Paper

Ultrahigh resolution integrated optic short range surface plasmon-polariton based sensor for aqueous environment

Published 10 February 2021 © 2021 IOP Publishing Ltd
, , Citation Vinod K Sharma 2021 Eng. Res. Express 3 015017 DOI 10.1088/2631-8695/abe27c

2631-8695/3/1/015017

Abstract

We propose a short length plasmonic integrated optic sensor with ultrahigh sensitivity and resolution for aqueous environment. The sensor section is a simple multilayer structure consisting of a dielectric planar waveguide separated from a thin metal film by a low index buffer layer. The sensor is based on the hybrid coupling between the guided mode and the short range plasmonic mode supported by the thin metal film. Periodic coupling between these modes takes place along the length of the sensor. For optimized layer thicknesses, resonant peak appears at very short length of sensing section. The spectral position of resonant peak is strongly dependent on the analyte index. We show theoretically that a spectral sensitivity of 3.2 × 104 nm RIU−1 with FWHM of 2 nm and resolution of 6.2 × 10–8 RIU can be achieved for aqueous environment.

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1. Introduction

The unique property of the surface plasmon polaritons (SPPs) supported by thin metal films, to confine the electromagnetic fields beyond the optical diffraction limits allows a range of potential plasmonic devices including sensors [13]. Surface plasmons are the collective excitations of the surface electrons in metals induced by the electromagnetic field. The simplest means to excite the surface plasmons is by using prism coupling technique and attenuated total reflection (ATR) method. Generally a thin metal film is deposited on the surface of a high index prism. Electromagnetic radiation entering the prism will be totally internally reflected. The internal reflection creates an evanescent wave which extends beyond the interface of prism-metal film into the surrounding analyte. In another prism based technique called the Otto geometry of ATR, a thin dielectric layer is inserted between the prism and metal film. The reflectivity plots exhibit dips associated with the transference of energy from photons to the surface plasmons. The plasmonic sensors based on prism coupling techniques can be operated in both angular and spectral modes [1, 47]. Although, prism based sensors have high sensitivity, these cannot be used for outdoor use. Another way to optically excite the surface plasmons is based on the diffraction of the radiation from a diffraction grating [810]. Radiation incident from dielectric medium on the metallic grating gets diffracted along different directions. The diffracted wave can couple to the surface plasmons when its propagation constant becomes equal to that of the surface plasmons. Surface plasmons can also be excited by the mode energy confined in the optical waveguide. Metal-clad optical waveguides absorb the TM mode. Surface plasmons being of TM nature , interact with guided TM mode and the energy transfers to the metal film where it gets absorbed. The application of plasmonic integrated optical (IO) waveguides in bio/chemical sensing offers large number of benefits including immunity to electromagnetic interference, high sensitivity, low cost, possibility of remote sensing and ruggedness. Numerous optical waveguide SPP sensors have been demonstrated [1, 1119]. The plasmonic IO sensors are based on the measurement of change in intensity or the change in spectral resonant position of the propagation modes when refractive index of the analyte changes. The properties of the guided modes of a plasmonic waveguide are largely dependent on the thickness of the metal film. For a thick metal film two surface plasmon modes exist on both sides of its interfaces with surrounding dielectric medium. When the metal film thickness is smaller than the penetration depth, the two modes start interacting with each other and give rise to two new modes. These modes are the symmetric and anti-symmetric surface plasmon polaritons ( on the basis of their magnetic field distribution being symmetric or anti-symmetric with respect to the middle of the metal film). Symmetric plasmonic mode is also called the long range surface plasmon-polariton (LRSPP) mode as it is characterized by small attenuation. It's mode effective index is also lower than the other mode. The anti-symmetric mode is known as the short range surface plasmon-polariton (SRSPP) because of its lossy behavior. Recently we have studied planar plasmonic multilayer waveguide structure supporting a LRSPP mode and a guided dielctric waveguide mode yielding a very high sensitivity and resolution for power mode operation for aqueous medium [14]. The coupling between the the hybrid LRSPP mode and the guided mode is very sensitive to the change in refractive index of the analyte. The design of the sensor was based on the supermode analysis of optical modes [14]. Refractive index sensor based on the coupling of SRSPP and a dielectric waveguide mode has also been reported [17, 18]. These sensors have sensitivity of 8880 dB/RIU and resolution of 7.3 × 10–6 RIU. The sensors were designed to operate for the analyte refractive index around 1.45 and 1.56. The spectral interrogation of these sensors has not been performed. Numerical study of IO plasmonic based sensors has been done for analyte index around 1.33 which is suitable for bio-sensing applications [19]. The major challenge for designing IO based plasmonic sensors is the material of the substrate so that effective mode index is close to index of water. As an alternative, if a high index layer is provided between the metal and the analyte, effective index of the plasmonic mode may be matched to that of the guided mode [14]. This reduces the sensitivity. Also, it is very difficult to excite the LRSPP mode without the use of a high index layer beneath the analyte. Using a low index substrate can possibly match the mode index to the plasmonic mode. High spectral sensitivity for these structures with buffer layer between the waveguide and the metal film has been obtained for aqueous medium [19]. However the reported length of the sensing section is quite large (6 mm). They have also reported the numerical results of the studied sensor using SRSPP with maximum sensitivity of 3700 nm RIU−1. Despite being much explored, detailed spectral interrogation of SRSPP based IO sensor has not been done so far, particularly, for aqueous medium.

