A refractive index sensor based on magneto-optical surface plasmon resonance

Journal Pre-proof A refractive index sensor based on magneto-optical surface plasmon resonance

Zhonghao Zheng, Jinhu Wang, Nengxi Li, Chaoyang Li, Jun Qin, Tingting Tang, Lei Wang, Zongzheng Liao, Yuxin Mo PII:

S0749-6036(19)31325-4

DOI:

https://doi.org/10.1016/j.spmi.2019.106286

Reference:

YSPMI 106286

To appear in:

Superlattices and Microstructures

Received Date:

27 July 2019

Accepted Date:

28 September 2019

Please cite this article as: Zhonghao Zheng, Jinhu Wang, Nengxi Li, Chaoyang Li, Jun Qin, Tingting Tang, Lei Wang, Zongzheng Liao, Yuxin Mo, A refractive index sensor based on magnetooptical surface plasmon resonance, Superlattices and Microstructures (2019), https://doi.org/10. 1016/j.spmi.2019.106286

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A refractive index sensor based on magneto-optical surface plasmon resonance Zhonghao Zheng1#, Jinhu Wang1#, Nengxi Li1#, Chaoyang Li2, Jun Qin3, Tingting Tang1,2*, Lei Wang1, Zongzheng Liao1, Yuxin Mo1 1. College of Optoelectronic Engineering, Chengdu University of Information Technology, Chengdu 610225, China 2. State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, No. 58, Renmin Avenue, Haikou, Hainan Province, 570228, China 3. National Engineering Research Center of Electromagnetic Radiation Control Materials, University of Electronic Science and Technology of China, Chengdu 610054, China # Zhonghao Zheng, Jinhu Wang and Nengxi Li contribute equally to this work. * Tingting Tang is the corresponding author. Abstract: A refractive index sensor based on magneto-optical surface plasmon resonance (MOSPR) is proposed. The device is made up of a prism coupling system consists of prism/TiN/SiO2/YIG/CeYIG/Au/liquid. System simulation is performed by 4 × 4 transfer matrix method. It is found that the structure has excellent sensing performance. Magnetron sputtering vacuum coating method is used to fabricate the dielectric film of the sample, and a MOSPR sensor detection platform is built to test the concentration of 0-30% alcohol solution. As the concentration of the alcohol solution increases, the refractive index of the sensing liquid increases and transverse Kerr effect is enhanced. Meanwhile the incident angle corresponds to the maximum value of the MOSPR curve (the difference of reflectance with opposite magnetic fields) also increases with the alcohol concentration. This structure of MOSPR is expected to be applied in the field of micro-measurement and biochemical sensing.

Keywords:

refractive index sensing design, magneto-optic effect, surface

plasmon resonance, self-assembly

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1.Introduction: Surface plasmon resonance (SPR) is a collective charge density oscillation that produces a highly restricted electromagnetic field at the interface of metal and dielectric material. SPR has characteristics different from those of optical waveguides, such as a large range of effective refractive index and a field enhancement effect. Moreover, the surface plasma wave is very sensitive to the change of the refractive index of the medium. By measuring the change of the reflected light intensity and phase in the waveguide, the change of the refractive index can be obtained. When linearly polarized light reflects from a magnetic material, angle momentum is transferred to reflected waves and induces a rotation of polarization plane, this phenomenon is called magneto-optical Kerr effect (MOKE). According to the direction of external magnetic field or magnetization, there are three types of magneto-optical Kerr effect, including polar magneto-optical Kerr effect (PMOKE), transverse magneto-optical Kerr effect (TMOKE) and longitudinal magneto-optical Kerr effect (LMOKE). SPR has a good enhancement of the MOKE[1] (Figure. 1), so the research on the refractive index sensing of magneto-optical Surface plasmon resonance (MOSPR) has attracted much attention. The high quality SPR enhances the transverse magneto-optic effect, and the over-damped SPR enhances the magneto-optical effects in the polar and longitudinal directions. If a magneto-optical signal is used instead of a reflected signal as a sensing parameter, this enhanced magneto-optical effect will double the biochemical sensing sensitivity of the SPR sensor[2]. Exploring the enhancement effect of SPR[3][4][5][6] on magneto-optical effect is a worthwhile challenge in the field of information transmission and biochemical sensing[7][8][9]. （a）

