Reduction of Excitation Volume in Fluorescence Spectroscopy with Localized Surface Plasmon

By introducing a sub-wavelength square nanohole in a semitransparent thin silver film, the penetration depth and the propagation length of the evanescent field can be reduced remarkably, confining the excitation field to a very small region. The excitation volume in the proposed structure is reduced to only 1.8% of the analyte volume excited in the conventional fluorescence spectroscopy structure based on surface plasmon coupled emission (SPCE) technique. Additionally, the proposed structure does not require hemispherical glass substrate for the wavevector match, since normally incident light beam can excite the surface plasmon mode at the aperture walls, rendering the device size to micrometer scale. Moreover, for the optimized dimensions of the nanohole, the peak excitation field is found to be similar to 3.3 times stronger than the peak evanescent field obtainable in the conventional arrangements. With reduced device size, increased resolution and enhanced field strength, the proposed structure will find applications in chip array implementation of florescence microscopy, and variants of SPCE based single molecule detection (SMD).