  We present experimental demonstration of electronically tunable metamaterial
perfect absorbers in the terahertz regime. By incorporation of active liquid
crystal into strategic locations within the metamaterial unit cell we are able
to modify the absorption by 30 percent at 2.62 THz, as well as tune the
resonant absorption over 4 percent in bandwidth. Numerical full-wave
simulations match well to experiments and clarify the underlying mechanism,
i.e. a simultaneous tuning of both the electric and magnetic response that
allows for the preservation of the resonant absorption. These results show that
the fundamental light interactions of surfaces can be dynamically controlled by
all-electronic means and provide a path forward for realization of novel
applications.
