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Reproducing the hierarchy of disorder for Morpho -inspired, broad-angle color reflection

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ABSTRACT

The scales of Morpho butterflies are covered with intricate, hierarchical ridge structures that produce a bright, blue reflection that remains stable across wide viewing angles. This effect has been researched extensively, and much understanding has been achieved using modeling that has focused on the positional disorder among the identical, multilayered ridges as the critical factor for producing angular independent color. Realizing such positional disorder of identical nanostructures is difficult, which in turn has limited experimental verification of different physical mechanisms that have been proposed. In this paper, we suggest an alternative model of inter-structural disorder that can achieve the same broad-angle color reflection, and is applicable to wafer-scale fabrication using conventional thin film technologies. Fabrication of a thin film that produces pure, stable blue across a viewing angle of more than 120 ° is demonstrated, together with a robust, conformal color coating.

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Optical analysis.(a) Normal reflectance spectra of the continuous (before ridge formation) film with regular and irregular layers, and the ridge structures with regular and irregular layers, and Morpho Rhetenor. (b) Optical images that compare an actual Morpho Rhetenor wing with different films fabricated in this paper under normal illumination. From left to right: the wing of Morpho Rhetenor, a continuous structure with irregular layer (as shown in the top of Fig. 3c), a ridge structure with regular layers (as shown in Fig. 3b), and a ridge structure with irregular layers (as shown in Fig. 3c). The viewing angles are, from the top, approximately 10, 40, 50, and 60 degrees. The direction of a ridge is aligned perpendicular with rotational plane of viewing angle. (c) Experimentally measured reflection spectra of Morpho Rhetenor under normal incident light conditions. A sketch of the corresponding structure is given on top. The experimental data are given in absolute reflectance values calibrated by an Al mirror. (d) Experimentally measured (middle) and calculated (bottom) reflection spectra of a fabricated, continuous structure with irregular layers under normal incident light conditions. The simulated values are normalized according to ref. 38 and multiplied by cosine to the reflection angle in order to convert to irradiance. Due to finite detector size, the relation between measurement and simulation are relative. (e) Same as (d), but with a fabricated, tapered ridge structure with regular layers. (f) Same as (d), but with a fabricated, tapered ridge structure with irregular layers. Simulated data has been Gaussian blurred as a simple way of incorporating the effect of the finite-sized detector in the measurement setup. Note that all graphs are plotted in log scale.
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f4: Optical analysis.(a) Normal reflectance spectra of the continuous (before ridge formation) film with regular and irregular layers, and the ridge structures with regular and irregular layers, and Morpho Rhetenor. (b) Optical images that compare an actual Morpho Rhetenor wing with different films fabricated in this paper under normal illumination. From left to right: the wing of Morpho Rhetenor, a continuous structure with irregular layer (as shown in the top of Fig. 3c), a ridge structure with regular layers (as shown in Fig. 3b), and a ridge structure with irregular layers (as shown in Fig. 3c). The viewing angles are, from the top, approximately 10, 40, 50, and 60 degrees. The direction of a ridge is aligned perpendicular with rotational plane of viewing angle. (c) Experimentally measured reflection spectra of Morpho Rhetenor under normal incident light conditions. A sketch of the corresponding structure is given on top. The experimental data are given in absolute reflectance values calibrated by an Al mirror. (d) Experimentally measured (middle) and calculated (bottom) reflection spectra of a fabricated, continuous structure with irregular layers under normal incident light conditions. The simulated values are normalized according to ref. 38 and multiplied by cosine to the reflection angle in order to convert to irradiance. Due to finite detector size, the relation between measurement and simulation are relative. (e) Same as (d), but with a fabricated, tapered ridge structure with regular layers. (f) Same as (d), but with a fabricated, tapered ridge structure with irregular layers. Simulated data has been Gaussian blurred as a simple way of incorporating the effect of the finite-sized detector in the measurement setup. Note that all graphs are plotted in log scale.

Mentions: The normal reflectance spectra of the films are shown in Fig. 4a. We find that formation of the tapered ridge highly suppresses the normal reflection, especially in the longer wavelength region. When the viewing angle is changed, however, the colors diverge dramatically, as is shown in Fig. 4b. The color of the ridged film with regular layers changes all over the visible spectrum as expected. In contrast, the color of the ridged film with inter-structural disorder remains blue at all viewing angles. For a more quantitative analysis, the angle-dependent reflection spectra for Morpho Rhetenor and the fabricated structures are shown in Fig. 4c–f respectively. Also shown for comparison are the theoretical reflection spectra of the fabricated structures calculated using the measured irregular shapes of layers and the actual shape of the ridges obtained from SEM images (See Supplementary Information, Fig. S2 for more detailed information on simulation structures). In both cases, we observe a good agreement between calculated and experimental results. Without inter-structural disorder, the spectrum is dominated by sharp peaks confirming that the array indeed is a grating, with a maximum reflection peak near 400 nm and a strong suppression of all reflection in the red (see Supplementary Information, Fig. S3 for the calculated normal reflectance spectra). With inter-structural disorder, we observe a nearly 100-fold reduction in the intensity of grating peaks, and the generation of broad, uniform reflection in the blue, confirming that the inter-structural disorder has reduced the coherence of reflection from the multilayered ridges to suppress the grating effect, despite the identical external shape of the ridges. Such broad-angle reflection is maintained for an oblique incidence angle as well (For the measured and calculated reflection spectra for a 45 ° incident angle and angle-resolved spectra of selected wavelengths, see Supplementary Information, Fig. S4).