（b）

（c）

Figure 1 (a) polar magneto-optical Kerr effect; (b) transverse magneto-optical Kerr effect; (c) longitudinal magneto-optical Kerr effect

In this paper, we designed a unified theoretical MOSPR[10][11] refractive index sensing design with prism/TiN/SiO2/YIG/CeYIG/Au/liquid new structure[12][13] based

Journal Pre-proof on the classification prism coupling system. The magneto-optical prism waveguide coupling system is studied. The physical mechanism of magneto-optical surface plasmon resonance in magneto-optical waveguide is studied, and the variation law between MOSPR and material parameters are analyzed. We will use the simulation methods based on transfer matrix method and to calculate the anisotropic medium, and derive the MOSPR calculation formula of the magneto-optical material waveguide with dielectric constant tensor. The parameters of the sensor, such as the wavelength of incident light, the angle of incident angle, the thickness of the metal film layer and the thickness of the magnetic material, are simulated and calculated to find the optimal structural parameters of the sensor. Then, we record the optical parameters which are obtained under different refractive index. After performing multiple experiments, we draw the refractive index sensing curve of the system.

2.Model and Theory 2.1 Principle of refractive index sensing: As for MOSPR,

[14]the

magneto-optic effect is essentially Zeeman splitting and

spin-orbit coupling. It is derived from the selection rule of different angular momentum photons when it appears. It is characterized by magnetic circular dichroism, that is, the material is polarized by left and right circularly polarized light under the action of magnetic field. The polarization frequencies of left-handed circular polarization and right-handed circular polarization are different under the action of magnetic field, which leads to the difference of real part and imaginary part in Lorentz model. Any beam of light can be seen as a bunch of left-handed circular polarized light and a righthanded circularly polarized light. When light propagates through a medium, its electric field causes motion of electrons in the medium. In the absence of an applied magnetic field, left-handed circularly polarized light drives the electrons to make left-handed motion. The right-handed circularly polarized light drives the electrons to make a righthanded motion, and the left-handed and right-handed electron orbitals have the same radius. At this time, the medium has the same dielectric constant as the left-handed circularly polarized light and the right-handed circularly polarized light, so that the Faraday effect is not generated. When there is an applied magnetic field, a Lorentz force is applied to each electron, which produces the difference between the left-handed and right-handed electron orbital radii. At this time, the dielectric constant of the medium

Journal Pre-proof will change relative to the left-handed circular polarization and the right-handed circularly polarized light，which causes the polarization plane of the outgoing light to deflect. Under the action of a magnetic field, the dielectric constant of an anisotropic material becomes a tensor, as in the following form (1)[13].

      - a z - a  y 

a z

 - a x

ay   a x   

(1)

The basic principle of the SPR structure as a refractive index sensor is based on the influence of the refractive index nd of the medium on the SPP (Surface Plasmon Polaritons) wave vector. In the SPR structure, the condition of surface plasmon resonance can be expressed by the formula (2): K SPP 

 C

 m d  Ksin SPR m  d

(2)

Kspp changes with the change of the refractive index nd of the medium, which causes the surface plasmon resonance to change condition, which is reflected as the overall movement of the reflection spectrum. Therefore, the refractive index n of the medium can be detected by detecting the reflection spectrum nd test. 2.2 TMOKE and transition matrix principle: The ξ in the matrix (1) denotes diagonal element, and the Π(x, y and z) is called the magneto-optical non-diagonal element of the material is the applied magnetic field. According to reflection and transmission, the magneto-optical effect can be divided into Faraday effect and Kerr effect. The Faraday effect is in transmission and the Kerr effect is in reflection. TMOKE which direction of the magnetic field is perpendicular to the plane of incidence and parallel to the surface of the material. Now, the permittivity tensor of the MO material will be (3)[15]: 0 0      0  a  0 -a     

