Resonant‐mode engineering for additive refective structural colors with high brightness and high color purity.pdf
1 Vol.:(0123456789) Scientific Reports |        (2024) 14:13694   | https://doi.org/10.1038/s41598-024-64176-4 www.nature.com/scientificreports Resonant‑mode engineering  for additive reflective structural  colors with high brightness  and high color purity Hojae Kwak 1,4, Incheol Jung 1,4, Dohyun Kim 1,4, Seongcheol Ju 1, Soyoung Choi 1,  Cheolhun Kang 1, Hyeonwoo Kim 1, Hyoung Won Baac 2*, Jong G. Ok 3* & Kyu‑Tae Lee 1* We present quad-layered reflective structural color filters generating vivid additive primary colors by  controlling a mode number in a Fabry–Perot (FP) cavity and an anti-reflective (AR) coating layer, thus  accomplishing high spectral contrast which is highly demanded in creating sharp colors. The reflection  brightness of fabricated structural color filters is over 78% and a color gamut is comparable to the  standard color gamut (sRGB). Higher-order resonant modes are exploited yielding a narrow passband  with strong suppression of the reflection at shorter and longer wavelength ranges for a green color,  while red and blue colors are produced by employing fundamental resonant modes. Besides, the  structural color filters maintain both high brightness and high color purity at oblique incidence angles  up to 40° due to a small angle of refraction by a cavity medium with high refractive index. Moreover,  a large-scale fabrication is enabled owing to the simplicity of a device structure, where thin film  deposition is used. The scheme presented in this work may open the door to a number of applications,  such as reflective displays, imaging devices, colored photovoltaics, and decorations. Keywords  Color filter, Multilayer, Fabry–Perot cavity, Higher-order resonance Color filters are widely used over various areas, such as liquid crystal displays (LCDs), white organic light- emitting diode displays (WOLEDs), image sensors, and anti-counterfeiting ­devices1–4. However, since the existing  color implementation is strongly dependent on the absorption of incident light with specific bands of wave- lengths, conventional color filters based on dyes or pigments exhibit poor colors ascribing to their deficiencies  in brightness, purity, and stability. To address the aforementioned issues, an enormous amount of effort has  been devoted to finding the alternative of the conventional color filters, and structural color filters generating  colors via light-matter interactions in which a narrow optical band can be achieved with relatively trivial losses  leading to simultaneous achievement of both high brightness and high color purity have gained increasing  attention. Numerous approaches based on multi-layered thin ­films5–10, guided-mode ­resonators11–13, plasmonic  ­nanostructures14–20, Mie ­resonators21–25, and ­metasurfaces26–31 have been experimentally demonstrated, all of  which present their superior color performances over the conventional color filters. Particularly, the multi- layered thin film structures are advantageous in that they can be created by a simple deposition process without  a series of complicated and expensive lithographic techniques, thus offering the distinct potential for large-scale  applications. While extensive research efforts have brought remarkable improvements in the brightness, purity,  and stability, much attention has been devoted to the development of transmission-type additive structural color  filters. Reflection-type additive structural color filters have remained unexploited although they play central  roles in a variety of applications, such as electro phoretic displays (EPDs), outdoor signages, imaging devices,  and automotive coatings. Although several approaches toward the reflective RGB color generation have been  demonstrated, they rely on complicated fabrication techniques, and they present low brightness and poor color  purity, thus imposing practical limitations on many potential applications. It is accordingly highly demanded  OPEN 1Department of Physics, Inha University, Incheon  22111, Republic of Korea. 2Department of Electrical and  Computer Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea. 3Department of Mechanical  and Automotive Engineering, Seoul National University of Science and Technology, Seoul  01811, Republic of  Korea. 4These authors contributed equally: Hojae Kwak, Incheol Jung and Dohyun Kim. *email: hwbaac@ skku.edu; jgok@seoultech.ac.kr; ktlee@inha.ac.kr 2 Vol:.(1234567890) Scientific Reports |        (2024) 14:13694  |  https://doi.org/10.1038/s41598-024-64176-4 www.nature.com/scientificreports/ to explore novel structures that are capable of generating additive colors in reflection with high brightness and  high color purity. In this work, we demonstrate engineering a resonant mode in a quad-layered structure consisting of a lossy  metal sandwiched by two dielectric layers on a reflective mirror for the structural color filters creating additive  primary colors of red (R), green (G), and blue (B) in reflection with high brightness and high color purity. An  order of the resonance in the quad-layered structures is well controlled to achieve greatly improved optical  properties, where higher-order resonances are employed for the G color whereas fundamental resonances are  used for the B and R colors. All three RGB filters show high reflectance over 78% with the improved color purity  exhibiting little discrepancy from the standard RGB color space. In addition, high index of refraction of a cavity  medium allows the reflective colors to be nearly invariant when the light is incident at large angles of incidence  up to 40˚. The simple design approach is expected to find a diversity of applications, such as reflective display  technologies, colored solar cells, decorations, imaging devices, and surface coatings for automobiles. Results and discussion Figure 1a shows a schematic view of the proposed reflective RGB structural color filters which consist of the  quad-layered thin films with a configuration of dielectric-metal-dielectric-metal (DMDM) on a glass substrate.  