Design Optimisation of Optical Planar Waveguide Sensor for Microplastics Detection in Water

doi:10.21203/rs.3.rs-2222268/v1

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Design Optimisation of Optical Planar Waveguide Sensor for Microplastics Detection in Water

https://doi.org/10.21203/rs.3.rs-2222268/v1

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Nowadays, microplastics pollution has become a global concern as it endangers the ecology and marine animals and causes health threats to human beings. This paper demonstrated a simulation of an optical planar waveguide sensor design for microplastics detection in water via Wave Optics Module-COMSOL Multiphysics®. The design optimisation of the sensor was conducted by changing the cladding thickness ranging from 0 to 5 µm. The change in analyte refractive index ranging from 1.480 to 1.500 refractive index unit (RIU) in reference to the microplastics’ refractive index has been performed to evaluate the simulated sensing performance. The simulation results showed that reducing the cladding thickness can vastly improve the sensor sensitivity. The sensor showed the highest sensitivity in the absence of cladding layer i.e.; the cladding thickness of 0 µm with 10⁷ difference in the order of magnitude of sensitivity compared to that of 5-µm thickness. Thus, the optimised sensor design with 0 µm cladding thickness assures an excellent sensor for microplastics detection in water.

Optical planar waveguide sensor

Microplastics detection

Cladding thickness

Simulation

Refractive Index

Plastics are widely used in almost every industry around the globe and are incredibly practical for everyday items like packaging, food storage, and drinking containers. However, improper recycling or reuse of post-consumer plastics can evidently cause environmental damage. Microplastics invasion of water generally occurs due to the degradation of plastics in water bodies such as rivers, lakes, and seas [1]. Not only does this put aquatic species in peril, it may also impose risks on human health.

In light of this, various studies on the subject of detecting microplastics in water have been carried out and published based on optical, visual, and chemical examinations, such as Raman, near-infrared (IR), and Fourier transform infrared (FTIR) spectroscopy [2]. However, there are some difficulties that seem contentious and impose hotly debated concerns on the methodologies of microplastics detection in water. Among other things, spectroscopic methods have drawbacks such as labour-intensive processing stages that result in limited yield at exorbitant expense.

Since the detection of the microplastics in water is intricate due to the aforementioned properties, a compact europium-aluminium (Eu-Al) doped polymer optical planar waveguide sensor, fabricated by using a micro-dispenser machine may be necessary to ascertain the presence of transparent and translucent microplastics in water. This is based on the fact that an optical polymer planar waveguide has superior properties [3–6], which include strong evanescent fields [7–9], highly flexible, small and compact, and cost-effective manufacturing for large scale production especially through the Mosquito method [10–13]. Aside from that, the europium-aluminium doped polymer exhibits sharp red emission at 617 nm with high brightness and intensity [2, 14, 15]. Hence, transparent microplastics information can apparently be revealed through the refraction of light emission.

In this paper, a circular core optical planar waveguide sensor for microplastics detection in water was simulated via Wave Optics Module-COMSOL Multiphysics® software. Despite the limited available literature on this particular sensor design, its fabrication process and advantages as mentioned before have proven its feasibility as a new optical waveguide sensor design. Previous works have never reported the design optimisation in terms of cladding thickness. Thus, this paper will examine the effect of cladding thickness on the sensitivity performance of the sensor. Furthermore, the electric field distribution throughout the sensor, the mode field diameter (MFD) and effective Refractive Index (RI) changes at different analyte RI mediums with their underlying physics were also studied and discussed.

The single-mode optical planar waveguide structure under investigation is illustrated in Fig. 1. It consists of a core (n = 1.510 RIU) diameter of 10 µm surrounded by cladding (n = 1.501 RIU) with width and thickness of 80 µm and 45 µm, respectively. The single-mode optical waveguide sensor was chosen to be simulated because it tends to have higher sensitivity than its multi-mode counterparts [16]. Figure 1(a) depicts the 3-dimensional (3D) structure of the newly developed europium-aluminium doped polymer composite optical planar waveguide sensor. A sensing medium i.e., analyte for simulation purposes, is placed above the structure as shown in the cross-section view in Fig. 1(b). The design was optimised at different cladding thicknesses ranging from 0 to 5 µm to access the penetration depth of the evanescent wave.

