Ultrahigh Field Enhancement Optimization Versus Rabi Splitting Investigated Using Au Nano-Bipyramids on Metal Films

When localized surface plasmons (LSPs) are coupled to extended surface plasmons (ESPs), higher density of particles causes stronger interaction, larger Rabi splitting (RS), and plasmonic energy band gap. Optimization of the field enhancement (FE) is addressed theoretically and experimentally particularly because the ultrahigh FE was found recently to exist when LSPs are excited via ESPs under the prism and grating coupling. The larger the RS, the weaker the FE, and at optimum density, RS almost vanishes. Under free-space excitation, for some periodicities of nano-bipyramids, ESPs-LSPs coupling occurs, causing RS and therefore two new hybrid modes, thus decreasing the field intensity by factor similar to x1.7; wherein under prism excitation, it drastically decreases by similar to x100. An additional effect is observed for free-space excitation far from the ESP-LSP coupling condition and that is the occurrence of a blueshift in the LSP wavelengths as the periodicity decreases, causing a decrease of FE by nearly an order of magnitude.