Photo source: arttanja / shutterstock.com
Written by Zhang Bo
01 reading guidance
Recently, Professor Zhang Shuang of the University of Birmingham, Professor Han min of Nanjing University, China, and many scientists of the University of Munich, Germany, integrated disordered silver nanoclusters, transparent lithium fluoride (LIF) films and silver films based on the coupled mode theory and three-dimensional full wave electromagnetic field simulation, and made an optical system that can regulate the reflection spectrum with limited bandwidth.
As a disordered plasma system, the silver nanoclusters can enhance the light field; the silver film is like a mirror, which can enhance the ability of LIF film to limit light; by changing the thickness of LIF film, the attenuation rate of optical mode can be changed, resulting in the mismatch with the coupling rate, resulting in a limited bandwidth light response and emitting a specific mode of light.
As shown in Figure 1, using a lif film whose thickness varies linearly along the diagonal, the system can emit "seven color light" of the rainbow.
Figure 1. Schematic diagram of optical system composed of silver film / lithium fluoride film / silver nanoclusters
This technology does not need to use the complex and time-consuming lithography process, it can produce the required structure color, and it can also be used to visualize external excitation, such as mechanical pressure, sound wave, and develop new sensing technology.
This study also broadens our understanding of disorder system, and provides a new scheme for the regulation of disorder system and the design of disorder photonics.
02 research background
As shown in Figure 2, from the red rose petals to the blue wings of butterflies, the animals and plants on earth contain almost all colors.
Part of the color comes from some chemicals, like anthocyanins;
The other part of color is produced by the interaction of light and micro nano structure of optical scale through interference, diffraction or scattering, the former is called pigment color, the latter is called structure color.
Compared with pigment color, structural color does not need pigment or dye, so it has good stability, high temperature resistance, corrosion resistance, no fading problem and little impact on the environment.
However, it is difficult to manufacture the required microstructure cheaply and in large quantities, which hinders the application of structural color.
Figure 2. Red rose petals and blue butterflies
Inspired by the structural colors of natural organisms, researchers want to produce controllable colors by controlling light through artificial disordered microstructure.
As shown in Figure 3, the disordered nanostructures are not easy to be affected by disturbances and initial conditions, and have the characteristics of broadband light absorption. They are superior to the traditional periodic structures in terms of solar energy utilization, broadband light capture, broadband transmission enhancement, light focusing, and black structure color generation. However, there is a lack of research on the regulation of disordered system.
Figure 3. Broadband absorption image of disordered nanostructures
As shown in Figure 4, the transparent LIF film is plated on the silver mirror as an external cavity to provide the required optical environment.
The silver mirror can improve the ability of limiting light in the external cavity, and enhance the light matter interaction, and then deposit the disordered silver nanoclusters on the LIF film, forming a structure similar to Fabry Perot.
By changing the thickness of the external cavity and the coupling efficiency of the optical mode, the optical characteristics of the disordered plasma system are successfully transformed from broadband absorption to limited bandwidth reflection.
And only by controlling the thickness of LIF film, we can produce a specific mode of light.
Figure 4. The change of light response of the system with the thickness of lithium fluoride film
03 innovation research
3.1 introducing rate mismatch based on coupled mode theory. Firstly, the author calculated the coupling efficiency η of the optical mode of the plasma system and the optical cavity shown in Figure 5 based on coupled mode theory (CMT).
They found that when the coupling rate of the optical mode 1 / τ E and the intrinsic decay rate 1 / τ K0 match, the coupling efficiency η reaches the maximum, and the system presents the characteristics of broadband absorption.
Figure 5. Schematic diagram of a simple optical cavity
However, as shown in Fig. 6, when the attenuation rate of a particular optical mode suddenly decreases, it will be mismatched with the coupling rate, resulting in a decrease in the coupling efficiency of the optical mode.
The light in this mode can't be absorbed, but reflected out, forming a limited bandwidth reflection.
Figure 6. Rate mismatch induced limited bandwidth photoresponse image
3.2 3D full wave simulation reveals the physical mechanism. Then, the author uses FDTD algorithm to simulate 3D full wave electromagnetic field.
As shown in figure 7A-C, the calculation model consists of spherical silver nanoparticles, LIF interlayer and silver film.