(3)

This Kerr effect only affects the component of the electromagnetic field in the plane of incidence. For a plane wave solution (4)：

E n   E0n  exp[i (t   n r )]

(4)

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the wave equation in a medium characterized by ξ(n) takes the form

 n 2 E0n    n   n  E0n   

 2 n  n   E0 c2

(5)

Here, ( E0(0) ) is the complex electric field amplitude, which specifies the wave

 

ˆ x  zN ˆ z  / c is the complex wave vector, t, ω, c, and r denote polarization,   n     xN the time, angular frequency, phase velocity of the wave in vacuum, and the position vector, respectively. According to the boundary conditions of electromagnetic field, the parallel components of each wave vector,  N zj ( n ) / c( j  1, 2,3, 4) .In an isotropic layer,

N zj ( n )  N z 0 ( n ) ( j  1, 2,3, 4) ,in which N z 0 ( n )  N ( n )2  N x ( n ) , N ( n )2    n  0 n  .Then the dynamic matrix is

D(n)

 1  (n)  N z0  0    0 

1  N z 0( n )

    n  1/ 2  (n) N z0 0   1/ 2   0 n  

0 0

0 0



0

N z 0 ( n )  0 n 

0

  0 n 









1/ 2



1/ 2



(6)



The propagation matrix of isotropic layers and magnetic layer is  ei ( / c ) N z 1   0  0   0 

(n) (n)

P

 n

d

0 ei ( / c ) N z 2

0 (n) (n)

d

0 (n) (n)

ei ( / c ) N z 3

0 0

   0  0  (n) (n)  ei ( / c ) N z 4 d  0

d

0

(7)

In which d ( n ) is the thickness of nth layer. Combining the D matrix and P matrix of each layer we can get M matrix which connects the electric field amplitudes of cover and substrate -1

-1

M  D（0） D (1) P (1) D（1） D (2)

(8)

There are four eigenmode amplitude solutions, and each solution will be of the form of

E（0 0）  ME（0 N 1）

(9)

where eigenmode amplitudes E0(0) and E0(n+1) are 4 × 1column vectors, M is the 4 × 4 total matrix and the product of the propagation matrices P and the dynamic matrices D. Equation (8) illustrates the transfer matrices of the proposed tree layers structure. With the 4 × 4 transfer matrix method the reflection coefficient can be calculated by

 E0( rp)  M M  M 13 M 41 rpp   ( i )   11 43  E0 p  E0( is) M 11M 33  M 13 M 31

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* R  rpp rpp  rpp

2

(11)

Where E0p(r) and E0p(i)are the p-polarized complex electric field amplitude of the reflect light and the incident light, respectively. When the s-polarized complex electric field amplitude E0s(i)=0 the reflectance can be calculated by equation (11), and all Mxy are the elements of the total matrix M[16][17]. In the MOSPR device, an additional Δksp (-Δksp) is added to the SPP wave vector when the applied magnetic field +H(-H), so the obtained SPR reflection spectrum has no applied magnetic field[18]. The time (denoted as R(0)) has a positive (negative) offset, denoted as R(+H) and R(-H), respectively, as shown in (10). The TMOKE signal can be defined according to the obtained R(+H) and R(-H). The TMOKE signal is defined as[19][20][21]: TMOKE 

R( H )  R( H ) R( H )+R( H )

(12)

The MOSPR signal recorded by TOMKE has a larger relative value and has a larger resolution. However, under this definition, when SPR is generated, R(+H) and R(-H) both approach 0, and the TOMKE signal will lose its meaning because it is extremely large.

