  The near-field and far-field spectral response of plasmonic systems are often
assumed to be identical, due to the lack of methods that can directly compare
and correlate both responses under similar environmental conditions. We develop
a widely-tuneable optical technique to probe the near-field resonances within
individual plasmonic nanostructures that can be directly compared to the
corresponding far-field response. In tightly-coupled nanoparticle-on-mirror
constructs with nanometer-sized gaps we find >40meV blueshifts of the
near-field compared to the dark-field scattering peak, which agrees with full
electromagnetic simulations. Using a transformation optics approach, we show
such shifts arise from the different spectral interference between different
gap modes in the near- and far-field. The control and tuning of near-field and
far-field responses demonstrated here is of paramount importance in the design
of optical nanostructures for field-enhanced spectroscopy, as well as to
control near-field activity monitored through the far-field of nano-optical
devices.
