Resonance mechanisms of coupled-particle plasmonic nano-antennas to maximize UV fluorescence enhancement of biological molecules

We investigated several geometric parameters such as the height, width and length, and the contribution of different plasmonic modes on the enhancement factors of aluminum (Al) bowtie nano-antennas (BNAs) on tryptophan's native fluorescence in the ultraviolet (UV) to visible range. The highest fluorescence enhancement was produced by the tallest BNAs. Analysis revealed that, in tall BNAs illuminated at normal incidence, phase retardation amplified quadrupole resonances which were exploited to obtain high excitation enhancement. The optimized oxide-free Al BNA predicted 331 x excitation enhancement, 74 x radiative enhancement, 993 x fluorescence net enhancement and the optimized oxidized Al BNA predicted 128 x excitation enhancement, 142 x radiative enhancement and 461 x fluorescence net enhancement. These enhancement factors are the largest reported for simulated UV plasmonic structures in literature using tryptophan as the model molecule. The effect of length and width on the different plasmonic modes were also studied and explained in depth. An oxide layer dampened the excitation enhancement but has negligible effect on emission enhancements. The numerical study conducted in this manuscript sheds light to light-matter interaction in the UV frequency range.