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3.Experiments and conclusions 3.1 Structural design of yttrium-doped garnet MOSPR waveguide

Prism （ SiO2（

θi

θr

H x

TiN

z

SiO2 YIG CeYIG Au Water

Figure 2 Schematic diagram of the MOSPR waveguide structure

Figure 2 shows the geometry of the proposed MOSPR waveguide. The structure consists of a SiO2 prism, a TiN layer, a SiO2 layer, an yttrium iron garnet (YIG) layer, a CeYIG[22][23][24] layer, an Au layer, and a sensing medium water. The bottom metal layer TiN is a refractory plasma material that withstands the high temperatures of the magnetic oxide during deposition. The intermediate 8 nm SiO2 layer is a non-crystalline diffusion barrier that prevents oxidation of TiN during high temperature annealing. This amorphous layer also prevents the templating effect of TiN on YIG while allowing the crystalline garnet to crystallize. The Au layer is to excite the SPR. Using the 4 × 4 transfer matrix method, we can discuss the MO effect generated by this structure. To calculate MOSPR, we need to know the dielectric constant of various dielectric materials. When the incident wavelength is 632.8 nm, the refractive indices of SiO2, TiN, YIG and water are equal to 1.45, 1.351+2.76i, 2.3761 and 1.33 respectively. Besides, the dielectric constant of Au is equal to -10.98+1.464i. The dielectric constant of CeYIG is a tensor which can be represented by a three-order matrix   c0  c   0  0 

0

 c0 - c1

0   c1   c 0 

Assuming that CeYIG is reached saturation magnetization in TMOKE, in CeYIG

(13)

Journal Pre-proof layer,  c 0  5.963  0.134i ,  c1  0.02027  0.003317i . Here,  c 0 are the principal diagonal and non-diagonal elements of the three order matrix of CeYIG dielectric tensor, as we mentioned in the previous section[25]. From the above data, we can calculate the SPR and MOSPR at this time.

Figure 3 Reflectivity curve of MOSPR waveguide structure. (a) MOSPR curves of the proposed structure. (b)the maximum ΔR with different refractive index of the sensing layer. (c)SPR curves of the of the structure consists of SiO2/YIG/CeYIG. (d) the maximum ΔR with different refractive index of the structure

Figures 3(a) and (b) represent the SPR and MOSPR curves of the proposed structure, respectively. When a reverse magnetic field is applied, there is a slight difference in the SPR reflectance curve of the magnetic field in both directions. As we increase the refractive index of the sensing layer, the MOSPR curve also changes, and the angle of incidence at which the maximum ΔR is obtained will also increase. The reason why the sensing structure is designed such that CeYIG must be grown on the implanted layer YIG in the actual coating process, and the thickness of the control layer and the surface roughness must be the thickness of the CeYIG and YIG. More than 30 nm. Figure 3(c) show the SPR curves of the film samples when the SiO2/YIG/CeYIG structure is employed. Among them, the thickness of YIG, CeYIG and Au are respectively 48nm, 45nm and 15nm. The refractive index of the sensing layer is still 1.33. As can be seen in the figure, the SPR curve at this time has a very large full width at half maximum and is not suitable for sensing. At the same time, it can be seen from