The RI of the sensing medium i.e., microplastics, was set from 1.480–1.500 RIU, with an RI step increment of 0.005 RIU. This range of analyte RI was chosen because 1.490 RIU in the middle range represents the well-known low-density polyethylene (LDPE) RI [17]. Besides, all the RI mediums were set to isotropic. Moreover, 617 nm was chosen as the operating wavelength, which is where the lowest attenuation window of the practical waveguide is established [18].

The circular core planar waveguide sensor has a similar working principle as D-shaped or C-shaped optical fibre sensor (OFS). Changes in the sensor structure and the analyte medium will change the speed of light propagation in the sensor and in turn, change the effective RI, ${n}_{eff}$ value of the sensor. Numerically, the ${n}_{eff}$ can be described as in Eq. 1 [19]:

$${n}_{eff}= \frac{c}{{v}_{g}}$$

(1)

where $c$ is the speed of light in vacuum and ${v}_{g}$ is the group velocity of light that propagates through the planar waveguide. Herein, the sensor depends on the interaction of the evanescent wave between the sensor and the analyte medium. The evanescent field senses the changes in the RI distribution caused by the analyte medium, thus inducing the ${n}_{eff}$ of the structure. In the analyte medium, the evanescent wave will decay exponentially with a distance. The distance of evanescent wave penetrate through the analyte is called penetration depth, ${d}_{p}$ which can be formulated as in Eq. (2) [20]:

$${d}_{p}= \frac{}{2\pi \sqrt{{n}_{eff}^{2}\theta -{n}_{a}^{2} }}$$

(2)

where $\lambda$ is the operating wavelength and ${n}_{a}$ is the analyte RI. Based on Eq. (2), The changes in ${n}_{a}$ and ${n}_{eff}$ will change the ${d}_{p}$ value. Thus, the RI of an unknown analyte mediums can easily be differentiatedvia the evanescent wave interaction. Then, to get the sensitivity for each cladding thickness, a graph of ${n}_{eff}$ vs ${n}_{a}$ will be plotted respectively.

Hence, in this work, the design optimisation of optical planar waveguide sensor for microplastics detection in water by modifying the cladding thickness is studied with the objective of achieving the maximum sensor sensitivity.

The outcome of the simulation is shown in two different formats. At the beginning, a coloured representation of the electrical field intensity through the sensor structure aids by visualising optical properties including light propagation, electric field distribution, light intensity, and evanescent wave. This is followed by a graph plotted to analyse the simulated device’s mode field diameter (MFD) and effective RI value.

The single mode profile is shown in Fig. 2. Here, the coloured visualisation of the electric field distribution through the planar waveguide sensor at various cladding thicknesses ranging from 0 to 5 µm when subjected to the analyte RI of 1.480 is presented. The maximum and minimum electric field intensities are represented by the colours red and blue, respectively. The electric field in the sensor structure for all three cladding thicknesses is concentrated at the centre and is almost evenly distributed, indicating that the light is strongly constrained in the core. Nonetheless, it can be observed that the electric field is slightly shifted away from the analyte when the cladding thickness is reduced due to the lower RI contrast between core/cladding compared to the core/analyte boundary. In every scenario, the colour changed from red to blue, demonstrating how the electric field gradually diminishes as it approaches the core/cladding and core/analyte boundaries due to the attenuation of light energy as it approaches the boundary of distinct RI mediums.

Figure 3 shows the MFD at a selective RI of 1.480 for three different cladding thickness, which can be used to analyse the electric field distribution of the planar waveguide sensor. Reducing the cladding thickness from 5 to 0 µm results in a significant change in MFD. The evanescent wave’s penetration at the boundary between the core and the analyte diminishes as the cladding thickness is reduced. The lowest penetration depth is at 0 µm cladding thickness, with an estimated depth of 2 µm, and the highest penetration depth is at 5 µm cladding thickness, with an estimated depth of 4 µm. It is important to note that the penetration depth depends on the RI difference between the core, cladding and the analyte medium. For example, the higher RI difference between core and analyte compared to core and cladding leads to lower penetration. Even though penetration depth at 0 µm cladding thickness is the lowest since the RI difference is higher, it is the most sensitive because the evanescent wave can fully interact with the analyte medium. Comparatively, the higher penetration depth at 5 µm cladding thickness did not contribute much to sensor sensitivity because most of the evanescent wave is in the cladding and only a tiny little part of it interacted with the analyte. Figure 4 provides a summary of this explanation.

