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Structured metal thin film as an asymmetric color filter: the forward and reverse plasmonic halos.

Ye F, Burns MJ, Naughton MJ - Sci Rep (2014)

Bottom Line: We explain this by a three-step process: coupling of photons to surface plasmon polaritons (SPPs), wave interference of SPPs forming resonant cavity modes, and out-coupling from SPPs to photons.Full wave electromagnetic simulations based on the finite element method support our findings.These results may have potential applications in areas such as optical color filtering and biosensing via dielectric detection within the step gap plasmonic cavity.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, Boston College, Chestnut Hill, MA, 02467, USA.

ABSTRACT
We observe asymmetric color filtering under unpolarized incidence in a structured metallic (Ag) film, where the center of an optically thick circular Ag disk surrounded by a step gap appears dark when observed from one side, and bright from the other. The latter situation corresponds to abnormally high optical transmission through the optically thick film. We explain this by a three-step process: coupling of photons to surface plasmon polaritons (SPPs), wave interference of SPPs forming resonant cavity modes, and out-coupling from SPPs to photons. Full wave electromagnetic simulations based on the finite element method support our findings. These results may have potential applications in areas such as optical color filtering and biosensing via dielectric detection within the step gap plasmonic cavity.

No MeSH data available.


Related in: MedlinePlus

Two dimensional full wave simulation with axial symmetry.(a) Schematic of simulated structure, with axis of symmetry labeled (r = 0). Perfectly matched layers (PML) on the right and bottom sides are indicated by hatched regions. Light is input from the top side. (b) & (c) Profiles of intensities of the vertical component /Ez/2 (b) and transverse component /Et/2 (c) of the electric field for 490 nm incidence, under the same color scale ranging from 0 to 1013 V2/m2.
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f3: Two dimensional full wave simulation with axial symmetry.(a) Schematic of simulated structure, with axis of symmetry labeled (r = 0). Perfectly matched layers (PML) on the right and bottom sides are indicated by hatched regions. Light is input from the top side. (b) & (c) Profiles of intensities of the vertical component /Ez/2 (b) and transverse component /Et/2 (c) of the electric field for 490 nm incidence, under the same color scale ranging from 0 to 1013 V2/m2.

Mentions: Two dimensional full wave electromagnetic simulations (using COMSOL MultiPhysics 4.3b) with axial symmetry were carried out to assist in our understanding of this phenomenon. As shown in Fig. 3, a plane wave is incident from the top side of the structure, with a k vector along the −z direction, H vector pointing out of plane, and E vector pointing in the horizontal, radial direction. When revolved 360 degrees, this corresponds to the situation of radially polarized electric field input. Although this excitation scheme is not identical to that employed in the experiments, it mimics the unpolarized nature of the incident light. The simulated circular cavity has a diameter of 4 μm, with other parameters defined in Fig. 1(c). The transverse electric field is obtained by /Et/2 = /Er/2 + /Eθ/2. The incident electric fields are first coupled to SPPs and travel to the bottom side of the Ag/ITO interface through the step gap region, as shown in Fig. 3(b). The SPPs then form resonant drumhead modes along the circular Ag/ITO interface, resulting in a center intensity maximum. This SPP focusing phenomenon along circular geometries is widely observed and studied71115. The down-propagating electric field has two origins: one is from direct tunneling through the optically thin step gap region, resulting in the bright outer rings similar to those seen in the plasmonic halo effect; the other is from coupling from the SPP component in the form of radiation loss of SPPs due to surface and grain-boundary scattering on the Ag/ITO interface16, resulting in the bright centers of the step gap cavities. The reasons for a much weaker radiation coupling effect on the Ag/air interface when illuminating from the bottom (the plasmonic halo) than on the Ag/ITO interface (the reverse halo) are twofold. First, the Ag/ITO interface on the bottom has an open optical boundary below it. With no other forms of modes to couple to, the SPPs along the Ag/ITO interface are scattered directly into free propagating photons, as opposed to the Ag/air interface on top, which is within a circular cavity formed by the step-gap, allowing the SPP-generated photons to interfere with photons directly tunneled through the step-gap11. Second, for SPPs, the Ag/ITO interface has a much larger effective radius than the Ag/air interface with the same physical dimension. For example, for incident light with 490 nm free space wavelength, the SPP wavelength along a Ag/air interface (λSPP(air) = 459 nm) is ~5 times that along a Ag/ITO interface (λSPP(ITO) = 87 nm), indicating a corresponding increase in the effective radius along the Ag/ITO interface. This much larger effective radius along the Ag/ITO interface assists the SPP-photon coupling process16. Furthermore, the higher dielectric constant of ITO effectively increases the surface roughness and grain boundary size of the Ag film along the Ag/ITO interface, enlarging the radiation loss of SPPs. In summary, the geometric difference and the dielectric environment difference result in a drastically different out-coupling pathway for bound SPPs, leading to the bright/dark contrast of the top and bottom Ag surface under two illumination schemes.


