Optical illusions of structural color.

It is said that the nano- and the microworld have 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 a lot of amazing examples in nature where the color is formed precisely by micro-structures, such as in butterflies or in crystals of opal.

If Nature took many millions of years of evolution to create all the variety of colors, the scientists of materials science and physics have learned in the last few decades literally "synthesize" colors in the laboratory. For the achievements of science in the field of meta-materials and colored, but colorless microstructures, welcome to the cat

Instead of the preface

What is meta-material? Metamaterial is a material whose properties are due not so much to the properties of its constituent components, but primarily to how these components are located in relation to each other in space, in other words, artificially created periodicity.

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

Classics of the genre: metallic 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" electron cloud around a "heavy" core of metal atoms under the action of light (or any other electro-magnetic wave).

This property is widely known, for example, for gold, copper, aluminum and silver, and it manifests itself in the form of a peak of resonance absorption of light. When two or more such particles are near, then there is an effect of mutual influence, which is expressed in the displacement of the peaks and the appearance of new ones. Thus, it is possible to "adjust" the wavelength of the absorbed radiation and, correspondingly, change the color of the structure consisting of such nanoparticles.

A group of Chinese scientists together with their colleagues from Switzerland proposed the use of silver disks, separated by an insulator – aluminum oxide, As structural blocks for creating meta-materials.

(a) Schematic representation of silver nanodiscs on the substrate surface and propagation of the electromagnetic wave between them. (B) An image of a real sample obtained by electron microscopy. (C) So-called plasmon modes of a separate nanodisk

Plasmonic particles in themselves have too wide a spectrum to be used directly to produce "pure" colors. However, the authors went to a certain trick, dividing plasmonic disks with a layer of dielectric, aluminum oxide. The basic principle behind this idea is to use hybrid modes that have a much narrower peak in the "reflection" mode and more complete absorption in the "pass" mode with the appropriate radius and period selection. Thus, by adjusting the size and spatial arrangement of composite discs (for example, the packing step) within the "pixel" of the image, you can create a full color gamut: from red to blue-violet

] Obtained samples in reflected (left) and transmitted (right) light: first line – optical photographs of samples, second line – simulation data and third line – experimentally obtained results

Unfortunately, the process of creating such structures Quite a lot of work and including into a plurality of stages of high technology, such as layer deposition, the creation of patterns using an electron beam, and even ionic etching. Nevertheless, scientists are confident that this work will be another step in the way of creating a platform for printing with the help of "structural" colors, as well as standards for high-precision color rendering.

] An example of the use of this technology: the abbreviations of two universities involved in the project, "printed" with plasmon colors

Original article " Full Color Generation Using Silver Tandem Nanodisks " 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 Harbin Institute of Technology (Harbin Institute of Technology) and Shanxi University. Instead of metallic nanoparticles, they proposed the use of titanium dioxide (TiO 2 ). One of the features of this material is a rather high refractive index (> 2) compared to other materials. This property of TiO 2 for example, is widely used in the creation of photonic crystals (once and twice).

The basic idea is to isolate from the spectrum the required wavelength, using the interference of the incident and reflected rays, which And gives the color to the structure. After carrying out the simulation and adjusting the structure parameters and the corresponding resonant modes to the visible part of the spectrum, the researchers were given the opportunity 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: Ray lithography to create a pattern and spraying a layer of titanium dioxide from the gas phase, followed by dissolution and removal of the photoresist. Thus, the production process is simpler than for the silver nanodisks described above, for which these stages are repeated several times.

The process of production of a meta-material based on TiO 2 . (A) Scheme of the process. (B-c) Micrographs of the resulting structures (low and high magnifications, scale marks 100 microns and 500 nm, respectively). (D-f) The resulting patterns and their corresponding colors (scale mark – 1 micron)

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

Reflection spectra and corresponding structural colors. (A) Simulated and experimentally obtained reflection spectra of one of the meta structures. (B) Calculated (black) and adjusted (red) according to CIE 1931 standard color. (C-d) Dependence of color on the period of the obtained structure and the distance between neighboring pyramids

But the authors of the work did not stop there. To show the applicability of technology to create complex multi-colored images, they "painted" the coat of arms of Harbin University.

An example of creating an image from meta structures of TiO 2 . (A) A micrograph obtained with an electron microscope. Images in reflected (b) and transmitted (c) light. (D) Polarized light image. Scale mark – 159 microns

Original article " All-Dielectric Full-Color Printing with TiO 2 Metasurfaces " published in ACSNano (DOI: 10.1021 / acsnano

How about the ordinary glass?

The last example to date is the use of conventional glass and not quite ordinary physics to create meta-materials and structural colors. The main idea of ‚Äč‚Äčthis work is that you can print using the "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 in properties to glass. Such a method is called nanotechnological lithography (1945).

The process of manufacturing a meta-material. (A) Preparation of a "stamp" using electron beam lithography. (B-d) Different stages of the process of nanotechnological lithography, as a result of which the pattern of the stamp is "imprinted" on the smooth glass during the polymerization of a special gel. (E) Optical micrograph of the obtained pattern. (F) The profile of the "printed" pattern (the resulting structures have a width of about 200 microns, and a height of only 50-60 nanometers)

The structures obtained can be of a wide variety of shapes: straight or curved lines, voids or massive bulges. If straight lines are chosen as a pattern, then we get the usual diffraction grating, which is depicted on the KPDP.

However, if several similar structures are combined into one (for example, with different periods, different strip thicknesses, etc.) And to bend, then the incomprehensible magic of physics begins to work. The light that is incident and absorbed in some sections of the structure is transferred inside it, as inside the fiber, and "leaves" on the surface at other sites.

Photos of two glass samples whose patterns are "tuned" to show different colors (a) or full-color images (b). Important! Under other conditions, such as lighting, viewing angle and so on, the image is not visible, see the video

Thus, the colors only "appear" only in specified areas and at specified viewing angles. To demonstrate the effect, the authors posted the following videos:

This technology, as the authors believe, can find its application in the field of security (for example, document and data protection), as well as creating smart displays

Original article " Color-Selective and Versatile Light Steering with up-Scalable Subwavelength Planar Optics " published in ACSPhotonics (DOI: 10.1021 / acsphotonics.7b00232). ]

Instead of confinement

In the past 10-15 years, significant progress has been made in the field of meta-materials. Scientists try to approximate the once-physical abstraction to the development by industry, to make meta-materials mass and to find their niche of applications every year.

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