Meta-materials: optical illusions of structural color

It is said that the nano- and microcosm has no color, because the particle size is smaller than the wavelength. Accordingly, the wave properties of light, such as diffraction and interference, prevail over ordinary absorption, reflection. However, there are in nature a lot of amazing examples when color is formed precisely due to micro-structures, such as, for example, butterflies or opal crystals.



If it took Nature millions of years of evolution to create all the variety of colors, then scientists of material scientists and physicists over the past several decades have learned in the literal sense of the word "synthesize" colors in the laboratory. For the achievements of science in the field of meta-materials and color, but colorless microstructures, welcome under the cat!

Instead of the preface

And what is a meta-material? A metamaterial is such a material, whose properties are caused not so much by the properties of its constituent components, but primarily by the way these components are located relative to each other in space, in other words, by artificially created periodicity.



Below is an overview of three interesting, in my opinion, works recently published in first-class scientific journals. All of them are devoted to the creation of metamaterials from various materials, both classical and exotic.

Classic genre: metal nanoparticles and plasmon resonance

Many metals in the nanodispersed state (read, nanoparticles) have a so-called plasmon resonance. Simply put, collective oscillations of a “light” cloud of electrons near the “heavy” core of metal atoms under the action of light (or any other electro-magnetic wave). In more detail wrote about it here .



This property is widely known, for example, for gold, copper, aluminum and silver, and it manifests itself in the form of a peak of resonant absorption of light. When two or more of these particles are close together, a mutual effect occurs, which is expressed in the shift of the peaks and the appearance of new ones. Thus, it is possible to “tune” the wavelength of the absorbed radiation, and accordingly, change the color of the structure consisting of such nanoparticles.



A group of Chinese scientists, together with their colleagues from Switzerland, proposed using silver disks, separated by a dielectric, aluminum oxide, as structural units for creating meta-materials.









(a) Schematic representation of silver nanodisks on the substrate surface and the propagation of an electromagnetic wave between them. (b) Electron microscopy image of a real sample. (c) So-called plasmon modes of a separate nanodisc



Plasmon particles themselves have too wide a spectrum to use them directly to produce “pure” colors. However, the authors went to a certain trick, dividing the plasmon disks with a dielectric layer, aluminum oxide. The basic principle underlying this idea is to use hybrid modes that have a much narrower peak in the “reflection” mode and a more complete absorption in the “transmission” mode with an appropriate choice of the radius and period. Thus, by adjusting the size and spatial arrangement of the composite discs (for example, the packing step) inside the “pixel” of the image, you can create a full range of colors: from red to blue-violet.









The obtained samples in reflected (left) and transmitted (right) light: the first line contains optical photographs of the samples, the second line contains the simulation data and the third line shows the experimentally obtained results.



Unfortunately, the process of creating such structures is quite laborious and involves many high-tech stages, such as layer-by-layer deposition, the creation of patterns using an electron beam, and even ion etching. Nevertheless, scientists are confident that this work will be another step towards creating a platform for printing using “structural” colors, as well as standards for high-precision color reproduction.









An example of the use of this technology: abbreviations of two universities involved in the project, “printed” with plasmon paints



The original article " Full Color Generation Using Silver Tandem Nanodisks " was published in ACSNano ( DOI: 10.1021 / acsnano.6b08465 ).

Non metallic particles and surfaces

Another example of the creation of structural colors was demonstrated by scientists from the Harbin Institute of Technology (Harbin Institute of Technology) and Shanxi University (Shanxi University). Instead of metallic nanoparticles, they proposed using titanium dioxide (TiO ₂ ). One of the features of this material is a rather high refractive index (> 2) compared with other materials. This property of TiO ₂ , for example, is widely used to create photonic crystals ( one and two times ).



The basic idea is to isolate the desired wavelength from the spectrum using the interference of the incident and reflected beam, which gives the color to the structure. After modeling and adjusting the structure parameters and the corresponding resonant modes to the visible part of the spectrum, the researchers were able to control the color by changing the size of the structure elements.



