  Optical limiters are nonlinear devices that feature decreasing transmittance
with increasing incident optical intensity, and thus can protect sensitive
components from high-intensity illumination. The ideal optical limiter reflects
rather than absorbs light in its active ("limiting") state, minimizing risk of
damage to the limiter itself. Previous efforts to realize reflective limiters
were based on embedding nonlinear layers into relatively thick multilayer
photonic structures, resulting in substantial fabrication complexity, reduced
speed and, in some instances, limited working bandwidth. We overcome these
tradeoffs by using the insulator-to-metal transition in vanadium dioxide (VO2)
to achieve intensity-dependent modulation of resonant transmission through
aperture antennas. Due to the dramatic change of optical properties across the
insulator-to-metal transition, low-quality-factor resonators were sufficient to
achieve high on-off ratios in device transmittance. As a result, our ultra-thin
reflective limiter (thickness ~1/100 of the free-space wavelength) is broadband
in terms of operating wavelength (> 2 um at 10 um) and angle of incidence (up
to ~50$\deg$ away from the normal).
