Parity-time symmetric plasmonics

Plasmons are hybrid photon-electron waves bound between conducting and dielectric materials. They have enabled strongly enhanced light-matter interactions for light emission and sensing, sub-wavelength structuring of photonic devices, and exotic optical phenomena such as optical-frequency magnetism. To date, most control over plasmon dispersion has been achieved through structuring of the real part of the refractive index, while losses have been viewed as detrimental. Photonic Parity-Time (PT) symmetry takes advantage of these inherent losses, utilizing them in conjunction with balanced gain media to control eigenmode evolution. In this chapter, we review progress in PT-symmetric plasmonics, focusing on planar and coaxial geometries. We show how inclusion of balanced loss and gain gives rise to exceptional points, enabling a multitude of phenomena including: (1) subwavelength mode multiplexing; (2) chiral molecule sensing and discrimination; (3) subwavelength polarization conversion; and (4) nonreciprocal, nonlinear optical metamaterials. We also show a route towards thresholdless symmetry breaking. These results provide a foundation for ultra-compact optical components with almost complete control over scattering, reflection, and transmission by tuning the PT potential.