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Ever since an experimental observation (1974) of large Raman scattering cross sections from pyridine molecules adsorbed onto silver electrodes, there have been considerable efforts to understand and produce materials demonstrating “surface-enhancement.” Metallic, nanostructured substrates have the ability to generate intense optical fields localized to subwavelength regions that are orders of magnitude larger than the incident optical fields. These enhanced fields can amplify certain optical effects, such as Raman scattering, fluorescence, harmonic generation, and four-wave mixing. We are working on developing different types of surfaces that enhance these processes, for applications in molecular sensing and spectroscopy.
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Silver-coated laser-nanostructured substrates are used for surface-enhanced Raman scattering |
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| Plainly speaking | When molecules are placed near a roughened metal surface, such as in the case of our femtosecond laser fabricated substrates, they can experience much greater electromagnetic fields than if they were on a flat surface. Consider two metal particles separated by several nanometers (as shown below) to illustrate the conceptual idea:
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An electromagnetic field with wavelength much longer than the particle dimensions causes charges to oscillate (also known as plasmons) back and forth on the surfaces of the particles. The resulting interparticle coupling can effectively form a capacitor within the gap, and this can lead to large local fields experienced by a molecule in close proximity.
As a result, scattered Raman signals can gain an extra boost from this local field. For many important applications, such as detection of biomolecules, contaminants, or explosive materials, this means that molecules can be identified with much greater sensitivity than if they were not near a nanostructure.
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