  Millimeter wave (mm-wave) communications and radar receivers capable of
processing small signals must be protected from high-power signals, which can
damage sensitive receiver components. Many of these systems arguably can be
protected by using photonic limiting techniques, in addition to electronic
limiting circuits in receiver front-ends. Here we demonstrate, experimentally
and numerically, a free-space, reflective mm-wave limiter based on a multilayer
structure involving a nanolayer of vanadium dioxide (VO2), experiencing a
thermal insulator-to-metal transition. The multilayer acts as a variable
reflector, controlled by the input power. At low input power levels, VO2
remains dielectric, and the multilayer exhibits resonant transmittance. When
the input power exceeds a threshold level, the emerging metallic phase renders
the multilayer highly reflective while dissipating a small portion of the input
power without damage to the limiter. In the case of a Gaussian beam, the
limiter has a nearly constant output above the limiting threshold input.
