Transient Color Filters and Superabsorbers Based on Magnesium

Photonic devices built upon ultrathin metal films have attracted rising research interest due to their structurally simple and mechanically stable design, which is highly scalable and time-efficient compared to the nanopatterning process, without compromising the superior optical performances. The current photonic thin film research depends heavily on conventional metals (e.g., Au, Ag and Cu) with limitations of high-cost, CMOS-incompatibility and bio-incompatibility, preventing their further usages in industrial applications. Here, we present Magnesium (Mg), the eighth most abundant material on the earth’s crust, as a promising candidate for its competitive low-loss optical properties with additional compatibilities in CMOS and biological devices. The first device presented is a Mg-based transient broad-band reflective color filter made from a tandem stack structure of two metal-insulator-metal (MIM) stacks, Mg/MgO/Mg and Ag/Al₂O₃/Ag, interconnecting by a thin MgO layer [1]. Our devices can successfully achieve distinctive hues across the CMY reflective color gamut and are insensitive to incident angles up to 50 degrees, which are conducive for color displays. The transient process takes place while these first vivid colors can vanish within ~40 seconds after immersing in water [2]. It is beneficial that the hue fades quickly for the security and protection of optical information. Further etching ultimately removes away the Mg-contained layers, leaving bottom Ag/Al₂O₃/Ag MIM with another reflective color which is maintained stably under ambient conditions. In contrast, the second device incorporates an ultrathin a-Si absorbing layer and exhibits a spectrally selective and tunable near-unity resonant absorption [3]. By varying the thickness of the semiconductor layers (a-Si and SiO₂), it can generate characteristic colors which cover all the primary RGB and CMY color spaces. An exposure to water can dissolve the Mg back reflection layer and the remaining bilayer transmissive color filters are ready for reconfiguration. To summarize, we propose a novel platform for transient color filters and superabsorbers with Mg-based thin films, which can attain diverse structural colors by tuning the layer thickness. Its unique feature of the single-time and irreversible transient color change process in a zero-power consumption manner, provides new opportunities for next-generation CMOS compatible, biodegradable, and low-cost photonic devices.<br/><br/><br/>[1] P. Lyu, et al. <i>Adv. Optical Mater</i>. 10, 2200159 (2022)<br/>[2] T. G. Farinha, T. Gong, P. Lyu, et al, <i>Optical Materials Express</i>, 11(5), 1555-1565 (2021)<br/>[3] T. Gong, P. Lyu, et al, In preparation (2022)