Journal Pre-proof Figure 6(d) that when the reverse magnetic field is applied to the structure, although there is a large ΔR, the angle at which the large reflectance difference occurs is far from the resonance angle of the SPR. The reflectance at this time is already close to 0.9. Therefore, there is no obvious MOSPR phenomenon at this time. Based on this, we added a layer of TiN and a layer of SiO2 to form a metal/insulator/metal structure. By comparison, we can find that this greatly enhances the MOSPR effect and is more conducive to sensing. 3.2 Sensing performance test of yttrium-doped garnet MOSPR waveguide After designing the MOSPR waveguide structure, we use a magnetron sputtering vacuum coating method to plate the dielectric film structure of the sample used. Firstly, we select a suitable SiO2 substrate, and perform a series of pretreatment work on the substrate, such as cutting, cleaning, soaking and drying. During the deposition process, since the quality of the film has a great influence on the sensing performance of the MOSPR waveguide, it is necessary to explore the optimum sputtering gas pressure and sputtering power. Finally, in the optimum sputtering gas pressure and sputtering power, we get the MOSPR waveguide sensor chip required by the design. However, the structure prepared by the experimental materials are not perfect, and these imperfections will affect the performance of the sensor. If the dielectric film structure is too thin or too thick may lead to the experimental values move left or right which means that the results can’t quite agree with the theoretical values.

Figure 4 Schematic diagram of MOSPR sensing test

Next, the MOSPR sensor detection platform will be built. We add an external magnetic field to the Kretschmann prism coupling structure to form a magneto-optical Kerr effect detection platform, and use the angle modulation type to detect the SPR

Journal Pre-proof signal. The schematic diagram of the experimental light path is shown in Figure 4. In the experiment, a He-Ne laser was selected as the light source in the experiment, and the incident light wavelength was 632.8 nm. The incident P-polarized light is incident on the surface of the sample. The direction of the external magnetic field is parallel to the surface of the sample and perpendicular to the entrance surface to ensure simultaneous excitation of the SPR effect and the TMOKE effect. The light reflected from the final sample is received by the photodetector. We are using the laser whose power is 3.0 mW, and the resulting beam is originally linearly polarized. Before the experiment, we need to adjust the horizontal position of the laser to ensure that the output laser beam is parallel to the optical platform. Then we adjust the Glan polarizer to a horizontal state while rotating and adjusting the angle of the laser cavity to ensure that the intensity of the transmitted light passing through the Glan polarizer is the largest. After that, we use a photodetector to detect the intensity of the light passing through the Glan polarizer and record the intensity of the light. By comparing this light intensity signal with the light signal reflected from the surface of the test sample, the reflection coefficient and reflectance at this time can be obtained. Due to the limitations of the materials used in the growth process, the prism material we use is fused silica, and the shape of the prism is an isosceles right triangle. It is SiO2 Amorphous and the refractive index is 1.45. Finally, we glued the side of the sensor chip substrate to the bottom surface of the prism with an index matching solution. The experimental test turntable is specially customized, and the turntable is controlled by a stepper motor with a minimum precision rotation angle of 0.0025 degrees. As can be clearly seen in the figure 4, the sample is fixed in the center of the rotating axis of the test platform, and the microfluidic channel is located on the other side of the sensing chip. When the SPR refractive index sensing is tested, the solution to be tested enters the microfluidic channel from the catheter. It is in full contact with the surface of the sensing chip and enables the spot of incident light to be present on the contact surface of the sample with the solution. In the test platform, there are three grooves that can be used to place the permanent magnets. Place the permanent magnets in different groove positions to adjust the direction of the magnetic field to adjust the magneto-optical Kerr effect in different directions. Based on this effect, we designed the dielectric film onto the fused silica substrate by magnetron sputtering. We used alcohol as the sensing solution and allocated alcohol solutions at concentrations of 0%, 5%, 10%, 20% and 30%, respectively.

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Figure 5 (a) the SPR curve of a film sample detected when a 0% alcohol solution is introduced into the microfluidic channel with H+ and H- magnetic fields. (b) the experimental and simulation comparison of the MOSPR curve .

