Scalable Superabsorbers and Color Filters Based on Earth-Abundant Materials

Abstract

Optical materials based on unconventional plasmonic metals (e.g., magnesium) have lately driven rising research interest for the quest of possibilities in nanophotonic applications. Several favorable attributes of Mg, such as earth abundancy, lightweight, biocompatibility/biodegradability, and its active reactions with water or hydrogen, have underpinned its emergence as an alternative nanophotonic material. Here, we experimentally demonstrate a thin film-based optical device composed exclusively of earth-abundant and complementary metal-oxide semiconductor (CMOS)-compatible materials (i.e., Mg, a-Si, and SiO₂). The devices can exhibit a spectrally selective and tunable near-unity resonant absorption with an ultrathin a-Si absorbing layer due to the strong interference effect in this high-index and lossy film. Alternatively, they can generate diverse reflective colors by appropriate tuning of the a-Si and SiO₂ layer thicknesses, including all the primary colors for RGB (red, green, blue) and CMY (cyan, magenta, yellow) color spaces. In addition, the reflective hues of the devices can be notably altered in a zero power-consumption fashion by immersing them in water due to the resulted dissolution of the Mg back-reflection layer. These compelling features in combination with the lithography-free and scalable fabrication steps may promise their adoption in various photonic applications including solar energy harvesting, optical information security, optical modulation, and filtering as well as structure reuse and recycling.

Figure 1

(a) Schematic of the trilayer device consisting of an ultrathin a-Si absorbing layer with varied thickness backed by an SiO₂-coated optically thick Mg substrate. When immersed in water, the Mg film gradually dissolves, leaving behind a bilayer structure with a different optical response. (b) Measured refractive indices of each material constituting the trilayer stack. Solid lines are the real indices n and dashed lines represent the imaginary indices k for a-Si (blue), SiO₂ (orange), and Mg (green), respectively.

Figure 2

(a) Measured and (b) calculated optical absorption of the trilayer superabsorbers with varying a-Si thicknesses between 10 and 30 nm. The SiO₂ spacer layer is fixed at 140 nm thickness.

Figure 3

(a) Photograph of the five trilayer superabsorber samples under white light illumination. The SiO₂ and Mg thicknesses are 140 and 160 nm, respectively, for all samples. The a-Si thicknesses are varied as 10, 15, 20, 25, and 30 nm, corresponding to samples S1, S2, S3, S4, and S5, respectively. (b) Simulated reflective colors of the CIE1931 chromaticity standard for the trilayer superabsorbers with varying a-Si and SiO₂ thicknesses. Red boxes highlight the thickness combinations corresponding to samples S1–S5. (c) Photograph of the corresponding bilayer samples without the Mg layer but with the same a-Si and SiO₂ thickness as the superabsorber samples. (d) Simulated reflective colors of the CIE1931 chromaticity standard for the bilayer devices without the Mg layer. Red boxes highlight the thickness combinations corresponding to samples S1–S5.

Figure 4

(a) Photograph of the six trilayer color filters under white light illumination. The Mg substrate thickness is 160 nm for all samples. The a-Si layer and SiO₂ layer thicknesses in the six samples are of various combinations. The upper row shows the cyan, yellow, and magenta colors while the lower row shows the red, green, and blue colors. (b) Simulated reflective colors of the CIE1931 chromaticity standard for the trilayer devices with varying a-Si and SiO₂ thicknesses (note: the color palette is identical to that in Figure 3b). Red boxes highlight the thickness combinations corresponding to samples C1–C6. (c) Photograph of the corresponding bilayer samples without the Mg substrate layer and with the same a-Si and SiO₂ thickness as the trilayer samples. (d) Simulated reflective colors of the CIE1931 chromaticity standard for the bilayer devices without the Mg layer (note: the color palette is identical to than in Figure 3d). Red boxes highlight the thickness combinations corresponding to samples C1–C6.

Figure 5

Sequential photographs over time of eight trilayer samples with different reflective hues during immersion in water (without heating and water temperature is 19 °C). The initial colors in the upper row are red (C4), green (C5), blue (C6), and light yellow (S1), and the colors in the lower row are cyan (C1), magenta (C3), yellow (S3), and orange (S5). The initial colors gradually transition into the those for the corresponding bilayer structures as Mg dissolves in water.