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A virtual optical probe based on localized Surface Plasmon Polaritons

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Abstract

A confined, evanescent nano-source based on the excitation of Surface Plasmon Polaritons (SPP) on structured thin metal films is proposed. With the help of a suitable cavity, we numerically demonstrate that it is possible to trap SPP over a spatial region smaller than the diffraction limit. In particular, the enhanced plasmonic field associated with the zero-order cavity mode can be used as a virtual probe in scanning near-field microscopy systems. The proposed device shows both the advantages of a localized, non-radiating source and the high sensitivity of SPP-based sensors. The lateral resolution is limited by the lateral extension of the virtual probe. Results from simulated scans of small objects reveal that details with feature sizes down to 50 nm can be detected.

©2005 Optical Society of America

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Figures (10)

Fig. 1.
Fig. 1. Excitation of Surface Plasmon Polaritons in the Kretschmann-Raether configuration. Light incident at angle θ with respect to the surface normal, hits the silver film deposited on the flat surface of a cylindrical glass lens and is then reflected. The reflected light is used to monitor the coupling of photons with plasmon polaritons at the metal interface.
Fig. 2.
Fig. 2. Calculated map of the reflection coefficient R(λ, θ) of a flat thin silver film (t = 50 nm) illuminated in the Kretschmann-Raether configuration at different wavelengths λ and incidence angles θ. Dark zones (low reflectivity) correspond to the excitation of SPP.
Fig. 3.
Fig. 3. Calculated map of the reflection coefficient R(λ, θ) of a thin silver film illuminated in the Kretschmann-Raether configuration at different wavelengths λ and incidence angles θ. The metal-air interface is corrugated with a sinusoidal profile of period Λ = 220 nm. A band gap at λ ≈ 470 nm emerges due to the presence of the corrugation.
Fig. 4.
Fig. 4. Topographic profiles of two possible configurations of the cavity. In case (a) the modulation is a raised profile on the surface of the metal layer, while in case (b) it is etched into the layer.
Fig. 5.
Fig. 5. Convergence of the field enhancement factor as a function of the number of DBR periods at each side of the flat region considered in the calculations.
Fig. 6.
Fig. 6. Reflection (R) and transmission (T) coefficients of a DFS for SPP for different lengths of the flat region. The two plots show a typical resonance profile.
Fig. 7.
Fig. 7. Squared amplitude of the magnetic field associated with the plasmonic zero-order mode excited in the two considered DFS configurations: (a) raised profile, (b) etched profile.
Fig. 8.
Fig. 8. Virtual probe excited in a cavity surrounded by a modified DBR (see inset for a detail). The squared amplitude of the magnetic field of the virtual probe is enhanced by a factor of 94 with respect to the incident plane wave. The effect of the overetched modulation is to reduce the intensity of the lateral lobes.
Fig. 9.
Fig. 9. A glass plate with a small defect is scanned by the virtual probe. The sample introduces a strong perturbation in the neighborhood of the metal surface that gives rise to a dramatic loss in the photon-plasmon coupling efficiency.
Fig. 10.
Fig. 10. (a) Reflection coefficient R and (b) transmission coefficient T calculated for different positions of the defect in the glass plate during the scan.

Equations (3)

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R = i η i r , T = i η i t .
Re { k } L ~ ( 2 n + 1 ) π 2
f ( x ) = { h exp [ ( x + w Δ ) 2 s 2 ] x < w h w x w h exp [ ( x - w Δ ) 2 s 2 ] x > w
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