In nature, colours often result from the interaction of light with periodic micro- or nano-structures. Scientists around the world are using Nanoscribe’s two-photon polymerisation (2PP)-based 3D printing technology to investigate various strategies for producing structural colours.
There is a strong interest in studying structural colour because in nature, this effect occurs in a variety of forms. For example, butterfly wings are of particular interest due to their vibrant and iridescent structural colours. Unlike synthetic dyes and pigments, structural colouration offers advantages such as brightness, usually angle dependent, stability in terms of fade resistance and environmental friendliness. Two-photon polymerisation-based 3D printing enables the precise and reliable fabrication of nature-inspired structures that generate structural colours. By exploiting the outstanding spatial resolution of the technology down to the submicron range, the dependence of structural colours on various structural parameters can be experimentally investigated.
Bi-grating structure for structural colors - optimierte Vorlage
A 3D printed bigrating nanostructure mimicking the blue iridescence of the Cynandra opis butterfly’s wings. The hue and colour purities can be adjusted via the structure parameters. Image: ETH Zürich.
Project 1: planar bigrating colour filters
Research team
- Institute for Chemical and Bioengineering (ICB), ETH Zürich, Switzerland
- Institute of Food, Nutrition and Health, ETH Zürich, Switzerland
- Institute of Environmental Engineering, ETH Zürich, Switzerland
- Guangzhou International Bio Island, China
- College of Science, Zhejiang University of Science & Technology, China
- Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, China
Researchers led by Professor Andrew deMello at the ETH Zürich used 2PP-based 3D printing to produce bigrating nanostructures that mimic those responsible for generating blue iridescence in the Cynandra opis butterfly’s wings1. The 3D structures consist of two grid layers stacked perpendicular to each other, also known as a ‘crossed double-grating’ structure. Such a structure consists of two diffractive planes, with an array of ridges forming the first and a perpendicular array of ridges below this forming the second. These orthogonal planes can diffract light in both the X and Y directions. The combined effects of diffraction and interference generate colouration.
Based on these observations, the scientists designed and printed bigrating nanostructures with different parameters. In this way, they were able to investigate the influence of incident angle, period and height on the structural colouration. Variations in the feature period and/or height of the ridges affected both hue and colour purity. The transparent substrate used to print the structures allowed the researchers to illuminate the structures from behind to create the colouration effect under different incident angles. Variations in the grating period between the first and second planes, while keeping the grating height constant, resulted in the generation of a full range of colour pixels in a plane covering the visible spectrum. This type of bigrating structure was used to print a metre-sized image scaled down as a thumbnail image with millimetre dimensions and micrometre pixel size. Such multicolour structures are likely to find applications, for example, in digital 3D displays, colour filtering and high-density data storage such as in microimage displays.
Moving from planar structural colouration to 3D objects that reflect structural colours is still a challenge. However, structural colours in 3D exploit the freedom to shape, control and display colours beyond the limitations of their 2D counterpart. Woodpile photonic crystals (WPCs) are promising structures that can be used as building blocks to form 3D shapes that display structural colours. However, these colours are generated when WPCs are illuminated from the top, and to achieve visible stop bands along the stacking direction, structural resolutions below 500 nm in all spatial directions are required. Manufacturing these structures via direct laser printing is challenging. Structural resolution can be improved, for example, by using advanced systems and novel materials or by post-processing steps such as heat shrinking.
Project 2: 3D shapes with structural colours
Research team
- Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), Singapore
- Engineering Product Development, Singapore University of Technology and Design
- School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore
Researchers collaborated on a new strategy for using WPCs to create 3D structural colours2. They investigated WPCs’ band structures under lateral illumination both theoretically and experimentally.
The 2PP-based 3D printing process was used to fabricate the WPCs, which allowed for the avoidance of post-processing steps and printing subwavelength lattice constants. It was possible to fabricate a wide range of WPCs with varying in-plane (from 750 to 1,300 nm) and out-of-plane (from 900 to 1,400 nm) pitches. The height of the rods was 380 nm, while the width was 130 nm. With this very small rod width, the WPCs generated high reflectance of bright structural colours, reaching up to 50 percent reflectance. This approach resulted in WPCs that reflected a wide range of colours, from blue, cyan, green, green-yellow, yellow, to red and purple. The vivid colours generated by these structures covered more than 85 percent of the sRGB colour space and exhibited excellent colour purity.
The researchers also validated the printing of arbitrary 3D shapes with colouration. They printed the Merlion, the official mascot of Singapore, and the 3DBenchy, a benchmark computer model for 3D printing performance. These 3D shapes were colourful, demonstrating the ability to tune the colours of different parts of the 3D shapes. Precise colour tuning at the voxel level was achieved by simultaneously changing in-plane and out-of-plane pitches, even in intricate geometries. The colours reflected by these structures enabled both gradual and abrupt colour changes. These findings are expected to open up applications for 3D freeform structural colours in fields such as colouration-based sensors, colour displays, light emitting devices and anti-counterfeiting technologies.
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Image 03 SEM of 3D-printed benchy - optimierte Vorlage
The 3DBenchy printed in woodpile photonic crystals (WPCs), which allow it to exhibit different structural colours. Image: Agency for Science, Technology and Research (A*STAR).
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Image 04 SEM benchy close-up - optimierte Vorlage
: A closeup of the printed 3DBenchy showing the WPCs in detail. Image: A*STAR.
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References
1Cao, X., Du, Y., Guo, Y., Hu, G., Zhang, M., Wang, L., Zhou, J., Gao, Q., Fischer, P., Wang, J., Stavrakis, S. and deMello, A. (2022). Replicating the Cynandra opis butterfly's structural color for bioinspired bigrating color filters. Advanced Materials, 34(9), p.2109161.Available at: https://bit.ly/3uNoB0i
2Liu, H., Wang, H., Wang, H., Deng, J., Ruan, Q., Zhang, W., Abdelraouf, O., Ang, N., Dong, Z., Yang, J. and Liu, H. (2022). High-order photonic cavity modes enabled 3D structural colors. ACS Nano, 16(5), pp.8244-8252.Available at: https://bit.ly/3P66MS5