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Enhancing the Surface Sensitivity of Metallic Nanostructures Using Oblique-Angle-Induced Fano Resonances

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ABSTRACT

Surface sensitivity is an important factor that determines the minimum amount of biomolecules detected by surface plasmon resonance (SPR) sensors. We propose the use of oblique-angle-induced Fano resonances caused by two-mode coupling or three-mode coupling between the localized SPR mode and long-range surface plasmon polariton modes to increase the surface sensitivities of silver capped nanoslits. The results indicate that the coupled resonance between the split SPR (−kSPR) and cavity modes (two-mode coupling) has a high wavelength sensitivity for small-angle incidence (2°) due to its short decay length. Additionally, three-mode coupling between the split SPR (−kSPR), substrate (+kSub) and cavity modes has a high intensity sensitivity for large-angle incidence due to its short decay length, large resonance slope and enhanced transmission intensity. Compared to the wavelength measurement, the intensity measurement has a lower detectable (surface) concentration below 1 ng/ml (0.14 pg/mm2) and is reduced by at least 3 orders of magnitude. In addition, based on the calibration curve and current system noise, a theoretical detection limit of 2.73 pg/ml (0.38 fg/mm2) can be achieved. Such a surface concentration is close to that of prism-based SPR with phase measurement (0.1–0.2 fg/mm2 under a phase shift of 5 mdeg).

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Optical properties of the silver capped nanoslits with oblique-angle incidence.(a) A schematic configuration depicts the geometrical parameters of the capped sliver nanoslits. (b) A schematic illustration demonstrates the Fano resonances (two- and three-mode coupling) in capped nanoslits under oblique-angle incidence. (c) The measured angular transmission diagram of 520-nm-period capped nanoslit arrays with an 80-nm-thick silver film in air for TM-polarized incident light. The green and blue dashed lines show the calculated resonance wavelengths (using equation (6)) for the SPR (the metal/air interface) and substrate modes (the metal/substrate interface), respectively. The white solid circle shows the calculated resonance wavelengths (using equation (7)) for first-order Wood’s anomaly at the metal/air interface. (d) The measured transmission spectra of 520-nm-period capped nanoslit arrays in air for different incident angles from 0° to 30°. The inset shows the bandwidth was 3.9 nm for normally incident TM-polarized light.
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f1: Optical properties of the silver capped nanoslits with oblique-angle incidence.(a) A schematic configuration depicts the geometrical parameters of the capped sliver nanoslits. (b) A schematic illustration demonstrates the Fano resonances (two- and three-mode coupling) in capped nanoslits under oblique-angle incidence. (c) The measured angular transmission diagram of 520-nm-period capped nanoslit arrays with an 80-nm-thick silver film in air for TM-polarized incident light. The green and blue dashed lines show the calculated resonance wavelengths (using equation (6)) for the SPR (the metal/air interface) and substrate modes (the metal/substrate interface), respectively. The white solid circle shows the calculated resonance wavelengths (using equation (7)) for first-order Wood’s anomaly at the metal/air interface. (d) The measured transmission spectra of 520-nm-period capped nanoslit arrays in air for different incident angles from 0° to 30°. The inset shows the bandwidth was 3.9 nm for normally incident TM-polarized light.

Mentions: Figure 1a shows a schematic configuration depicting the geometrical parameters of the capped sliver nanoslits and the direction of the transverse magnetic (TM)-polarized incident light. Figure 1b shows a schematic illustration that demonstrates the Fano resonances in capped nanoslits under oblique-angle incidence. There are three resonance modes in the capped nanoslit arrays. One is the gap plasmon resonance (cavity mode) in the slit gaps, and the others are Bloch wave surface plasmon polaritons (BW-SPPs) on both sides of the periodic sliver surface (the silver/medium and silver/substrate interfaces). The cavity mode is coupled to the BW-SPP waves from the edges of the top and bottom interfaces. The light transmits through the nanoslits and capping layer, leading to a broadband transmission within the cavity spectrum. The resonance condition is estimated using a Fabry-Perot cavity32. The resonance wavelength is determined by the gap width and cavity length. The BW-SPP occurs on the periodic metallic surface when the Bragg condition is satisfied. The BW-SPP wave is scattered by the periodic grooves, resulting in narrowband transmission within the SPR spectrum. Under oblique-angle incidence, the Bragg condition for one-dimensional arrays can be described by ref. 1