Figure 5(a) shows the SPR curve of a film sample detected when a 0% alcohol solution is introduced into the microfluidic channel (i.e. the sensing solution is water), where H+ and H- represent Positive and negative magnetic fields are applied separately during the detection process. When the incident angle nears the resonance angle, a slight difference which is caused by the magneto-optical effect can be observed in reflectance. Figure 5(b) shows the experimental and simulation comparison of the MOSPR curve at this time. The solid line represents the experimental measurement value and the dotted line represents the theoretical simulation value. It can be seen from the figure that the theoretical and experimental values of the MOSPR curve exhibit the same trend and are basically consistent in numerical values. However, there are certain deviations between experimental values and theoretical values. For example, the experimental maximum value of MOSPR is smaller than the theoretical maximum value, and the experimental curve width is also larger than the theoretical value. The reason for these occurrences may be that the thickness of the magneto-optical CeYIG layer and the metal Au layer is deviated during the sputtering process of the film sample, and is not equal to the theoretical thickness, resulting in a change in structure.

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Figure 6 MOSPR sensing test chart of the sample. (a) the MOSPR curve of the sample with a concentration of 0%, 10%, and 20% alcohol solution. (b) ΔR of SPR and MOSPR.

Figure 6(a) shows the change in the MOSPR curve of the sample when a concentration of 0%, 10%, and 20% alcohol solution is introduced into the microfluidic channel. As the concentration of the alcohol solution increases, the refractive index of the sensing liquid increases, and the incident angle of the maximum value of the MOSPR curve (ie, the difference between the forward and reverse magnetic field reflectance is the largest) also increases. However, this method is relatively cumbersome in the actual sensing process. Typically, we fix the angle of incidence to the angle at which ΔR is maximal when the sensing solution is water. In this structural sample designed, this angle appeared near 67.3 degrees. Subsequently, we introduced different concentrations of alcohol solution into the microfluidic channel and recorded the ΔR size at this time. As can be seen in Figure 6(b), as the liquid concentration increases, the ΔR detected at the same angle at this time also decreases. In order to eliminate the influence of source noise, we can normalize this signal and record it as formula (10). Comparing the ΔR signal with the TMOKE signal in Figure 6(b), it can be seen that the MOSPR signal recorded by TMOKE has a larger relative value and has a larger resolution. However, under this definition, when SPR is generated, R(+H) and R(-H) both approach 0, and TOMKE signals can also be meaningless because of their magnitude.

4.Conclusion A refractive sensor based on magneto-optic surface plasmon resonance is proposed, which consists of prism/TiN/SiO2/YIG/CeYIG/Au/liquid. A layer of TiN and a layer of SiO2 are added to the traditional SiO2/YIG/CeYIG structure to form a

Journal Pre-proof metal/insulator/metal structure, which greatly enhances the MOSPR effect and is more conducive to sensing. The reflectivity curve and △ R curve are calculated by simulation. It is found that the structure has excellent sensing performance. The △R structure is 1-2 orders of magnitude larger than the ferromagnetic MOSPR structure. Magnetron sputtering vacuum coating method was used to fabricate the dielectric film of the sample, and a MOSPR sensor detection platform was built to test the concentration of 0-30% alcohol solution. When the wavelength of the incident light is 632.8 nm, the refractive index of the sensing liquid increases with the increase of the concentration of alcohol solution, and the incident angle of the maximum value of the MOSPR curve also increases. In addition, the reasons for the errors between the experimental and theoretical values of MOSPR are analyzed. This structure of MOSPR is expected to be applied in the field of micro-measurement and biochemical sensing. Acknowledgements This work is supported by Sichuan Science and Technology Program (2019JDJQ0003), Open Project Program of State Key Laboratory of Marine Resource Utilization in South China Sea (2019010). Zhonghao Zheng, Jinhu Wang and Nengxi Li contribute equally to this work. Tingting Tang is the corresponding author.

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Journal Pre-proof Competing financial interests: The authors declare no competing financial interests.

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Highlights 1. A refractive index sensor based on magneto-optical surface plasmon resonance (MOSPR) is proposed. 2. System simulation is performed by transfer matrix method. 3. It is found that the structure has excellent sensing performance. 4. The refractive index of the sensing liquid increases and transverse Kerr effect is enhanced.