Designing a Compact Plasmonic Pressure Sensor for Nanophotonic Applications

Published in the journal nanomaterials, a team of researchers has developed a small plasmonic pressure sensor that could help to advance nanophotonic devices. 

Image Credit: narong sutinkham/Shutterstock.com

Surface plasmon polaritons (SPP's) are electromagnetic (EM) waves that mediate light-matter interactions between metal nanoparticles (MNPs) and a dielectric device, which is polarised when exposed to an applied electric field. These waves travel along the dielectric-metal border and form collective oscillations of photons and electrons at resonant wavelengths. 

SPPs’ exceptional optical properties make them suitable for applications in numerous areas, particularly nanophotonics. 

A MIM (metal-insulator-metal)-based plasmonic pressure sensor is an optical element that is capable of undergoing exerted pressure and converting it into optical or electrical signals. 

To improve integrated optical circuits (IOC) compatibility with the pressure sensor, its size has to be reduced. Although extensive research has been conducted on this topic, complicated fabrication processes are often involved. On top of this, researchers are faced with improving sensitivity due to advancements in current pressure sensor applications. 

In this article, a plasmonic pressure sensor was constructed at the nanoscale with a significant increase in sensitivity compared to previous efforts. These results, as well as the innovative sensors ease of design, are indicative of its great potential for advancing nanophoptonic devices. 

Methodology

Figure 1a, b depicts the schematics of the sensor. The golden and white parts in the figure depict silver (Ag) and air. In case an applied pressure F is given on the top of the Ag layer, it would bend downwards with deformation of d.

Figure 1. (a) Schematic diagram of the designed sensor. (b) Schematic diagram of an applied pressure F exerted on the Ag layer. Image Credit: Chao, et al., 2021

A TM-polarized incident light pairs with the fundamental SPP mode into the input port of the bus waveguide's (WG), and the transmission power attains the output port.

This research uses Ag as the plasmonic metal as it is cost-effective, and its EM wave response has the least imaginary part of relative permittivity within the near-infrared level resulting in low power consumption. The transmittance spectrum and EM wave distributions were stimulated with FEM-based COMSOL Multiphysics with ultrafine mesh sizes.

The transmittance spectrum for the developed structure without exerting pressure on the system is depicted in Figure 2.

Figure 2. Transmittance spectrum for the designed structure without pressure on the system (i.e., d = 0 nm). Image Credit: Chao, et al., 2021

The advancements in nanofabrication enabled easy fabrication of the designed pressure sensor. The MIM waveguide comprising of a rectangular shape was achieved with stripping and ion beam lithography processes. 

Results

The bus WG’s width was retained as w = 50 nm to ensure that the TM mode travels in bus WG. The resonator size of the developed sensor is compact and very smaller than earlier designs.

Table 1 compares the resonance wavelength (λ_res), dipping strength (∆D), FWHM, and QF of the structure at corresponding resonance modes. It was noted that the resonance dips in the developed sensor had a more profound ∆D, a higher QF, and a narrower FWHM. Moreover, the transmission loss has the least influence on sensing performance as the designed device has fewer transmission loss values at resonance modes.

Table 1. Comparison of λ_res, FWHM, ΔD, and QF of the designed structures at corresponding resonance modes. Source: Chao, et al., 2021

The normalized magnetic field intensity and electric field intensity at the corresponding resonance modes is illustrated in Figure 3.

Figure 3. Normalized (a) magnetic field intensity and (b) electric field intensity at the corresponding resonance modes and one of the off-resonance modes. Image Credit: Chao, et al., 2021

The transmittance spectra of the developed structure with a variation of d from 0 to 10 nm is shown in Figure 4.

Figure 4. Transmittance spectra of the designed structure with a variation of d from 0 to 10 nm with an interval of 2 nm in the wavelength of (a) 700–3500 nm for modes 1–3, (b) 1100–1300 nm for mode 3, (c) 1300–1700 nm for mode 2, and (d) 2500–3000 nm for mode 1, respectively. Image Credit: Chao, et al., 2021

A pressure influence on the upper horizontal slot indicates deformation. Figure 5 illustrates the linear relationship between λ_res and d.

Figure 5. Resonance wavelength versus variation of d from 0 to 10 nm with an interval of 2 nm. Image Credit: Chao, et al., 2021

The transmittance spectrum of the designed structure when the deformation d  in various nm is illustrated in Figure 6.

The transmittance dip would redshift as d increases and fade when d = 50 nm also, the FWHM enlarges and the dipping strength decreases with an increase in d. This results in asymmetricity impacting the linearity of the resonant wavelength vs. d.

Figure 6. Transmittance spectrum of the proposed structure when deformation d = 20 nm, 30 nm, 40 nm, and 50 nm, respectively. Image Credit: Chao, et al., 2021

The observations show that the structural parameters have a major impact on the sensing performance of developed plasmonic pressure sensors (see Figure 7).

Figure 7. Transmittance spectra of the designed structure with variation of the height of stubs (b), (a) b = 50 nm, (b) b = 100 nm, (c) b = 150 nm, and (d) b = 200 nm, respectively. Image Credit: Chao, et al., 2021

The resonance wavelength shift (∆λ_res) and sensitivity S(nm/MPa) were compared and listed in Table 2.

Table 2. Comparison of [∆λ_res(nm), S(nm/MPa)] of the designed structure for b = 50, 100, 150, and 200 nm. Source: Chao, et al., 2021

The transmittance spectra of the developed structure containing variation in the number of stubs (N), when N = 3, 5, 7, and 9 are shown in Figure 8.

Figure 8. Transmittance spectra of the designed structure with a variation of the number of stubs (N), (a) N = 3, (b) N = 5, (c) N = 7, and (d) N = 9, respectively. Image Credit: Chao, et al., 2021

Table 3 lists the (∆λ_res, S) of the developed structure for N = 3, 5, 7, and 9.

Table 3. Comparison of (∆λ_res(nm), S(nm/MPa)) of the designed structure for N = 3, 5, 7, and 9. Source: Chao, et al., 2021

Table 4 shows the comparisons between the developed plasmonic pressure sensor with the published designs. The sensor shows increased sensitivity (≈23.32 times).

Table 4. Comparison of the designed plasmonic pressure sensor with the published designs. Source: Chao, et al., 2021

Here, a compact and simple plasmonic pressure sensor structure featuring a bus waveguide and a resonator with a horizontal slot and numerous stubs was developed. This sensor can be applied for small-scale ultrahigh-pressure sensitivity.

The observations show that the resonance wavelength redshift comprises a linear relationship with the resonator’s deformation. The sensor developed possesses an ultra-high sensitivity of 592.44 nm/MPa having the highest value and achieving 23.32 times more sensitivity. 

Continue reading: Using Surface Enhanced Raman Spectroscopy for Molecular Sensing.

Journal Reference:

Chao, C.-T. C., Chau, Y.-F. C., Chen, S.-H., Huang, H. J., Lim, C. M., Kooh, M. R. R., Thotagamuge, R., Chiang, H.-P. (2021) Ultrahigh Sensitivity of a Plasmonic Pressure Sensor with a Compact Size. Nanomaterials, 11(11), p. 3147. Available online: https://www.mdpi.com/2079-4991/11/11/3147/htm.

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