Enhancing the Surface Sensitivity of Metallic Nanostructures Using Oblique-Angle-Induced Fano Resonances
Optical properties of the silver capped nanoslits with oblique-angle incidence.(a) A schematic configuration depicts the geometrical parameters of the capped sliver nanoslits. (b) A schematic illustration demonstrates the Fano resonances (two- and three-mode coupling) in capped nanoslits under oblique-angle incidence. (c) The measured angular transmission diagram of 520-nm-period capped nanoslit arrays with an 80-nm-thick silver film in air for TM-polarized incident light. The green and blue dashed lines show the calculated resonance wavelengths (using equation (6)) for the SPR (the metal/air interface) and substrate modes (the metal/substrate interface), respectively. The white solid circle shows the calculated resonance wavelengths (using equation (7)) for first-order Wood’s anomaly at the metal/air interface. (d) The measured transmission spectra of 520-nm-period capped nanoslit arrays in air for different incident angles from 0° to 30°. The inset shows the bandwidth was 3.9 nm for normally incident TM-polarized light.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5016831&req=5

f1: Optical properties of the silver capped nanoslits with oblique-angle incidence.(a) A schematic configuration depicts the geometrical parameters of the capped sliver nanoslits. (b) A schematic illustration demonstrates the Fano resonances (two- and three-mode coupling) in capped nanoslits under oblique-angle incidence. (c) The measured angular transmission diagram of 520-nm-period capped nanoslit arrays with an 80-nm-thick silver film in air for TM-polarized incident light. The green and blue dashed lines show the calculated resonance wavelengths (using equation (6)) for the SPR (the metal/air interface) and substrate modes (the metal/substrate interface), respectively. The white solid circle shows the calculated resonance wavelengths (using equation (7)) for first-order Wood’s anomaly at the metal/air interface. (d) The measured transmission spectra of 520-nm-period capped nanoslit arrays in air for different incident angles from 0° to 30°. The inset shows the bandwidth was 3.9 nm for normally incident TM-polarized light.
Mentions: Figure 1a shows a schematic configuration depicting the geometrical parameters of the capped sliver nanoslits and the direction of the transverse magnetic (TM)-polarized incident light. Figure 1b shows a schematic illustration that demonstrates the Fano resonances in capped nanoslits under oblique-angle incidence. There are three resonance modes in the capped nanoslit arrays. One is the gap plasmon resonance (cavity mode) in the slit gaps, and the others are Bloch wave surface plasmon polaritons (BW-SPPs) on both sides of the periodic sliver surface (the silver/medium and silver/substrate interfaces). The cavity mode is coupled to the BW-SPP waves from the edges of the top and bottom interfaces. The light transmits through the nanoslits and capping layer, leading to a broadband transmission within the cavity spectrum. The resonance condition is estimated using a Fabry-Perot cavity32. The resonance wavelength is determined by the gap width and cavity length. The BW-SPP occurs on the periodic metallic surface when the Bragg condition is satisfied. The BW-SPP wave is scattered by the periodic grooves, resulting in narrowband transmission within the SPR spectrum. Under oblique-angle incidence, the Bragg condition for one-dimensional arrays can be described by ref. 1

View Article: PubMed Central - PubMed

ABSTRACT

Surface sensitivity is an important factor that determines the minimum amount of biomolecules detected by surface plasmon resonance (SPR) sensors. We propose the use of oblique-angle-induced Fano resonances caused by two-mode coupling or three-mode coupling between the localized SPR mode and long-range surface plasmon polariton modes to increase the surface sensitivities of silver capped nanoslits. The results indicate that the coupled resonance between the split SPR (−kSPR) and cavity modes (two-mode coupling) has a high wavelength sensitivity for small-angle incidence (2°) due to its short decay length. Additionally, three-mode coupling between the split SPR (−kSPR), substrate (+kSub) and cavity modes has a high intensity sensitivity for large-angle incidence due to its short decay length, large resonance slope and enhanced transmission intensity. Compared to the wavelength measurement, the intensity measurement has a lower detectable (surface) concentration below 1 ng/ml (0.14 pg/mm2) and is reduced by at least 3 orders of magnitude. In addition, based on the calibration curve and current system noise, a theoretical detection limit of 2.73 pg/ml (0.38 fg/mm2) can be achieved. Such a surface concentration is close to that of prism-based SPR with phase measurement (0.1–0.2 fg/mm2 under a phase shift of 5 mdeg).

No MeSH data available.


Related in: MedlinePlus