  Plasmonic systems have attracted remarkable interest due to their application
to the subwavelength confinement of light and the associated enhancement of
light-matter interactions. However, this requires light to dwell at a given
spatial location over timescales longer than the coupling rate to any relevant
loss mechanism. Here we develop a general strategy for the design of
stopped-light plasmonic metasurfaces, by taking advantage of the conformal
symmetry which underpins near-field optics. By means of the analytical
technique of transformation optics, we propose a class of plasmonic gratings
which is able to achieve ultra-slow group velocities, effectively freezing
surface plasmon polaritons in space over their whole lifetime. Our method can
be universally applied to the localization of polaritons in metallic systems,
as well as in highly doped semiconductors and even two-dimensional conductive
and polar materials, and may find potential applications in nano-focusing,
nano-imaging, spectroscopy and light-harvesting.