By changing the diameter and position of nanoparticles, different degrees of disorder (α) are introduced. For systems with different degree of disorder, the corresponding reflection spectrum and the change of reflection spectrum with the thickness of LIF interlayer (T) are shown in figure 7d-f.
It can be seen that when t = 50 nm, the reflection ability of the system decreases with the increase of disorder degree.
As shown in Fig. 7F, for the system with large degree of disorder (α = 0.5), with the increase of the thickness of LIF interlayer, an obvious reflection band appears, and the position of the reflection peak strongly depends on the thickness of the interlayer.
However, as shown in Fig. 7d, for a system with a small degree of disorder, there is no reflection band depending on the thickness of the external cavity in the whole frequency band of the visible spectrum.
Therefore, the disordered plasma system is essential in the system of limited bandwidth reflection.
Fig. 7. A-c optical system with different degree of disorder.
The reflection spectrum of d-f platform varies with the thickness of LIF interlayer.
The spatial distribution of electric field intensity of the system with the same degree of disorder and different thickness of the interlayer is shown in Figure 8.
Fig. 8g and H correspond to the electric field intensity distribution of the system on the x-z plane and the Y-Z plane when t=50 nm and 110 nm respectively.
It is clear that when t = 50 nm, most photons are captured and dissipated by the plasma system; when t = 110 nm, a large part of photons are reflected.
Therefore, the light response of the system can be adjusted by changing the thickness of the interlayer.
Figure 8. Spatial distribution of electric field intensity of the system under different thickness of the bay.
3.3 experimental fabrication of an optical system that can adjust the reflection spectrum. The LIF thin film is coated on the silver mirror by thermal evaporation technology, and the silver clusters are deposited on the LIF thin film by gas-phase cluster beam technology.
The thickness of LIF film can be precisely controlled by thermal evaporation technology, and the precision can reach the nanometer level.
As shown in Figure 9, the author found in the experiment that the average reflection spectrum in the visible light range increases monotonically with the thickness of the LIF film, and then oscillates, and the position of the reflection peak is directly proportional to the thickness of the film.
Fig. 9. The average reflection spectrum and the position of the reflection peak vary with the thickness of the lithium fluoride film.
In order to prove the key role of disorder structure in experiments, the author studies the change of reflection spectrum of the system with disorder degree.
As shown in Figure 10F, when the density of nano clusters is low, the interaction between nano particles is weak, and the shape disorder plays a leading role; with the increase of density, the nonlinear interaction between nano particles is enhanced, the disorder degree of the system is increased, and the bandwidth of the reflection spectrum of the system is significantly reduced, as shown in Figure 10g. Figure 10. The scanning transmission electron microscope (stem) image (f) of the silver nanoclusters and the corresponding reflection spectrum (g) of the samples.
As shown in Fig. 11a, the author has made a platform with a thickness gradient of LIF film.
The optical image of the platform is shown in Figure 11b. Along the diagonal direction, the system produces rainbow like colors, which proves that the system has the ability to continuously adjust the reflection spectrum, which can be used to easily generate structural colors.
Figure 11. Schematic diagram (a) and optical image (b) of the sample composed of silver mirror / gradient lithium fluoride film / silver nanoclusters.
The author also uses their system to reproduce Mr. Qi Baishi's peony, as shown in Figure 12. The original peony contains red, magenta, yellow, green and black, while the traditional periodic structure is not easy to produce black.
In their system, the author only adjusts the thickness of the interlayer to produce various colors in the original.
Figure 12. The peony of Qi Baishi
04 application and Prospect
This study shows that the disorder system can also be effectively regulated to produce the structure color independent of the material.
As Professor Zhang Shuang said, the way in which colors are produced in nature is fascinating. If we can effectively use these ways, we can produce more vivid and bright colors.
Dr. Liu Changxu, co-author of the paper, added that in physics, we often think randomness in nanofabrication is harmful.
However, their research shows that in some cases, random structure can produce better results than ordered structure, such as for new sensing technology.
The topic of this paper is managing ordered cellular systems by external activity with transition from broadband absorption to reconfigurable reflection, published in nature communications.
Download address:
https://doi.org/10.1038/s41467-020-15349-y
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? source: light academic publishing center, Changchun Institute of Optics and mechanics, Chinese Academy of Sciences
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