ABSTRACT
Plasmonics [1-13], the study of the interaction between electro-
magnetic field and free electrons in a metal, has drawn increasing
attention recently due to its huge potential for solving many
eminent issues encountered by our world. Up to now, exciting
plasmonic applications, for instance, super-resolution imaging [14-
19], optical cloaking [20-23], and energy harvesting [24-29],
have been reported. Many other potential applications are under
development. All these developments are attributed to the advanced
nanofabrication techniques. Top-down nanofabrication techniques
such as electron-beam lithography [30-33] and focus ion beam
milling [34-40] allow the accurate fabrication of structures at
nanoscale with desirable trade-off on the high equipment expenses,
as well as the considerably long time during sample preparation.
Bottom-up techniques like self-assembly [41-43] can easily achieve
regular patterns at a large scale at a rather low cost but are
not preferred for the cases that require accurate positioning
and alignment with nanometer precision. Despite their strengths
and weaknesses, both top-down and bottom-up techniques make
their unique contributions to plasmonics by providing various
nanostructures for plasmonic applications for different purposes.
However, most plasmonic devices fabricated through top-down
and bottom-up techniques are passive, which greatly limits their
application because of the additional investments required in
fabricating another similar device with little change in the sample
design. Thus, the new research field of active plasmonics emerges,
which deals with reconfigurable function after the devices are
fabricated, with the help of active mediums responsive to certain
stimulus. The first original active plasmonic device is gallium-
coated plasmonic waveguides proposed by Krasavin and Zheludev
[44]. Since then, many other active mediums have been used
to build active plasmonic devices, including liquid crystals [45-
60], molecular machines [61, 62], elastic polymers [63-65], and
chemical oxidation/reduction [66-68]. Among all the mentioned
active mediums, liquid crystal stands out from all the rest because
of its large birefringence on refractive index, low threshold on
transition among different states, and versatile driven methods to
cause the transitions.
