fig. 1(b), based on the transfer matrix method [24]. Assuming an N-layer structure of the sensor, the amplitude of the electric and the magnetic fields (E and H) can be expressed using the characteristic matrix equation as [25]

(3)$$\left[ {\begin{array}{{c}} {{E_1}}\\ {{H_1}} \end{array}} \right] = M\left[ {\begin{array}{{c}} {{E_N}}\\ {{H_N}} \end{array}} \right],$$

where E₁ and H₁ are the components of electric and magnetic fields of the first layer, E_N and H_N are the components of electric and magnetic fields of the Nth layer, and M denotes the transfer matrix of the multilayered structure. M can be obtained using the continued product of M_k, which was the transfer matrix for the boundary between the kth and (k + 1)th layer:

(4)$$M\textrm{ = }\prod\limits_{k = 1}^{N - 1} {{M_k}} = \left[ {\begin{array}{{cc}} {{M_{11}}}&{{M_{12}}}\\ {{M_{21}}}&{{M_{22}}} \end{array}} \right],$$

with

(5)$${M_k} = \left[ {\begin{array}{{cc}} {\cos {\phi_k}}&{ - {{i\sin {\phi_k}} \mathord{\left/ {\vphantom {{i\sin {\phi_k}} {{\eta_k}}}} \right.} {{\eta_k}}}}\\ { - i{\eta_k}\sin {\phi_k}}&{\cos {\phi_k}} \end{array}} \right],$$

where ${\phi _k} = ({{{2\pi } \mathord{\left/ {\vphantom {{2\pi } \lambda }} \right.} \lambda }} )({{n_k}\cos {\theta_k}} ){d_k}$ represents the phase factor of the kth layer, $\lambda$ is the wavelength, ${\theta _k}$ is the angle of the incidence for the kth layer, and ${\eta _k}\textrm{ = }{{\cos {\theta _k}} \mathord{\left/ {\vphantom {{\cos {\theta_k}} {{n_k}}}} \right.} {{n_k}}}$ denotes the optical admittance for the kth layer. Therefore, the reflectance R can be calculated as

(6)$$R = {|r |^2} = {\left|{\frac{{({M_{11}} + {M_{12}}{\eta_N}){\eta_1} - ({M_{21}} + {M_{22}}{\eta_N})}}{{({M_{11}} + {M_{12}}{\eta_N}){\eta_1} + ({M_{21}} + {M_{22}}{\eta_N})}}} \right|^2},$$

The normalized transmitted power, P_trans, in the all optic-fiber CPWR sensor can be calculated as

(7)$$\mathop P\nolimits_{trans} = \frac{{\int_{{\theta _{cr}}}^{{\pi \mathord{\left/ {\vphantom {\pi 2}} \right.} 2}} {{R^{{N_{ref}}(\theta )}}\frac{{n_1^2\sin \theta \cos \theta }}{{{{({1 - n_1^2{{\cos }^2}\theta } )}^2}}}d\theta } }}{{\int_{{\theta _{cr}}}^{{\pi \mathord{\left/ {\vphantom {\pi 2}} \right.} 2}} {\frac{{n_1^2\sin \theta \cos \theta }}{{{{({1 - n_1^2{{\cos }^2}\theta } )}^2}}}d\theta } }},$$

where ${N_{ref}}(\theta )= {L \mathord{\left/ {\vphantom {L {{D_c}\tan \theta }}} \right.} {{D_c}\tan \theta }}$ represents the total number of reflections in the sensing region with L = 10 mm and D_c=600 µm denoting the length of the exposed sensing region and the diameter of the fiber core, respectively, and ${\theta _{cr}} = {\sin ^{ - 1}}({{{{n_{cl}}} \mathord{\left/ {\vphantom {{{n_{cl}}} {{n_1}}}} \right.} {{n_1}}}} )$ is the critical angle of the fiber with n_cl denoting the refractive index of the fiber cladding.