The proposed manufacturing process of such a meta-material includes a smaller number of technologically complex processes: electron-beam lithography to create a pattern and deposition of a layer of titanium dioxide from the gas phase, followed by dissolving and removing the photoresist. Thus, the production process is simpler than for the silver nanodiscs described above, for which these steps are repeated several times.









The production process of a meta-material based on TiO ₂ . (a) Process flow. (bc) Micrographs of the structures obtained (low and high magnification, scale marks 100 microns and 500 nm, respectively). (df) Received patterns and corresponding colors (scale mark - 1 micron)



As a result, samples were obtained that cover a significant part of the color gamut: there are blue, red and green shades. If we continue the comparison with plasmon particles, then TiO2-based meta-materials have demonstrated a wider color gamut.









Reflection spectra and corresponding structural colors. (a) The simulated and experimentally obtained reflection spectra of one of the meta-structures. (b) Calculated (black) and corrected (red) in accordance with CIE 1931 standard colors. (cd) Color dependence on the period of the structure obtained and the distance between adjacent pyramids



But the authors of the work did not stop there. To show the applicability of the technology for creating complex multi-colored images, they “painted” the coat of arms of Harbin University. Different colors and shades were used.









An example of creating images from TiO ₂ meta-structures. (a) Electron microscope micrograph. Images in reflected (b) and transmitted (c) light. (d) Image in polarized light. Scale label - 159 microns



The original article " All-Dielectric Full-Color Printing with TiO ₂ Metasurfaces " was published in ACSNano ( DOI: 10.1021 / acsnano.7b00415 ).

How about ordinary glass?

The latest example for today is the use of ordinary glass and not quite ordinary physics for creating meta-materials and structural colors. The main idea of ​​this work is that you can print using a “master” stamp on almost any transparent surface, including glass. Under such a stamp with a given pattern, a special gel is poured, which hardens under the action of ultraviolet and turns into a material similar to the properties of glass. This method is called nanoprinted lithography (nanoimprint lithography).









The process of making a meta-material. (a) Preparation of a “stamp” using electron-beam lithography. (bd) Various stages of the process of nanopress lithography, as a result of which the stamp pattern is “imprinted” on a smooth glass during the polymerization of a special gel. (e) Optical micrograph of the obtained pattern. (f) Profile of the “printed” pattern (the structures obtained are about 200 microns wide, and only 50-60 nanometers in height)



The structures obtained can be of the most diverse forms: straight or curved lines, voids, or vice versa, massive bulges. If straight lines are chosen as a pattern, then we get the usual diffraction grating, which is shown on the CDRV.



However, if we combine several similar structures into one (for example, with different periods, different strip thicknesses, and so on) and even bend, the inscrutable magic of physics begins to work. Light falling and absorbed in some parts of the structure is transferred inside it, as inside an optical fiber, and it "comes out" to the surface in other parts.









Photographs of two glass samples, the patterns of which are “tuned” to show different colors (a) or full images (b). Important! under other conditions, such as lighting, viewing angle, and so on, the image is not visible, see video



Thus, colors literally “appear” only in specified areas and at given angles of view. To demonstrate the effect of the authors of the work posted the appropriate video:

• Three color pixels appear together only at a certain angle (video 5 on the publisher's website and a mirror on Y.disk )
• Two completely different objects can be located at the same place depending on the angle of view (video 6 on the publisher's website and a mirror on Y.disk )

This technology, according to the authors of the work, will be able to find its application in the field of security (for example, protection of documents and data), as well as when creating displays for smart glasses.



The original article " Color-Selective and Versatile Light-Steering with an Up-Scalable Subwavelength Planar Optics " is published in ACSPhotonics ( DOI: 10.1021 / acsphotonics.7b00232 ).

Instead of conclusion

Over the past 10-15 years, there has been significant progress in the field of meta-materials. Every year, scientists are trying to bring the physical abstraction closer to industrial development, to make mass materials massive and to find their niche for applications.



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