Reproducing the hierarchy of disorder for Morpho -inspired, broad-angle color reflection
Optical analysis.(a) Normal reflectance spectra of the continuous (before ridge formation) film with regular and irregular layers, and the ridge structures with regular and irregular layers, and Morpho Rhetenor. (b) Optical images that compare an actual Morpho Rhetenor wing with different films fabricated in this paper under normal illumination. From left to right: the wing of Morpho Rhetenor, a continuous structure with irregular layer (as shown in the top of Fig. 3c), a ridge structure with regular layers (as shown in Fig. 3b), and a ridge structure with irregular layers (as shown in Fig. 3c). The viewing angles are, from the top, approximately 10, 40, 50, and 60 degrees. The direction of a ridge is aligned perpendicular with rotational plane of viewing angle. (c) Experimentally measured reflection spectra of Morpho Rhetenor under normal incident light conditions. A sketch of the corresponding structure is given on top. The experimental data are given in absolute reflectance values calibrated by an Al mirror. (d) Experimentally measured (middle) and calculated (bottom) reflection spectra of a fabricated, continuous structure with irregular layers under normal incident light conditions. The simulated values are normalized according to ref. 38 and multiplied by cosine to the reflection angle in order to convert to irradiance. Due to finite detector size, the relation between measurement and simulation are relative. (e) Same as (d), but with a fabricated, tapered ridge structure with regular layers. (f) Same as (d), but with a fabricated, tapered ridge structure with irregular layers. Simulated data has been Gaussian blurred as a simple way of incorporating the effect of the finite-sized detector in the measurement setup. Note that all graphs are plotted in log scale.
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f4: Optical analysis.(a) Normal reflectance spectra of the continuous (before ridge formation) film with regular and irregular layers, and the ridge structures with regular and irregular layers, and Morpho Rhetenor. (b) Optical images that compare an actual Morpho Rhetenor wing with different films fabricated in this paper under normal illumination. From left to right: the wing of Morpho Rhetenor, a continuous structure with irregular layer (as shown in the top of Fig. 3c), a ridge structure with regular layers (as shown in Fig. 3b), and a ridge structure with irregular layers (as shown in Fig. 3c). The viewing angles are, from the top, approximately 10, 40, 50, and 60 degrees. The direction of a ridge is aligned perpendicular with rotational plane of viewing angle. (c) Experimentally measured reflection spectra of Morpho Rhetenor under normal incident light conditions. A sketch of the corresponding structure is given on top. The experimental data are given in absolute reflectance values calibrated by an Al mirror. (d) Experimentally measured (middle) and calculated (bottom) reflection spectra of a fabricated, continuous structure with irregular layers under normal incident light conditions. The simulated values are normalized according to ref. 38 and multiplied by cosine to the reflection angle in order to convert to irradiance. Due to finite detector size, the relation between measurement and simulation are relative. (e) Same as (d), but with a fabricated, tapered ridge structure with regular layers. (f) Same as (d), but with a fabricated, tapered ridge structure with irregular layers. Simulated data has been Gaussian blurred as a simple way of incorporating the effect of the finite-sized detector in the measurement setup. Note that all graphs are plotted in log scale.
Mentions: The normal reflectance spectra of the films are shown in Fig. 4a. We find that formation of the tapered ridge highly suppresses the normal reflection, especially in the longer wavelength region. When the viewing angle is changed, however, the colors diverge dramatically, as is shown in Fig. 4b. The color of the ridged film with regular layers changes all over the visible spectrum as expected. In contrast, the color of the ridged film with inter-structural disorder remains blue at all viewing angles. For a more quantitative analysis, the angle-dependent reflection spectra for Morpho Rhetenor and the fabricated structures are shown in Fig. 4c–f respectively. Also shown for comparison are the theoretical reflection spectra of the fabricated structures calculated using the measured irregular shapes of layers and the actual shape of the ridges obtained from SEM images (See Supplementary Information, Fig. S2 for more detailed information on simulation structures). In both cases, we observe a good agreement between calculated and experimental results. Without inter-structural disorder, the spectrum is dominated by sharp peaks confirming that the array indeed is a grating, with a maximum reflection peak near 400 nm and a strong suppression of all reflection in the red (see Supplementary Information, Fig. S3 for the calculated normal reflectance spectra). With inter-structural disorder, we observe a nearly 100-fold reduction in the intensity of grating peaks, and the generation of broad, uniform reflection in the blue, confirming that the inter-structural disorder has reduced the coherence of reflection from the multilayered ridges to suppress the grating effect, despite the identical external shape of the ridges. Such broad-angle reflection is maintained for an oblique incidence angle as well (For the measured and calculated reflection spectra for a 45 ° incident angle and angle-resolved spectra of selected wavelengths, see Supplementary Information, Fig. S4).

View Article: PubMed Central - PubMed

ABSTRACT

The scales of Morpho butterflies are covered with intricate, hierarchical ridge structures that produce a bright, blue reflection that remains stable across wide viewing angles. This effect has been researched extensively, and much understanding has been achieved using modeling that has focused on the positional disorder among the identical, multilayered ridges as the critical factor for producing angular independent color. Realizing such positional disorder of identical nanostructures is difficult, which in turn has limited experimental verification of different physical mechanisms that have been proposed. In this paper, we suggest an alternative model of inter-structural disorder that can achieve the same broad-angle color reflection, and is applicable to wafer-scale fabrication using conventional thin film technologies. Fabrication of a thin film that produces pure, stable blue across a viewing angle of more than 120 ° is demonstrated, together with a robust, conformal color coating.

No MeSH data available.


Related in: MedlinePlus