Multiple concentric rainbows induced by microscale concave interfaces for reflective displays

Abstract

In our current world, active display technology creates a glut of energy demand to meet its illumination needs. This demand can be stymied by using reflective display technologies that require no active illumination, with some examples including: electronic paper using electrophoretic motion of ink particles, electrowetting of water/oil droplets, and interferometric modulators, all of which have been commercialized. However, due to the lack of active light sources, it is difficult to implement these reflective display technologies in low light environments (e.g. nighttime display). In this work, we report an experimental observation of multiple concentric circular rainbows from reflective microscale concave interfaces (MCIs), which are introduced by the reflection of optical rays within a polymer-embedded microsphere. Exit rays from a single edge and opposite edges will introduce completely different interference mechanisms depending on the illumination and observation conditions, which will result in different angle-dependent colors. By clarifying the mechanism behind this coloration phenomenon, as well as quantitatively mapping the generated color, the implementation of the MCI in smart signs and pixelated displays are demonstrated, showing angle-dependent color-changing reflected images that can be observed over a wide spatial angle range. This structural material will serve as a building block for the development of new platforms for light-matter interactions, on-chip sensors, anti-counterfeiting tools, and passive and smart color reflective displays. Intriguingly, we also demonstrate a smart MCI traffic sign for both visible and infrared wavelengths that will introduce extra signals for pattern and image recognition in order to enhance the safety of future autopilot/autonomous systems.

Introduction

Display technology, as one of the most important branches of optoelectronics associated with modern life (e.g. televisions, smart phones, smart watches and glasses), has opened up huge markets along the way (e.g. electronic-paper [1], [2], [3], three-dimensional displays [4], virtual and augmented reality applications [5], electronic-skin display [6], etc.). However, as electronic/optoelectronic devices become more ubiquitous within our daily life, so too does the need to reduce the energy consumption of these devices, especially within display technologies. For instance, passive reflective displays that rely on external light sources stand out because of their low energy consumption (e.g. e-paper, electronic-wetting [2] and MEMS interferometer displays [3]), especially when there is plenty of ambient light (such as sun light during the day). However, displays under low-light environments (e.g. nighttime outdoor display) still require active light emitting devices (e.g. LED displays), for example, nighttime advertisements and illuminated billboards on commercial streets and highways. In recent decades, global modernization and urbanization processes have increased the levels of ambient light pollution, enabling the use of nighttime satellite imagery to characterize the amount of regional development [7]. However, the increase in light pollution has also led to harmful impacts on ecosystems [8] and has even been affiliated with the spread of viruses [9] and cancers [10]. Therefore, a smart, passive display technology especially for nighttime use is essential for offsetting this ambient light pollution and its associated negative impacts on society. This article will report a new multiple concentric rainbow reflection from microscale concave interfaces (MCIs). By clarifying this new coloration mechanism and integrating these new structures with display components, we demonstrated a new reflective color display which is particularly useful in nighttime smart traffic signs and billboards on highways, reflective safety vests, anti-counterfeiting labels, entertainment toys, etc.

Recently, a total internal reflection (TIR) interference mechanism introduced by MCIs was proposed [11] and enabled new optical microscopic imaging technologies [12]. However, it was believed that the incorporation of this new color creation mechanism into large scale displays and sensors is exciting but challenging to achieve [13]. Independently, a similar, but large-scale structure was realized by partially embedding a monolayer array of polymer microspheres into a transparent tape [14]. This structure was responsible for generating iridescent and vivid retroreflective colors, which was utilized as the building block for smart traffic signs [14,15]. Although both pioneering works [11,14] attributed the coloration mechanism to TIR (different from rainbow in nature [16,17]), neither work was able to definitively prove the exact nature of the color generation, which is essential for a color display application. In order to explain the physics of this reflective structural coloration strategy [11], [12], [13], [14], [15], it is necessary to introduce the key results reported by the pioneering works first.

Within Ref. [11], a multi-bouncing TIR model was proposed to explain the generated color (Fig. S1a in Section S1). When a beam of collimated light enters the microdroplets, it will experience many reflections within the droplet, due to the concave structure of the bottom surface. Furthermore, the beam will be split into different components, which will bounce a number of times, m, within the concave structure. Due to the different optical paths for m = 2, 3, 4 …, an interference spectrum will be formed because of the constructive and destructive interference experienced by the beam components. Using this hypothesis, the TIR-induced spectral interference patterns were modeled analytically using ray optics and classical interference equations. In order to realize the color that is a result of a given spectrum, these interference patterns were then converted into a point on the Commission Internationale de l'Eclairage (CIE) color chart. However, colorimetric information does not have a one-to-one correspondence with the spectral feature of light. The same color found on the CIE chart can be a result of two totally different spectra (a concept known as metamerism [18]). Therefore, the utilization of the modeled color to definitively characterize the given sample should not be regarded as conclusive. On the other hand, within Ref. [14], a direct measurement of the reflected spectrum was obtained, but did not agree well with the multiple ray interference coupled from a single side of the microscale concave structure proposed by Ref. [11]. Instead, this work attributed the new color creation to thin-film interference introduced by an air gap between the microspheres and adhesive polyacrylate (i.e., the tape) interface (Fig. S1b in Section S1) [14]. However, the extracted air gap within the range of ∼100 nm was not clearly observed in the microscopic characterization. Therefore, both reported mechanisms should be considered incomplete or inaccurate. In this work, we will begin by presenting a systematic experimental investigation to reveal the unambiguous complete mechanism for the colored reflection from the new MCI.