2.4 Simulation and analysis of the CPWR sensor

Using the basic model mentioned above, the transmission characteristic of the CPWR sensor have been investigated. Since the diameter of the optical fiber core was fixed, the thicknesses of gold and ZrS₂ will affect the performance of the CPWR sensor.

The deionized water was used as the medium under test, and its refractive index was 1.333. When the thickness of the ZrS₂ layer was set as 150 nm, the normalized transmission spectrum corresponding to different thicknesses of the gold layer was calculated as shown in Fig. 4(a). It can be found that there was an obvious absorption valley in the spectrum due to the SPR phenomenon. With the increase of the thickness of the gold layer, the depth of the valley will decrease. When the thickness of the gold layer exceeded 60 nm, the SPR absorption valley became negligible. This was because that in such case there was not enough energy coupled into the test media through the ZrS₂ layer. On the other hand, the thickness of the gold layer cannot be too small, otherwise the spectrum will be significantly broadened. Compromising the amplitude and the width of the spectrum, an optimal thickness of the gold layer will be 30-40 nm.

Fig. 4. The influence of the thickness of the dielectric layer on the transmission characteristics of the CPWR sensor. (a) The case for: Au layer with different thicknesses and ZrS₂ layer with the thickness of 150 nm; (b) The case for: ZrS₂ layer with different thicknesses and Au layer with the thickness of 30 nm.

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When the thickness of the gold layer was set as 30 nm, the normalized transmission spectrum corresponding to different thicknesses of the ZrS₂ layer is calculated in Fig. 4(b). It can be seen that with the increment of the thickness of the ZrS₂ layer, the SPR valley will have a red shift and an increased absorption. But when the thickness of the ZrS₂ layer is too large, e.g. 280 nm, more absorption valleys will appear, resulting in difficulties in the demodulation. Compromising the spectrum amplitude and the demodulation performance, an optimum thickness of the ZrS₂ layer will be 160-240 nm.

According to the above analysis, the thicknesses of the gold and the ZrS₂ layers were set as 30 nm and 200 nm (within the optimal ranges), respectively. The SPR transmission spectrum of the CPWR sensor is shown in Fig. 5(a), with the refractive index of the test media changing from 1.333 to 1.403. When the refractive index increased, the SPR valley will show a red shift. The nonlinear relationship between the SPR absorption wavelength and the refractive index of the test media is shown in Fig. 5(b). This showed that the CPWR sensor scheme can be used for refractive index sensing.

Fig. 5. The sensing characteristic of the CPWR sensor. (a) The transmission spectrum of the CPWR sensor with different RIU values; (b) The sensing fitting curve of the CPWR sensor.

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3. Experimental discussion

3.1 Sensor fabrication

Based on the numerical simulation, an all optic-fiber CPWR sensor was fabricated built on a plastic-cladding silica-core multimode fiber provided by Scitlion Technology, with a numerical aperture of 0.37 and a core diameter of 600 µm, respectively. In the middle of the fiber, there is a ∼10 mm unclad section, which was cleaned by acetone and ethanol successively in an ultrasonic cleaner. The unclad fiber section was then coated with a 35 nm-thick gold layer, using the vacuum sputtering coating device. The working pressure was 1 Pa (7.5 mTorr) in pure Ar atmosphere. The RF power was set to be 33 W for the gold target, which had a diameter of 2 inch and the purity of 99.999%, purchased from ZhongNuo Advanced Material. The thickness of the gold layer was monitored using a crystal film thickness detector, and the deposition rate was experimentally determined.

