Photon-plasmon coupling: Dye guides light through perforated metal foil

Jan 27, 2011

(PhysOrg.com) -- Just as photons are bundles of light energy, plasmons are energy packets of plasma oscillations—oscillations of the electron density in a solid body, which are known as surface plasmons when occurring at a metal interface. Surface plasmons introduce new possibilities for the manipulation and transmission of light for applications in a variety of areas, from modern data processing to biomedical sensing.

In the journal Angewandte Chemie, Thomas W. Ebbesen, James A. Hutchison, and a team from the University of Strasbourg (France) introduce an interesting new effect based on the coupling of photons and plasmons: dye molecules help light pass through holes in metal foils that are so small that conventional theory predicts the light should not actually be able to pass through at all.

According to classical aperture theory, light should not be able to pass through tiny holes when the diameter is significantly smaller than the wavelength of the light. However, as reported by Ebbesen’s group over a decade ago, light transmission can be much higher than predicted for regular arrays of holes owing to the involvement of surface plasmons. In essence, light is converted into surface plasmons, and in this coupled state the photons can pass though the holes to the other side of the metal as plasmons. They can then uncouple and reappear as light.

The French team has now described another phenomenon: if dye molecules are placed directly on the perforated metal surface, they significantly increase its transparence. Contrary to expectation, the additional windows of transparency can occur at wavelengths that are strongly absorbed by the molecules. Interestingly, this also occurs if the arrangement of holes in the foil is irregular; even a single hole is enough.

The researchers propose that two complementary effects are at play. On one hand, the in the holes generate a large index variation in the hole favoring the transmission near the absorption band. On the other, the dye molecule generates a kind of “mirror image” of its electric dipole in the metal’s free electron , and the dipole and mirror-image dipole interact. If the molecule then absorbs light, it is not re-emitted; instead, the is completely transferred to the metal surface, where it couples with surface plasmons helping the transmission process. This combination enables the light to pass efficiently to the other side of the metal foil.

This discovery represents a new approach for making perforated metal films with tailored transmission of visible light by simply applying a dye that absorbs with the desired wavelength, which would have application in solar energy technology, filters, and sensing. That the transient excited states of molecules have absorption properties that are very different to their ground state adds a further dynamic dimension to these films, with all-optical, ultra-fast switches another possible application.

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More information: Thomas W. Ebbesen, Absorption-Induced Transparency, Angewandte Chemie International Edition, Permalink to the article: dx.doi.org/10.1002/anie.201006019

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Nik_2213
not rated yet Jan 27, 2011
How thick is the foil ??
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Uh, cue 'transparent aluminium'...
Macksb
1 / 5 (1) Jan 27, 2011
I wonder if there is a useful connection between this work and the experiment by Cavalleri of Oxford in which laser pulses created superconductivity (briefly) in a cuprate material that was otherwise not a superconductor, though its structure was close to that of superconducting cuprates. (See recent Phys Org article in Superconductivity section.) Both experiments suggest that organized light (organized by frequency, for example) can create surprising organization within material, even to the extent of defeating the normal "habits" of the material. Both suggest some relationship to Art Winfree's theory of coupled or synchronized oscillators, as described in Sync by Steven Strogatz.
electrodynamic
not rated yet Jan 27, 2011
Would different shaped holes make a difference. Ultra thin slots could also be used to kind of further restrict the light ( have the width of the slot much thinner than a wavelength, but the length of the slot about the same, or shorter than the wavelength)Just curious because, I remember seeing something about differently shaped objects on the nano scale(objects created from the same material) can create different wavelengths of light on the macro scale.

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