  The ability of curved gratings as sectors of concentric circular gratings to
couple linearly polarized light into focused surface plasmons is investigated
by theory, simulation and experiment. Curved gratings, as sectors of concentric
circular gratings with four different sector angles, are etched into a 30-nm
thick gold layer on a glass coverslip and used to couple linearly-polarised
free space light at nm into surface plasmons. The experimental and simulation
results show that increasing the sector angle of the curved gratings decreases
the lateral spotsize of the excited surface plasmons, resulting in focussing of
surface plasmons which is analogous to the behaviour of classical optical
lenses. We also show that two faced curved gratings, with their groove radius
mismatched by half of the plasmon wavelength (asymmetric configuration), can
couple linearly-polarised light into a single focal spot of concentrated
surface plasmons with smaller depth of focus and higher intensity in comparison
to single-sided curved gratings. The major advantage of these structures is the
coupling of linearly-polarised light into focused surface plasmons with access
to and control of the plasmon focal spot, which facilitates potential
applications in sensing, detection and nonlinear plasmonics.