The ZrS₂ layer was then immobilized on the gold-coated fiber. The completed sensor was immersed in the ultrasound treated ZrS₂ isopropanol nano-dispersion until the solution fully evaporated. The size of the ZrS₂ nanosheet was from dozens of nanometers to hundreds of nanometers, and the thickness of the ZrS₂ nanosheet was usually several nanometers, because of the ultrasound treatment on the isopropanol solution of ZrS₂. The full evaporation of the nano-dispersion indicated the completion of one deposition cycle. The procedure of annealing at 50 °C for 5 hours was necessary to ensure the adhesion strength. Deposition cycles from 0 to 4 were carried out by repeating the above evaporation process for different CPWR sensors with various thicknesses of the ZrS₂ layer. Taking the sensor with ZrS₂ of two layers as example, the scanning electron micrograph (SEM) pictures of the CPWR sensors are shown in Fig. 6. The gold layer was very smooth, according to the SEM image shown in Fig. 6(a). The nano-sheets of ZrS₂, with size of several hundreds of nanometers, can be clearly observed in Fig. 6(b), as they were supposed to be. The thickness of the layers including gold and ZrS₂ was measured as 50.6 nm by the SEM shown in Fig. 6(c).

Fig. 6. The SEM pictures of the CPWR sensors. (a) SEM images of Au layer; (b) SEM images of ZrS₂ with two layers; (c) Thickness of the layers.

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3.2 Experimental setup

To measure the sensing characteristic of the CPWR sensor, an all optic-fiber refractive index sensing system was implemented. As shown in Fig. 7(a), the system included a tungsten halogen light source (HL 2000, Ocean Optics), all optic-fiber CPWR sensor, a spectrophotometer (HR 4000, Ocean Optics) and a PC. The light from the tungsten halogen lamp, with a broadband spectrum covering 360-2000 nm and an input power of 100 W, was injected into the multimode fiber and passed through the CPWR sensor. The transmitted optical signals out of the sensor were measured using the spectrophotometer with a detecting range of 200-1100 nm and a resolution of 20 pm, and were eventually stored and analyzed in the computer.

Fig. 7. The all optic-fiber CPWR sensing system. (a) Block diagram of the experimental setup; (b) Picture of the practical CPWR sensor.

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The sensor was implemented by inserting the optical fiber into a glass tube, as shown in Fig. 7(b). The glass tube with upper and lower openings was used for the flow control of the alcoholic solution with different concentrations. The CPWR sensor was inserted into the tube, in order to prevent the evaporation of the solution and to guarantee the sensing time. Both ends of the tube were blocked by self-made stoppers to prevent the leakage.

3.3 Results and discussion

As the aforementioned descriptions, the measurement of the refractive index can be realized using the above experimental sensing system. As we know, the SPR sensor was usually modulated by the ambient temperature, and there will be a blue shift with the increase of the temperature [26]. In order to eliminate the temperature influence on the sensor, the practical CPWR sensor was set in a glassware, which was immersed in a thermotank. The temperature of the thermotank was kept at 37 °C, which was the common temperature for most immunoreaction. The resonance spectrum of the CPWR sensor can be obtained by collecting and comparing the spectra of the light source with and without the use of the CPWR sensor [27]. Then the CPWR wavelength of the sensor can be calculated, according to the absorption valley in the spectrum. Alcoholic solutions with different refractive indices can be provided, by varying the concentration of the alcoholic. The resonance spectrum of the CPWR sensor with two layers of ZrS₂ is shown in Fig. 8, corresponding to different refractive index of the alcoholic solution.

Fig. 8. The resonance spectrum of the CPWR sensor corresponding to different refractive index.