Figure 5 shows the penetration depth at different analyte RI for different cladding thickness. For 0 µm and 2 µm cladding thicknesses, all analyte RIs from 1.480 – 1.500 RIU, with an RI step increment of 0.005 RIU can be ‘sensed’ distinctively with a great sensitivity by the sensor. This is proven in Figure 5(a) and 5(b) as the curve for each analyte RI does not overlap and can be differentiated clearly. It also can be observed that the curves separation for 0 µm cladding thickness is higher compared to the case of 2 µm. However, for the same scale of y-axis in the case of 5 µm cladding thickness, all the curves are overlapping as shown in Figure 5(c). Only when scaled down 100 times with respect to the y-axis, the curves can be differentiated, as depicted in the inset of Figure 5(c).

Figure 6 shows the registered ${n}_{eff}$ changes plotted at different analyte RIs for each cladding thickness. The magnified graph is shown in the inset, demonstrating the two different curves of 2 µm and 5 µm cladding thicknesses. The ${n}_{eff}$ changes increase nonlinearly as the analyte RI rises from 1.480 to 1.500. The non-linear change happens due to the nonlinearity of evanescent wave energy when interacting with the analyte medium [21]. However, there is an extremely small ${n}_{eff}$ change that occurred at the 5 µm cladding thickness. The significant ${n}_{eff}$ changes can be seen when reducing the cladding to the minimum of 0 µm thickness.

Despite the curve nonlinearity, Xiao et al. suggested an easy determination of sensor sensitivity by evaluating the changes in linear fit [21]. Thus, Table 1 shows the sensitivity changes towards the cladding thickness determined from the slope of the linear fit. It can be concluded that the cladding thickness extremely affects the sensitivity of the planar waveguide sensor. The highest sensor sensitivity is at 0 µm cladding thickness whereas 5 µm cladding thickness shows the smallest sensitivity. The difference in sensitivity between these two cases is enormous with a 10⁷ difference in the order of magnitude. This is expected since the evanescent wave fully ‘senses’ the disturbance from the analyte medium in the absence of cladding layer, which allowed more light interaction between the sensor and analyte medium.

Table 1

Sensitivity of the optical waveguide sensor at different cladding thickness
Cladding thickness (µm)	Sensitivity
0	2.75 x 10^− 4
2	3.98 x 10^− 7
5	8.40 x 10^− 11

The design and simulation of a circular core optical planar waveguide at analyte RIs ranging from 1.480 to 1.500 have been demonstrated in this work. The best planar waveguide sensor sensitivity could be determined by conducting design optimisations at various cladding thicknesses. The optimised 0 µm cladding thickness waveguide sensor measuring various analyte RI exhibits distinct electric field distribution and evanescent field without any overlapping during simulation evaluation. The waveguide sensor's highest estimated sensitivity is at 0 µm of cladding thickness, where there is a 10⁷ order of magnitude difference in sensitivity when compared to the 5 µm of cladding thickness. Essentially, the circular core optical planar waveguide sensor concept as demonstrated in the simulation is deemed reliable for water microplastics detection.

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Conflicts of interests

The authors have no conflicts of interest to declare.

Authors’ Contributions

Ahmad Izzat Mohd Hanafi: Investigation, Methodology, Simulation and Writing - original draft. Nur Najahatul Huda Saris: Conceptualization, Supervision, Writing - review & editing. Sevia Mahdaliza Idrus: Supervision, Writing - review & editing. Nazirah Mohd Razali: Methodology and Simulation

Funding

This work was supported by UTM Fundamental Research (UTMFR) grant vot number Q.J130000.3851.21H76 and Leave a Nest Research Grant.

Availability of Data and Materials

Data available on request from the authors.

Acknowledgement

This research is a collaborative project of Faculty of Electrical Engineering and Malaysia Japan International Institute of Technology (MJIIT), Universiti Teknologi Malaysia(UTM), partially funded by UTM Fundamental Research (UTMFR) grant vot number Q.J130000.3851.21H76 and Leave a Nest Research Grant. The authors would like to thank Lightwave Communication Research Group (LCRG) and Optical Device System Laboratory (ODESY) members for their help and encouragement.

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No competing interests reported.

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Design Optimisation of Optical Planar Waveguide Sensor for Microplastics Detection in Water

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