Silver (Ag) is selected as a bottom layer whose thickness is optically thick providing high reflectivity. Tungsten  trioxide ­(WO3) is chosen owing to negligible absorptions and high refractive index in the visible wavelength  regime for the dielectric layers that comprise a Fabry–Perot (FP) cavity ­(WO3 #1) and an anti-reflective (AR) coat- ing ­(WO3 #2). The lossy metal, chromium (Cr), is inserted between the two dielectric layers where constructive  interference between light waves reflected from a surface of Cr and those reflected from the cavity occurs yielding  a peak in the ­reflection32,33. Despite the high reflectance of a bulk Ag that is used at the bottom of the structure  contributing to narrowing the full width at half maximum (FWHM), the high absorption of Cr leads to the low  reflection and hence a broad reflectance peak thus causing the color performance to be significantly degraded.  For the purpose of improving the color purity, the AR layer is well designed which suppresses the reflection of  Figure 1.   (a) Schematic diagram of proposed reflective RGB structural color filters. (b) Calculated resonant  mode number, which is the net phase shift divided by 2π, in the ­WO3 #1 FP cavity and the ­WO3 #2 AR layer  for the RGB. (c) Measured and simulated reflectance spectra of the reflective RGB structural color filters.  (d) Optical images of the fabricated RGB color filter samples. (e) Color coordinates calculated from both the  measured and simulated reflectance spectra along with the standard RGB color space (sRGB). 7 Vol.:(0123456789) Scientific Reports |        (2024) 14:13694  |  https://doi.org/10.1038/s41598-024-64176-4 www.nature.com/scientificreports/ Conclusion In conclusion, we have demonstrated the improved brightness and color purity of the quad-layered reflec- tive structural color filters by controlling the order of the resonant mode in the FP cavity and the AR layer.  The presented devices attain high brightness over 78% and generates pure reflective RGB colors whose color  gamut is analogous to that of the sRGB. Moreover, rigorous analyses on the device structures by exploiting the  net phase analysis, optical admittance loci, and electric field intensity distributions are conducted to provide a  better understanding. Furthermore, the angular performance of the color filters is investigated, exhibiting that  the angle-tolerant characteristic is maintained up to 40° of the incident angle owing to the reduced refraction  angle into the cavity by high refractive index of the cavity medium. Additionally, only the thin film deposition is  required for the device fabrication, bolstering the readiness of mass-production of the large-scale devices. The  demonstrated strategy may pave the way to various applications, such as reflective displays, imaging devices,  optical sensors, color solar panels, and decorations. Data availability The datasets used and/or analyzed during the current study available from the corresponding author on reason- able request. Received: 2 April 2024; Accepted: 5 June 2024 References 	 1.	 Sabnis, R. W. Color filter technology for liquid crystal displays. Displays 20, 119–129 (1999). 	 2.	 Chang, C. H. et al. 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H.K., I.J., D.K., S.J., S.C., C.K., H.K., H.W.B., J.G.O., and K.-T.L. analyzed the results. H.K., H.W.B., J.G.O., and  K.-T.L. wrote the manuscript. All authors reviewed the manuscript. Competing interests  The authors declare no competing interests. Additional information Supplementary Information The online version contains supplementary material available at https://​doi.​org/​ 10.​1038/​s41598-​024-​64176-4. Correspondence and requests for materials should be addressed to H.W.B., J.G.O. or K.-T.L. Reprints and permissions information is available at www.nature.com/reprints. 9 Vol.:(0123456789) Scientific Reports |        (2024) 14:13694  |  https://doi.org/10.1038/s41598-024-64176-4 www.nature.com/scientificreports/ Publisher’s note  Springer Nature remains neutral with regard to jurisdictional claims in published maps and  institutional affiliations. Open Access   This article is licensed under a Creative Commons Attribution 4.0 International  License, which permits use, sharing, adaptation, distribution and reproduction in any medium or  format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the  Creative Commons licence, and indicate if changes were made. The images or other third party material in this  article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the  material. If material is not included in the article’s Creative Commons licence and your intended use is not  permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from  the copyright holder. To view a copy of this licence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/. © The Author(s) 2024
Title: Resonant‑mode engineering for additive reflective structural colors with high brightness and high color purity
Authors: Hojae Kwak, Incheol Jung, Dohyun Kim, Seongcheol Ju, Soyoung Choi, Cheolhun Kang, Hyeonwoo Kim, Hyoung Won Baac, Jong G. Ok, Kyu‑Tae Lee ​
Publisher: Scientific Reports
Date: 0
Abstract: The paper discusses engineering resonant-mode reflective structural color filters for high brightness and color purity, with applications in reflective displays, imaging devices, colored photovoltaics, and decorations. ​ It introduces quad-layered reflective structural color filters that generate vivid additive primary colors by controlling a mode number in a Fabry–Perot (FP) cavity and an anti-reflective (AR) coating layer, achieving high spectral contrast, reflection brightness, and a color gamut comparable to the standard sRGB. ​