We propose an integrated optic plasmonic sensor for aqueous medium which is based on hybrid coupling between SRSPP mode and dielectric optical waveguide. The sensitivity and the resolution come out to be 3.2 × 104 nm RIU−1 and 6.2 × 10–8 RIU respectively.

2. Structure and analysis method

The proposed sensor is shown in figure 1. z is the direction of the mode propagation. The sensor structure consists of three sections. The substrate material, the waveguide material and its thickness are kept same for the three sections. In the central section the waveguide is covered with a low index buffer layer above which a thin layer of metal is considered. Semi-infinite analyte covers the metal film. The cover material for the input and output waveguides is same as that of the buffer layer. For aqueous medium ( as in bio-sensors) the waveguide mode effective index itself should be close to the index of the plasmonic mode supported by the metal film. This requires the substrate with a lower refractive index than the analyte index. We choose Teflon grade AF 1300 (ns = 1.305) as the substrate material, MgF2 (nf = 1.37) as the waveguide material and Teflon grade 2400 (nb = 1.275) as the buffer layer [20]. There are many low index polymer materials which can also be considered as waveguide materials in place of MgF2. The metal considered is gold (Au), which has high chemical stability. The refractive index of gold is taken from the fitted data of Johnson and Christy [21]. The sensor is designed to operate near wavelength (λ) of 1.55 μm and can be used for sensing aqueous medium including bio- molecules/tissues. The losses in biological tissues are small around this wavelength [22]. The waveguide layer thickness is taken as 0.75 μm (for all three sections) which ensures single mode operation. The sensor section supports two mode, the guided mode and the plasmonic mode supported by the metal film. The asymmetry of index does not allow the metal film to support LRSPP mode at small buffer thicknesses. However, the asymmetric mode is always supported. For small buffer thicknesses the guided mode and plasmonic mode do not merely interact but change the mode profile of each other and the coupled mode theory cannot be applied to study mode coupling. We now consider the sensing structure as a single waveguide supporting two modes (guided and plasmonic). Supermode analysis can be applied to study the mode interaction in sensor section. The total field pattern at any position along the sensing section is given by the summation of these super modes. The propagation constants of the modes supported by the sensing section and those of the input and output waveguides are obtained by using the Transfer matrix method [8]. It is a fast and efficient method to find the eigenvalue equation of the multilayer waveguide structure. The refractive index of each layer is assumed to be constant. The magnetic (TM) field in ith layer can be expressed as sum of forward and backward propagating fields and is given by

Equation (1)

where, ${k}_{i}=\sqrt{{\beta }^{2}-{\left({k}_{0}{n}_{i}\right)}^{2}},$ ${A}_{i}$ and ${B}_{i}$ are the complex field coefficients, ${k}_{0}$ is the free space wavevector and $\beta =\,{k}_{0}n,$ $n$ being the mode effective index and ${x}_{i}$ is the position of ith layer. Eigenvalue equation of the multilayer structure can be obtained by matching the magnetic fields at various layer interfaces. The roots of this equation are the propagation constants of the supermodes. Since there are two discontinuities ( at z = 0 and z = L), we apply the mode expansion and propagation method to find input-output mode power relation. The mode in the input waveguide excites the modes supported by the sensor section. The mode energy from the central section is fed to the output waveguide where it is detected. The single mode input waveguide, with field distribution $\psi \left(x,z=0\right),$ provides the power to the sensing section and field at z = 0 can be expressed as [14]

