Reconfigurable Structural Color Display Based on Effective Index Modulation of GST for Color Purity Enhancement

Abstract

Phase-change materials (PCMs) have been emerged owing to their reconfigurable physical properties such as refractive index, light absorption, electrical conductivity, and Young's modulus by switching between amorphous and crystalline phases [1]. Ge2Sb2Te5 (GST), which consists of germanium, antimony, and tellurium, is one of the representative PCM materials and is widely studied for a dynamic color display or phase change memory by utilizing its great tunability in both optical and electrical properties. Due to the nature of the refractive index variation depending on the phase of GST, there have been tremendous suggestions for reconfigurable photonic structures, including photonic memory, thermal radiation modulator, and tunable structural color [2]. Among them, the high color purity and feasible tuning functions of structural coloration has attracted attention due to its promising applicability as a next generation color display. Despite its great potential application, low chromaticity and limited intermediate step over phase change have been treated as a barrier to the efficient operation as a color display [3]. In this study, we present an ultrathin tunable color filter comprising GST. We modulate the effective index of GST by applying porosity (Pr) with anisotropic geometry, resulting in color purity enhancement and enhanced controllability based on polarization-driven modulation. Specifically, we deposit GST on top of an Au film using glancing angle deposition (GLAD) to manufacture self-aligned porous nanocolumns (PNCs) with varying Pr (0% 40%, 75%). In the amorphous GST/Au reflection spectrum, dip intensity decreases as Pr increases, and we find that color purity can be enhanced by controlling the Pr and phase. Particularly, the amorphous phase of GST exhibits a higher color purity and a wider color range than the crystalline phase. Furthermore, we observe that the slanting plane direction of the PNCs has a higher Pr due to the atomic shadowing effect and that the Pr of the PNCs varies with the polarized light angle, causing a structural color change. We confirm that the color purity can be improved with the Pr, phase, thickness of GST, and polarization angle. We believe that our suggestions can be used as a way to improve optical properties in applications such as optical coating, optoelectronic displays, and optical storage devices.