  Plasmon hybridization between closely spaced nanoparticles yields new hybrid
modes not found in individual constituents, allowing for the engineering of
resonance properties and field enhancement capabilities of metallic
nanostructure. Experimental verifications of plasmon hybridization have been
thus far mostly limited to optical frequencies, as metals cannot support
surface plasmons at longer wavelengths. Here, we introduce the concept of
'spoof plasmon hybridization' in highly conductive metal structures and
investigate experimentally the interaction of localized surface plasmon
resonances (LSPR) in adjacent metal disks corrugated with subwavelength spiral
patterns. We show that the hybridization results in the splitting of spoof
plasmon modes into bonding and antibonding resonances analogous to molecular
orbital rule and plasmonic hybridization in optical spectrum. These hybrid
modes can be manipulated to produce enormous field enhancements (larger than
5000) by tuning the separation between disks or alternatively, the disk size,
which effectively changes the relative gap size. The impact of the radiation
loss is considered to find out the optimum disk size that maximizes field
enhancement capabilities. Our investigation not only extends the range of
applicability of the hybridization model, but also provides insightful guidance
to exporting the exciting applications associated with plasmon hybridization to
lower spectral range.