Equation (2)

m is the number of modes supported by the sensing section and ${\phi }_{i}$ is the field of ith eigenmode of the waveguide. The sum includes all the eigenmodes. Power transferred to the radiation modes has been ignored. The eigenmodes of the sensor are complex orthogonal and the normalized expansion coefficients ${a}_{i}$ are defined as

Equation (3)

The power from the sensing section is coupled to the output waveguide which is also z-invariant with eigenvalue $\psi ^{\prime} .$ Again, if we assume that there is no reflection, the coupling coefficients ${b}_{i}$ between eigenmodes of the sensor (z = L) can be expressed as

Equation (4)

The total power in the sensor of length L is

Equation (5)

The resultant field due to the two modes can be expressed as

Equation (6)

Figure 1.

Figure 1. SPP sensor structure.

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3. Results and discussions

The most important parameters for designing the integrated optic plasmonic sensor are the buffer and metal thicknesses. We choose a metal (Au) thickness as 0.03 μm. At this thickness, the SRSPP mode index for buffer/metal/analyte structure is close to that of the input and output waveguides. Since the structure is asymmetric, LRSPP mode is not supported at this metal thickness for the present structure. The mode effective index and the losses (of SRSPP mode) increase with decrease in the metal thickness, which is it's unique characteristic [1, 2]. With metal layer thickness fixed, the buffer layer thickness is varied so that the effective index and losses of the SRSPP and the guided mode are nearly equal. Mode indices of both the modes change with variation in buffer thickness [23]. At the buffer thickness at which the effective indices and the losses of both the modes are nearest, periodic coupling between them takes place along the sensor length and the output power is modified accordingly . Further slight optimization of buffer/metal layer can provide a resonant dip (not necessarily at first maximum) . However the resonant dip position can be shifted to the smallest distance by decreasing the metal layer thickness. The optimized buffer and metal thicknesses for the present structure are 1.3 μm and 0.031 μm respectively. The modes supported by the sensor section interfere to produce beats which arise due to periodic coupling between the two modes. Figure 2 shows the output power for varying length of the sensor section. There is a very sharp resonant dip in the transmitted power curve at L = 109.4 μm for analyte index na = 1.319, which is the refractive index of water at λ = 1.55 μm.

Figure 2.

Figure 2. Variation of output power with sensor length for two analye indices.

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Although there is still periodic coupling between the modes, there is strong dip at the first position (at smallest sensor length) and the coupling becomes weak as the modes travel further. At L = 109.4 μm the interference of the guided mode and the SRSPP mode completely eliminates the power from the dielectric waveguide. The beat period between the two supermodes is expressed as π/(Re(β1)—Re(β2)) and is independent of the amplitude phases. The mode amplitudes are complex quantities and contribute to accumulation of phases of the two modes. The amplitude phases do not vary periodically resulting in sharp resonances only at the resonant length of the sensor section. For a small change in analyte index, the resonant dip disappears for na = 1.3193 as shown in figure 2. The field profile of the two supermodes is shown in figure 3.

Figure 3.

Figure 3. Field profiles of the eigenmodes of sensing section for (a) L = 0 μm and (b) L = 109.4 μm.

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Figure 3(a) shows the field profile for the modes at L = 0 μm. We can observe that the asymmetric field distribution with reference to middle of the metal film is a representation of the SRSPP mode. The two mode are in phase in the dielectric region and thus the mode energy lies within the dielectric waveguide. At L = 109.4 μm , the modes meet in opposite phase in the dielectric guide region and hence the mode energy transfers to the metal film where it is absorbed almost completely as shown in figure 3(b). The power exchange from the waveguide mode to the plasmonic mode is shown in figure 4.

Figure 4.

Figure 4. Simulated mode power at optimized layer thicknesses.

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It can be observed from the figure 4 that at L = 109.4 μm the power from the guide is completely eliminated. The power is transferred to the metal film where it is absorbed. The color bar on the left side represents the colour scale for relative power. The sensing structure is very sensitive to the variation in the analyte index. For a small change in analyte index, the resonant dip disappears. We can get it again at some other operating wavelength. For an increase in analyte index the resonant dip reappears at larger wavelength. The change in resonant wavelength with the change in analyte index can be applied to sense the index variation in analyte medium. The variations of output power for na = 1.3190, 1.3193 and 1.3196 are shown in figure 5.

