Plasmon Localization Assisted by Conformal Symmetry

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 time scales 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 ultraslow group velocities, effectively freezing surface plasmon polaritons in space up to 31 time cycles. 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 nanofocusing, nanoimaging, spectroscopy, and light harvesting.