Metal Enhanced Fluorescence (MEF) Metal Enhanced Fluorescence (MEF): Nanoparticle Arrays: By using e-beam lithography to fabricate arrays of nanoparticles of varying sizes, separation, and shapes, MEF can be systematically studied and the underlying physics is revealed. A small sample of the research done at LPS is presented here.
In the above figure, the MEF enhancement factor of square shaped nanoparticle pillars is measured as a function of lateral size and separation. The peak in the curves represent the presence of a plasmon resonance. Since the excitation/emission wavelengths are longer for the fluorescent molecule Cy5 than they are for Cy3, larger nanoparticles with longer plasmon wavelengths are required to maximize MEF in (c) compared to (b). Suggestive of an enhanced electric field between particles, the enhancement factor increases as the gap between particles decreases. However, as the particles become closer in space, their plasmon modes couple, and the degeneracy between modes in broken. Thus in (b), two plasmon resonances are observed. Presumably, one mode overlaps the laser excitation and the other overlaps the Cy3 emission spectrum.
In the above figure, the MEF enhancement factor of triangular nanoparticle pillars is compared to that of square pillars. While both particle types have plasmon resonances, the larger enhancements of the triangular particles suggest that electric field concentrated between the triangular points effectively increase the excitation cross-section. Although the enhancement factors presented here are rather modest, the systematic study at LPS is producing a deeper understanding of MEF that should lead to the ability to maximize enhancement. Preliminary experiments on nearly overlapping metal structures have produced enhancements of about 400. Ultimately the practicality of MEF will depend on the technology to cheaply fabricate MEF arrays with precise geometrical control. |