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With each refractive index, the resonance spectrum was measured 100 times using the spectrophotometer. The averaged resonance wavelengths and the mean squared errors (MSE) for each refractive index were calculated. The relationship between the CPWR wavelength of the sensor and the refractive index of the alcoholic solutions is shown in Fig. 9(a), together with MSE values for each refractive index. It can be seen that the curves was monotonic and nonlinear, which had the same trend as the theoretical simulation. The MSE values are no more than 0.25 nm, which shows the measurement repeatability and precision of the sensor. The sensitivity of the sensor had been investigated using inverse proportional function fitting with the lowest residual error, and the sensitivity response curves were shown in Fig. 9(b). The sensitivity will be higher than 8000 nm/RIU when the refractive index exceeded 1.4. So this measurement of refractive index can achieve a resolution of better than 2.5×10⁻⁶ RIU, with a resolution of 20 pm for the spectrophotometer. It can be seen that the CPWR sensors with different numbers of the ZrS₂ layers had a similar response in the refractive index sensing. The senor with two layers of ZrS₂ had a relatively higher sensitivity, with a moderate thickness of about 200 nm according to numerical simulation results. If the thickness was smaller than it, the sensitivity will become lower due to the insufficient SPR resonance. If the thickness was larger than it, the sensitivity will also get decreased because of the insufficient energy coupled into the measured medium. It is noted that the detected CPWR wavelengths in experiments in Fig. 9(a) deviated significantly from the theoretical simulation results in Fig. 5(a). That is because the theoretical model was developed based on a thin dielectric layer, while the practical sensor in the experiment was coated with nano-sheets of ZrS₂. Additionally, we can also see that the sensitivity of the practical sensor was higher than the simulation results. It is because that we can only simulated calculation the optical characteristic for the materials as the dielectric layer. In the practical experiments, the ZrS₂ will capture more molecules because of the absorption characteristic as the 2-D materials. This absorption characteristic cannot be expressed in the basic model of the CPWR sensor.

Fig. 9. The sensing characteristic of the CPWR sensor for refractive index measurement with different dielectric layers. (a) The CPWR wavelength of the sensor; (b) The sensitivity of the sensor.

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The CPWR sensors with other TMDCs based dielectric layers were also investigated. As a comparison, vanadium diselenide (VSe₂) and hafnium disulfide (HfS₂) were employed as examples. The relationship between the CPWR wavelength of the sensor with different materials based dielectric layers and the refractive index is shown in Fig. 10(a), and the curves of the sensitivity response are shown in Fig. 10(b). All sensors had similar sensing characteristics for two-layer 2D materials. It can be found that the CPWR sensor with ZrS₂ based dielectric layer had a relatively higher sensitivity, compared to the CPWR sensors using other materials based dielectric layers. In addition, since the mechanism of the proposed sensitivity enhancement was independent of the structure optimization process, this technique can be applied together with other enhancement methods to further improve the sensitivity.

Fig. 10. The sensing characteristic of the CPWR sensor for refractive index measurement with different dielectric materials. (a) The CPWR wavelength of the sensor; (b) The sensitivity of the sensor.

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4. Conclusion

An all optic-fiber CPWR sensor was proposed and implemented using ZrS₂ based dielectric layer. A generic process for evaluating dielectric constants was developed to obtain the ZrS₂ parameters using first-principles calculations. Based on the transfer matrix method, the theoretical model of the CPWR sensor was established. The parameters of the sensor were optimized using theoretical simulations. The sensor was fabricated by depositing gold layer and immobilizing the ZrS₂ layer on top of it. The experimental setup was implemented for measurement the refractive index of alcoholic solutions with different concentrations. The sensor with two cycles showed the optimal performance, with a sensitivity of more than 8000 nm/RIU for the refractive index of larger than 1.4. This proposed method can also be applied together with other structure optimization approaches to further enhance the sensitivity of the SPR sensor.

Funding

National Natural Science Foundation of China (61735011, 61775161, 61922061); Tianjin Science Fund for Distinguished Young Scholars (19JCJQJC61400); Ministry of Science and Technology of the People's Republic of China (2013YQ030915).

Acknowledgments

Portions of this work were presented at the 8th Asia-Pacific Optical Sensors Conference in Auckland, New Zealand on 19-22 November 2019. The title of the paper is “All optic-fiber coupled plasmon waveguide resonance sensor based on disulfide zirconium as the dielectric layer”, and the conference paper number is Oral 154.

Disclosures

The authors declare no conflicts of interest.

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All optic-fiber coupled plasmon waveguide resonance sensor using ZrS₂ based dielectric layer

Abstract

1. Introduction