  We develop a new concept for active plasmonics exploiting nanoscale
structural transformations which is supported by rigorous numerical analysis.
We show that surface plasmon-polariton signals in a metal-on-dielectric
waveguide containing a gallium section a few microns long can be effectively
controlled by switching the structural phase of gallium. The switching may be
achieved by either changing the waveguide temperature or by external optical
excitation. The signal modulation depth could exceed 80 percent and switching
times are expected to be in the picosecond-microsecond time scale.