Figure 5.

Figure 5. Wavelength dependence of output power for various analyte indices.

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It can be noted that the resonant dip shifts to the higher wavelength as the analyte index is increased. The spectral sensitivity ( defined as the ratio of change in resonant wavelength to the change in analyte index, s) calculated from the curves of figure 5 is 3.2 × 104 nm RIU−1, which is about seven times higher than that of reported in [19]. For a baseline noise of 1.4 pm the resolution is 6.2 × 10–8 RIU. The Full width at half maximum (FWHM, w) for these curves is 2 nm resulting in a figure of merit (FOM = s/w) of 1.6 × 104 RIU−1.

A comparative study of some efficient sensors based on SPR is presented in table 1. The proposed sensor has much higher sensitivity than most of these sensors reported recently. Only the sensors based on LRSPP have higher sensitivity (than that of presented here). This is due to low loss nature and stronger dependence on index symmetry on both sides of the metal film of the LRSPP mode. We hope that LRSPP excitation in our proposed geometry will produce more efficient sensors. The study is underway and the results will be communicated separately.

Table 1. Comparison for spectral interrogation with some recently published work.

Sensor StructureAnalyte index rangeWavelength region (μm)Sλ (nm RIU−1)FWHM (nm)Resolution (RIU)References.
SPR with topological insulator1.33–1.3330.4–1.05333[5]
Waveguide controlled LRSPP1.361–1.36111.54–1.592.6 × 105 35.38 × 10–9 [7]
Diffraction grating coupled to thin metal film1.33–2.01.3–2.01000[8]
LRSPP in H-shaped optical fiber1.36–1.390.4–1.607540271.3 × 10–5 [11]
Symmetrical dual D-shaped PCFs1.36–1.410.65–1.50146606.82 × 10–6 [12]
Asymmetric metal clad dielectric waveguide1.3–1.41.0–1.807724.9[13]
Waveguide based SRSPP refractometer1.32–1.340.625–0.655370065[19]
Waveguide based LRSPP refractometer1.3209–1.33031.03–1.081.2 × 105 18[19]
Integrated optic SRSPP1.3190–1.31961.542–1.5583.2 × 104 26.2 × 10–8 Present work

'-'represents not mentioned in the article

The sensor can also be operated in power interrogation mode. The analyte index is varied and corresponding change in mode power is observed, keeping the operating wavelength fixed (1.55 μm, here). The output power variation with change in analyte index is shown in figure 6.

Figure 6.

Figure 6. Output power variation with analyte index.

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The extremely small width of the curve indicates very high intensity sensitivity. The sensitivity(Sp) comes out to be 6.2 × 106 dB/RIU which is an order of magnitude higher than that obtained by using the LRSPP mode in our previous publication [14]. The resolution of the sensor (∆p/Sp, where ∆p = 0.01 dB, the power resolution of optical power meter) is about 1.58 × 10–9. This mode of sensing is useful when the changes in the index is extremely small (e.g. to continuously observe the real time bio/chemical reactions). When the index change is large ( in fourth or fifth decimal place), spectral interrogation is preferred.

4. Conclusions

We theoretically propose highly compact integrated optic plasmonic sensor for aqueous environment which is based on the hybrid coupling of the dielectric guided mode and the SRSPP mode supported by a thin metal film. The sensor operates in both spectral and intensity measurement mode. In particular, sensor based on spectral configuration for aqueous medium with very high sensitivity is presented. The sensor is designed to operate at communication wavelength (1.55 μm) at which the losses of many biological tissues are the lowest. The obtained spectral sensitivity is 3.2 × 104 nm RIU−1 with a resolution of 6.2 × 10–8 RIU. This sensor can also be operated in intensity interrogation mode with very high sensitivity. The presented design procedure can be adopted to design a compact refractive index sensor for various analyte materials and wavelengths.

Acknowledgments

I thankfully acknowledge Prof. Madhu Pruthi, the Principal, Keshav Mahavidyalaya, for granting me the sabbatical leave, during which this work was carried out.

Data availability statement

All data that support the findings of this study are included within the article (and any supplementary files).

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