We present several designs and experimental implementations of optical power diodes – devices that are designed to be transparent from one direction, but opaque from the other, when illuminated by a beam with sufficient intensity. Optical power diodes can be used to protect optical devices that both detect and transmit light.
Our designs are based on phase-change material vanadium dioxide (VO2), which undergoes an insulator-to-metal transition (IMT) that can be triggered thermally or optically. Here, VO2 films serve as nonlinear elements that can be transformed from transparent to opaque by intense illumination. We build thin-film metallic structures on top of the VO2 films such that the optical absorption becomes asymmetric – light impinging from one direction is absorbed at a higher rate than from the other direction, triggering the transition, and turning the device opaque. This results in asymmetric transmission.
The designs are optimized with finite-difference time-domain (FDTD) simulations, using optical constants of VO2 extracted using ellipsometry, and are shown to be scalable across the near- and mid-infrared. Our initial experimental results using a simple design comprised of metal and VO2 films on sapphire, designed for an operating wavelength of 1.35µm, show a transmission asymmetry ratio of ~2, and experiments with superior designs are ongoing. Future work will include the use of defect-engineered VO2 to engineer the intensity threshold of optical power diodes.