Highly directive plasmonic structure with double resonance at excitation and emission for molecule-enhanced fluorescence

An improved bull’s eye nanostructure is proposed as a substrate for surface-enhanced fluorescence. Optimized by finite-difference time-domain, annular corrugation on both upper and lower surfaces is placed around a nanobow tie in the middle. The whole structure is designed for the excitation and emission process of a certain fluorescence simultaneously, which can enhance the fluorescence intensity enormously (35 times for an excitation field and 120 times for radiation, independently). Meanwhile, it is able to confine the far-field emission energy within a divergence angle of 3.5°. The dual-resonant enhancement and directivity of such a substrate allows higher sensitivity during fluorescence detection. Furthermore, the designed resonance and directional light control at emission may effectively reduce the required energy at excitation, which is ideal for those molecules with low laser damage threshold.