Additionally, further interference effects or diffraction via irregularities in the microstructure heights and differences in the laser radiation exposure during the production process can be avoided.
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The scanning electron microscope SEM images give an overview of the similarities among the individual structures inside the relevant area see Fig. The photonic structures of all the samples were fabricated with the same number of layers. The density was controlled by the variation in the lateral x , y -distances between each microstructure see Fig.
A microscope with a standard halogen lamp was used for the first estimation of the light reflection from the samples. The microstructure top layer in sample A see Fig. However, a thin polymerization junction between single branches can be identified for all samples. This indicates the potential to generate nanometer lamellas and a hierarchical design inside the microstructures. Nevertheless, a variation in the actual microstructure shape did not affect the light reflection properties.
All arrays ensured blue color formation see Fig. Furthermore, the blue color hue was independent of the density of microstructures inside the array, as illustrated by the microscope images in Fig. A decrease of the microstructure density see.
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Artificial blue coloration fabricated with photonic structures using 2PP. A section view of the polymerized array of three different laterally scaled photonic structures from the actual structure geometry see Fig. In comparison to sample A , the variation in the lateral distances between each microstructure is illustrated in the SEM images for samples B and C. The microscope images in D — F demonstrate the color formation results from those arrays.
To study the color formation as a function of the viewing angle, the spectrum of sample A see Fig. Surprisingly, the measured spectra did not resemble either of the expected results. In contrast, the spectrum was independent of the observation angle, which is similar to the property demonstrated by the biological butterfly template 6 , 7 , 8.
The sample was homogeneously illuminated. The blue coloration was independent of the observation angle. Light scattering properties of photonic structures. B The normalized to their area reflection spectra are shown at different observation angles. Upright light microscopy images C at different sample tilt angles were obtained using a microscope color camera.
The simulation results of reflection spectra for the artificial photonic structure using two different approaches is illustrated in D. A specular reflection for multilayer system consisting of 5 layers of the polymer separated by air layers is shown by solid line.
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Furthermore, the dash-line represents the specular reflection of the real photonic structure. Here we evaluate different effects influencing the light reflection on our fabricated photonic structures by the combination of two models of light reflection see Fig. An ideal periodic structure would act as a bragg mirror and the reflection will be angle-dependent and described by the maximum constructive interference condition We suppose that the mainly part of the incident light intensity is reflected on the periodic structures.
However, there is also a scattering of the incident light on the random Rayleigh scattering centers. Thus, the resulting field is a superposition of these two processes. The calculations of these two components of the scattered field can be analyzed as follows. The first order reflection should appear for observation angles around to the surface normal and demonstrate the gradual color change from blue to red. The second order should be observed in the backward direction. These properties were not observed for the structures in the real experiments.
The next interpretation of the optical properties of the artificial photonic structures is the interference in thin films. The reflection angle should be equal to the incident angle, but because of the interference the specular reflection should be colored. Considering the presence of gaps between individual printed layers on the edges of the structures, and assuming that the thickness of both gaps and polymer layers is equal, the spectrum of specular reflection could be calculated see solid line in Fig.
Finally, the reflection less dependent on observation angle could be caused by Rayleigh scattering.
In this case the reflection intensity is inversely proportional to the forth degree of wavelength. To determine the existence of the microstructure hierarchical design, including the nanoscaled lamellas and interlayered air cavities, the samples were scratched and examined with an SEM. An uniform five-layer lamination consisting of air cavities and a thin polymer material was identified in all three samples and is shown for sample A in Fig.
The cross section of the lamellar 2PP structures was comparable to the shape of a Christmas tree compare Fig. The layer dimensions of the air cavities and artificial lamellas see Fig. As a result of the similar design properties, the angle-independence of the coloration and a suitable polymer refractive index value, the artificial blue color formation has Morpho -like characteristics.
Therefore, the color formation and its properties are based on a multilayer interference, diffraction and scattering. The resulting coloration based on 2PP fabricated photonic structures are shown in A — C.
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The oblique views of the microstructures demonstrates the hierarchical design containing air cavities and polymer lamellas. Since the color formation depends mainly on the aforementioned interference condition for a multilayer stack, the coloration can be changed by varying the thickness of the polymer lamellas and air cavities The overall size for both features fits to the theoretical values calculated with the interference condition of a multilayer stack. The variation of those parameters was controlled by different laser power values. In conclusion, those results strongly support that the natural blue coloration of the Morpho largely depends on the effect of multilayer interference.
The angle-independence of the artificial coloration can be explained by the imperfectness of the artificial hierarchical system.
Structural coloration with similar optical properties as the Morpho butterfly was clearly achieved in our experiments. Fully cured layers with the same thickness and cube-shaped polymerization only showed a high transparency. The primary microstructures written directly using 2PP see Fig. Non-trivial light scattering was observed see Fig. Although the primary microstructure see Fig.
The nanostructures result from nanoscaled polymerizations between single branches via diffusion of radicals 45 and they are spaced by an artificial ridge layer see Fig. The generated coloration is non-iridescent. While the effect of iridescence depends on long-range order, e. Therefore, a disorder in the orientation of the polymer lamellas and air cavities see.
Since iridescence is a great feature for organisms in nature to generate display functions 2 , 3 , this optical property is disadvantageously for the most real world applications, e. Although high precision devices are obviously better suited for improving the 2PP process, the stage deviation contributes to an addition of disorder in the color system which can also be observed in nature.
Therefore, the complex modeling of the exposure paths or the integration of complex algorithms to mimic disorder can be avoided. However, the degree of disorder in one of our artificial photonic structures is less in comparison to the disorder in natural color systems.
All microstructures are highly ordered oriented in the fabricated area and the disorder can only be identified in the artificial periodic nanostructures. In contrast, the wing of the Morpho butterfly, for example, is more complex. The wing contains many hierarchically structured scales on different levels see Fig. Our results demonstrate that bioinspired non-iridescent coloration can be generated by specific biomimetic disorder.
Thus far, replicas of the Morpho lamellas can be artificially created using multilayer deposition 23 , 24 , FIB-CVD 25 , synthesis of nanostructures 26 , 27 , 28 , 29 , and e-beam lithography 30 , 31 , However, these fabrication processes require cumbersome tools and workflows that are neither suitable for research and development of a uniform physical concept to fabricate different structural color hues or for establishing a broad variety of recipes and rules to fabricate structural colors with different hues. The other methods are highly complex due to the usage of vacuum atmosphere, individual masks for the fabrication of different structure geometries 32 , different materials 23 , 24 , 31 , or multiple manufacturing process steps 26 , 27 , 28 , 29 , We demonstrated for the first time in the present paper an effortless method for the generation of structural color that is much simpler than the aforementioned fabrication tools.
This method uses 2PP to create an adapted cross-sectional geometry of the Morpho Christmas tree structure in a single photosensitive material.