  Plasmonics is an interdisciplinary field focusing on the unique properties of
both localized and propagating surface plasmon polaritons (SPPs) -
quasiparticles in which photons are coupled to the quasi-free electrons of
metals. In particular, it allows for confining light in dimensions smaller than
the wavelength of photons in free space, and makes it possible to match the
different length scales associated with photonics and electronics in a single
nanoscale device. Broad applications of plasmonics have been realized including
biological sensing, sub-diffraction-limit imaging, focusing and lithography,
and nano optical circuitry. Plasmonics-based optical elements such as
waveguides, lenses, beam splitters and reflectors have been implemented by
structuring metal surfaces or placing dielectric structures on metals, aiming
to manipulate the two-dimensional surface plasmon waves. However, the abrupt
discontinuities in the material properties or geometries of these elements lead
to increased scattering of SPPs, which significantly reduces the efficiency of
these components. Transformation optics provides an unprecedented approach to
route light at will by spatially varying the optical properties of a material.
Here, motivated by this approach, we use grey-scale lithography to
adiabatically tailor the topology of a dielectric layer adjacent to a metal
surface to demonstrate a plasmonic Luneburg lens that can focus SPPs. We also
realize a plasmonic Eaton lens that can bend SPPs. Since the optical properties
are changed gradually rather than abruptly in these lenses, losses due to
scattering can be significantly reduced in comparison with previously reported
plasmonic elements.
