Limiting Optical Diodes Enabled by the Phase Transition of Vanadium Dioxide

A limiting optical diode is an asymmetric nonlinear device that is bidirectionally transparent at low power but becomes opaque when illuminated by sufficiently intense light incident from a particular direction. We explore the use of a phase-transition material, vanadium dioxide (VO<sub>2</sub>), as an active element of limiting optical diodes. The VO<sub>2</sub> phase transition can be triggered by optical absorption, resulting in a change in refractive index orders of magnitude larger than what can be achieved with conventional nonlinearities. As a result, a limiting optical diode based on incident-direction-dependent absorption in a VO<sub>2</sub> layer can be very thin, and can function at low powers without field enhancement, resulting in broadband operation. We demonstrate a simple thin-film limiting optical diode comprising a transparent substrate, a VO<sub>2</sub> film, and a semitransparent metallic layer. For sufficiently high incident intensity, our proof-of-concept device realizes broadband asymmetric transmission across the near-infrared, and is approximately ten times thinner than the free-space wavelength.