Structured metal thin film as an asymmetric color filter: the forward and reverse plasmonic halos.

Ye F, Burns MJ, Naughton MJ - Sci Rep (2014)

Two dimensional full wave simulation with axial symmetry.(a) Schematic of simulated structure, with axis of symmetry labeled (r = 0). Perfectly matched layers (PML) on the right and bottom sides are indicated by hatched regions. Light is input from the top side. (b) & (c) Profiles of intensities of the vertical component /Ez/2 (b) and transverse component /Et/2 (c) of the electric field for 490 nm incidence, under the same color scale ranging from 0 to 1013 V2/m2.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4248275&req=5

f3: Two dimensional full wave simulation with axial symmetry.(a) Schematic of simulated structure, with axis of symmetry labeled (r = 0). Perfectly matched layers (PML) on the right and bottom sides are indicated by hatched regions. Light is input from the top side. (b) & (c) Profiles of intensities of the vertical component /Ez/2 (b) and transverse component /Et/2 (c) of the electric field for 490 nm incidence, under the same color scale ranging from 0 to 1013 V2/m2.
Mentions: Two dimensional full wave electromagnetic simulations (using COMSOL MultiPhysics 4.3b) with axial symmetry were carried out to assist in our understanding of this phenomenon. As shown in Fig. 3, a plane wave is incident from the top side of the structure, with a k vector along the −z direction, H vector pointing out of plane, and E vector pointing in the horizontal, radial direction. When revolved 360 degrees, this corresponds to the situation of radially polarized electric field input. Although this excitation scheme is not identical to that employed in the experiments, it mimics the unpolarized nature of the incident light. The simulated circular cavity has a diameter of 4 μm, with other parameters defined in Fig. 1(c). The transverse electric field is obtained by /Et/2 = /Er/2 + /Eθ/2. The incident electric fields are first coupled to SPPs and travel to the bottom side of the Ag/ITO interface through the step gap region, as shown in Fig. 3(b). The SPPs then form resonant drumhead modes along the circular Ag/ITO interface, resulting in a center intensity maximum. This SPP focusing phenomenon along circular geometries is widely observed and studied71115. The down-propagating electric field has two origins: one is from direct tunneling through the optically thin step gap region, resulting in the bright outer rings similar to those seen in the plasmonic halo effect; the other is from coupling from the SPP component in the form of radiation loss of SPPs due to surface and grain-boundary scattering on the Ag/ITO interface16, resulting in the bright centers of the step gap cavities. The reasons for a much weaker radiation coupling effect on the Ag/air interface when illuminating from the bottom (the plasmonic halo) than on the Ag/ITO interface (the reverse halo) are twofold. First, the Ag/ITO interface on the bottom has an open optical boundary below it. With no other forms of modes to couple to, the SPPs along the Ag/ITO interface are scattered directly into free propagating photons, as opposed to the Ag/air interface on top, which is within a circular cavity formed by the step-gap, allowing the SPP-generated photons to interfere with photons directly tunneled through the step-gap11. Second, for SPPs, the Ag/ITO interface has a much larger effective radius than the Ag/air interface with the same physical dimension. For example, for incident light with 490 nm free space wavelength, the SPP wavelength along a Ag/air interface (λSPP(air) = 459 nm) is ~5 times that along a Ag/ITO interface (λSPP(ITO) = 87 nm), indicating a corresponding increase in the effective radius along the Ag/ITO interface. This much larger effective radius along the Ag/ITO interface assists the SPP-photon coupling process16. Furthermore, the higher dielectric constant of ITO effectively increases the surface roughness and grain boundary size of the Ag film along the Ag/ITO interface, enlarging the radiation loss of SPPs. In summary, the geometric difference and the dielectric environment difference result in a drastically different out-coupling pathway for bound SPPs, leading to the bright/dark contrast of the top and bottom Ag surface under two illumination schemes.

Bottom Line: We explain this by a three-step process: coupling of photons to surface plasmon polaritons (SPPs), wave interference of SPPs forming resonant cavity modes, and out-coupling from SPPs to photons.Full wave electromagnetic simulations based on the finite element method support our findings.These results may have potential applications in areas such as optical color filtering and biosensing via dielectric detection within the step gap plasmonic cavity.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, Boston College, Chestnut Hill, MA, 02467, USA.

ABSTRACT
We observe asymmetric color filtering under unpolarized incidence in a structured metallic (Ag) film, where the center of an optically thick circular Ag disk surrounded by a step gap appears dark when observed from one side, and bright from the other. The latter situation corresponds to abnormally high optical transmission through the optically thick film. We explain this by a three-step process: coupling of photons to surface plasmon polaritons (SPPs), wave interference of SPPs forming resonant cavity modes, and out-coupling from SPPs to photons. Full wave electromagnetic simulations based on the finite element method support our findings. These results may have potential applications in areas such as optical color filtering and biosensing via dielectric detection within the step gap plasmonic cavity.

No MeSH data available.


Related in: MedlinePlus