New metamaterials come near stealth technology

A Harry Porter invisibility cloak is just a product of imagination and, in fact, to create a similar product requires the use of negative refractive index materials for all. Optical wavelengths range from red to purple.

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To date, optical materials with such artificial structures are only available in some very narrow wavelengths, thus limiting the ability to cover many color bands. However, this obstacle may soon be lifted by optical engineers at Stanford University having successfully designed a wide-range metamaterial with a negative refractive index for 7 colors. basic of the rainbow.

Duke's first "cloak" was created based on the principle of bending light around a covered object . However, this mechanism is not simple because the light escaping from the coating must correspond to the polarity and phase of the light beam passing through. If not, the shirt will reveal the whole picture and in this case, you may not be able to see what is covered but it is possible that "there is a shirt here".

According to Associate Professor Jennifer Dionne: "All natural materials have a positive refractive index . " For example, air at standard conditions has the lowest refractive index in nature, ie above a little. Meanwhile, the refractive index of water is 1.33, of diamond is 2.4. The higher the refractive index material, the more deflected the light is than the original path. If the refractive index is close to zero or negative, a very interesting physical phenomenon will occur. We can imagine the following, if you insert a straw to tilt in the cup of water, if the refractive index of the water is negative, the image of the straw that we see will reverse. The top part of the straw (on the water surface) is tilting from right to left but the lower part (below the water surface) will tilt from left to right instead of the same direction (below image).

In order to obscure an object or limit refraction, the material must be able to precisely control the path of light according to the hypothesis. However, very few natural materials can meet and the answer is optical metamaterials. Unlike natural materials, metamaterials inherit optical properties from the geometric shapes of nano-cell units that make it known as " artificial atoms" . By changing the shape of these cell units, the refractive index of the material can reach a positive value, close to zero or negative.

In addition, such materials need to interact with both the magnetic field and the electric field of light. In the past, many researchers have tried to create a metamaterial containing artificial atoms including an element that interacts with an electric field and the other component that interacts with the magnetic field. Each component interacts with different colors of light and is difficult to interact with multiple wavelengths at the same time.

As a result, the wave range or wavelength range for each component's activity is often limited. The invisible cloak can only work with golden light after all and of course is not useful at all unless you want to hide a yellow, banana-colored banquet.

Therefore, Jennifer Dionne and graduate students Hadiseh Alaeian, Ashwin Atre and pre-doctoral student Aitzol Garcia designed a new metamaterial with a unified structure that allowed it to interact effectively with both electric fields. and the magnetic field of light on a variety of light colors.

Dionne's team used a technique called " conformal transformation" to convert a two-dimensional metamaterial structure with desirable optical properties into a three-dimensional form of billions. Nanoscale with optical properties remains the same.

The transformed nanostructures are shaped like a crescent moon, narrow at the ends and thin at the center and they are artificial atoms that make metamaterials. Atoms are arranged on a periodic network and with the current design, metamaterials have a negative index of refraction above a wavelength of 250nm in many visible spectral regions and near-infrared spectra. The team says that with just a few structural adjustments, metamaterials can be used on multiple wavelengths.

Atre said: "We can adjust the shape of the crescent or shrink the atomic size, so the metamaterial can cover visible spectra, from 400 to 700nm".

Of course, from Stanford's research to Harry Porter's invisibility shirt is a long way off. However, the aforementioned metamaterial has laid the groundwork for new layers of materials that could one day be used to make a truly invisible cloak or coat, at least with the human eye. Stanford University research has been published in Advanced Optical Materials.