  Structured metallic tips are increasingly important for optical
spectroscopies such as tip-enhanced Raman spectroscopy (TERS), with plasmonic
resonances frequently cited as a mechanism for electric field enhancement. We
probe the local optical response of sharp and spherical-tipped atomic force
microscopy (AFM) tips using a scanning hyperspectral imaging technique to
identify plasmonic behaviour. Localised surface plasmon resonances which
radiatively couple with far-field light are found only for spherical AFM tips,
with little response for sharp AFM tips, in agreement with numerical
simulations of the near-field response. The precise tip geometry is thus
crucial for plasmon-enhanced spectroscopies, and the typical sharp cones are
not preferred.
