A Simple Route for Generating Iridescent Retroreflective Color Filter

Iridescent colors found in the body of various animals and insects, such as tails of birds or wings of butterflies, exhibit eye-catching visual information, enabling camouflage, predation, courtship, and communication in various environmental changes. In particular, certain living, <i>Papilio palinurus</i><i>,</i> which has multilayer photonic structures on the concave structure of the wing surface, exhibits conspicuous light-matter interaction with both iridescent color reflection and retroreflection, which delivers vibrant visible information in the direction of light input¹. Conventionally, retroreflectors employ geometric structures, such as corner-cube shapes, to redirect incident light back toward its source. However, these structures typically yield non-iridescent colors and are challenging to apply in various applications due to these limitations. Moreover, the use of organic materials such as dyes and pigment for colorization compromises the color purity and durability of retroreflective materials. Based on a lesson from natural photonic structure, specific examples have been introduced to make an iridescent retroreflector inducing gap interference within biologically inspired microsphere structures or distributed Bragg reflector (DBR)². However, these methods still suffer from limitations such as intricate structures and time-consuming manufacturing processes. In this study, inspired by the structural properties of butterflies, we present a conspicuous iridescent retroreflective color filter that dynamically changes in response to the incident angle. The filter aims to overcome limitations in dynamic coloration and durability by utilizing an ultra-thin resonant layer (UTRL) on a metal film.<br/>To implement the iridescent retroreflective filter, a metal film is deposited on a micro-concave array (MCA) designed to implement retroreflective behavior to form a UTRL. The round curvature of the MCA possesses different deposition angles at various locations, which consequently allows the UTRL to deposit with a gradual change in porosity, resulting in sequential color changes under modulated resonance conditions [4]. The fabricated UTRL converts white light into iridescent light that gradually changes with the curvature of MCA by turning the direction of light to the original light source. The resonance conditions were regulated by controlling the porosity of the absorption medium in the metal reflector, which means that when resonance occurs in the visible wavelength range, the change in porosity causes a color change³. The dynamic angle-sensitive color changes resulting from the difference between this porosity and the effective index are experimentally confirmed by the measurement of the reflection spectrum and its spectral color at a wide angle. Additionally, the retroreflective filter can analyze using a rotational linear polarizer. When the polarizer was located in front of the light source and the image sensor, the retroreflection can be confirmed by observing the near-field pattern. When the polarizer in front of the image sensor rotated 90°, the retroreflected light was detected in four segments of the concavity. As a result, we implemented a sustainable retroreflective structural color filter that is constructed using a self-graded porous medium comprising concave structures. The iridescent color filters can be used in various visual applications such as load signals or optical devices.<br/>[1] Kolle, Mathias, et al., “Mimicking the colourful wing scale structure of the Papilio blumei butterfly,” Nat. Nanotechnol. <b>5</b>, 511–515 (2010)<br/>[2] Kats, Mikhail A., et al., “Nanometre optical coatings based on strong interference effects in highly absorbing media,” Nat. Mater. <b>12</b>(1), 20–24 (2012).<br/>[3] Ko, Joo Hwan, et al., " Flexible, Large-Area Covert Polarization Display Based on Ultrathin Lossy Nanocolumns on a Metal Film," Adv. Funct. Mater. <b>30</b>, 1908592 (2020).