Chenghao Wan

Design considerations for the enhancement of human color vision by breaking binocular redundancy

To see color, the human visual system combines the response of three types of cone cells in the retina—a compressive process that discards a significant amount of spectral information. Here, we present designs based on thin-film optical filters with the goal of enhancing human color vision by breaking its inherent binocular redundancy, providing different spectral content to each eye. We fabricated a set of optical filters that “splits” the response of the short-wavelength cone between the two eyes in individuals with typical trichromatic vision, simulating the presence of approximately four distinct cone types. Such an increase in the number of effective cone types can reduce the prevalence of metamers—pairs of distinct spectra that resolve to the same tristimulus values. This technique may result in an enhancement of spectral perception, with applications ranging from camouflage detection and anti-counterfeiting to new types of artwork and data visualization.

Thermally tunable VO2-SiO2 nanocomposite thin-film capacitors

We present a study of co-sputtered VO2-SiO2 nanocomposite dielectric thin-film media possessing continuous temperature tunability of the dielectric constant. The smooth thermal tunability is a result of the insulator-metal transition in the VO2 inclusions dispersed within an insulating matrix. We present a detailed comparison of the dielectric characteristics of this nanocomposite with those of a VO2 control layer and of VO2/SiO2 laminate multilayers of comparable overall thickness. We demonstrated a nanocomposite capacitor that has a thermal capacitance tunability of ∼60% between 25 °C and 100 °C at 1 MHz, with low leakage current. Such thermally tunable capacitors could find potential use in applications such as sensing, thermal cloaks, and phase-change energy storage devices.We present a study of co-sputtered VO2-SiO2 nanocomposite dielectric thin-film media possessing continuous temperature tunability of the dielectric constant. The smooth thermal tunability is a result of the insulator-metal transition in the VO2 inclusions dispersed within an insulating matrix. We present a detailed comparison of the dielectric characteristics of this nanocomposite with those of a VO2 control layer and of VO2/SiO2 laminate multilayers of comparable overall thickness. We demonstrated a nanocomposite capacitor that has a thermal capacitance tunability of ∼60% between 25 °C and 100 °C at 1 MHz, with low leakage current. Such thermally tunable capacitors could find potential use in applications such as sensing, thermal cloaks, and phase-change energy storage devices.

Optical power diodes based on phase-transition materials (Conference Presentation)

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.