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Blue-shift ultrasensitivity using rhombus-shaped plasmonic crystal on Si3N4 membrane

Foozieh Sohrabi, Dordaneh Etezadi, Yasaman Jahani, Ershad Mohammadi, Bahareh Ghadiani, Mahdi Tamizifar, and Seyedeh Mehri Hamidi

Open Access Open Access

Abstract

Harnessing ultrasensitivity from optical structures to detect tiny changes in the targeted samples is the main goal of scientists in the field of sensor design. In this study, an uncommon rhombus-shape plasmonic structure is proposed for providing blue-shift ultrasensitivity. The physical origin of this optical response relies on multi-faces of gold rhombus and their electromagnetic coupling with their induced images in a high-refractive-index substrate (Si3N4). A characteristic of blue-shift emerges as the Fano resonance in the reflection spectrum. We have experimentally shown that this novel structure has the surface sensitivity to the refractive index difference in the order of 10−5. These characteristics have been applied for non- and conditioned- cell culture medium with refractive differences in this order.This level of sensitivity is interesting for enhanced fingerprinting of minute quantities of targeted molecules and interfacial ion redistribution.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Figures (5)
Fig. 1.
Fig. 1. (a) The evolution of the structures before reaching the optimized rhombus array. Fabrication of a rhombus-shape plasmonic crystal on the Si3N4 membrane: (b) The process flow of the fabrication of Si3N4 membrane from double Si3N4-coated wafer using photolithography, dry and wet etches. (c) The process flow of the fabrication of rhombus-shaped plasmonic crystal on this Si3N4 membrane. (d) Structural parameters of the fabricated structure. (e) Real-photo of the fabricated chip and the SEM of the chip showing the average diagonal of 125 nm for the gold nanoparticle.

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Fig. 2.
Fig. 2. (a)-(b) The simulated reflection spectrum of the proposed structure in dry and wet cases using FDTD solutions, Lumerical. By increasing the incident angle from 30° to 60°, the Fano resonance becomes more conspicuous in both dry and wet cases. (c) The schematic of the inverted microscope used for recording the normal transmission responses. (d) The sensitivity of the fabricated structure for water (n=1.33297) and the different concentrations of sucrose solutions with refractive indices of n=1.33721 (50 mg/1.5 ml total vol.), n=1.33938 (75 mg/1.5 ml total vol.) and n=1.34210 (100 mg/1.5 ml total vol.). The quantitative data of the resonance wavelengths are mentioned in Table 1. By increasing the concentration of the sucrose, the restoring columbic force increased that was required for the blue-shift emergence.

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Fig. 3.
Fig. 3. (a)-(h) The transmission spectrum of the patterned rhombus plasmonic crystal normalized to non-patterned Si3N4 substrate for water and various concentrations of sucrose. There were four rows of membranes and two membranes were patterned in each row. The columns were named “a” and “b”.

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Fig. 4.
Fig. 4. (a) Simulated transmission spectrum for various sucrose concentrations (b) enlarged image of A showing the splitting of resonance wavelengths (c) the simulated unit cell of the rhombus structure (d) the corresponding experimental results to the simulated structure.

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Fig. 5.
Fig. 5. (a)-(h) The transmission spectrum of the patterned rhombus plasmonic crystal normalized to non-patterned Si3N4 substrate at the presence of fresh and conditioned primary cell medium. There were four rows of membranes and in each row, two membranes were patterned. The columns are called “a” and “b”. The analytes of fresh and primary cell secreted culturing medium (i.e. conditioned medium) with refractive indices of 1.33541 and 1.33556 had the difference in their refractive index around 0.00015. For fresh medium and secreted medium with this low level of difference in their refractive indices, we generally had a blue-shift of 1 nm which gave us a large surface sensitivity.

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Tables (1)

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Table 1. Plasmonic resonance wavelength for patterned Si3N4 membrane. There were four rows of membranes, and two membranes were patterned in each row. The columns were named “a” and “b”.

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