  Nanophotonic (nanoplasmonic) structures confine, guide, and concentrate light
on the nanoscale. Advancement of nanophotonics critically depends on active
nanoscale control of these phenomena. Localized control of the insulator and
metallic phases of vanadium dioxide (VO2) would open up a universe of
applications in nanophotonics via modulation of the local dielectric
environment of nanophotonic structures allowing them to function as active
devices. Here we show dynamic reversible control of VO2 insulator-to-metal
transition (IMT) locally on the scale of 15 nm or less and control of
nanoantennas, observed in the near-field for the first time. Using
polarization-selective near-field imaging techniques, we monitor simultaneously
the IMT in VO2 and the change of plasmons on gold infrared nanoantennas.
Structured nanodomains of the metallic VO2 locally and reversibly transform
infrared plasmonic dipolar antennas to monopole antennas. Fundamentally, the
IMT in VO2 can be triggered on femtosecond timescale to allow ultrafast
nanoscale control of optical phenomena. These unique capabilities open up
exciting novel applications in active nanophotonics.
