  Plasmonic nano-antennas can focus light to nanometre length-scales providing
intense field enhancements. For the tightest optical confinements (0.5-5 nm)
achieved in plasmonic gaps, the gap spacing, refractive index, and facet width
play a dominant role in determining the optical properties making tuning
through antenna shape challenging. We show here that controlling the
surrounding refractive index instead allows both efficient frequency tuning and
enhanced in/output-coupling through retardation matching as this allows dark
modes to become optically active, improving widespread functionalities.
