Dr. Don Gregory and Seyed Sadreddin Mirshafieyan

Schematic diagram of the electrically adjustable perfect light absorber.

Newswise — In recent years, technology for controlling the absorption of light at selected wavelengths in nanostructures has attracted much attention; however, dynamically adjusting the absorption wavelength without changing the structure's geometry is somewhat elusive. Dr. Don Gregory, Distinguished Professor of the Department of Physics and Astronomy at the University of Alabama Huntsville (UAH) recently published a scientific report and his doctoral thesis in the journal Nature. Student Seyed Sadreddin Mirshafieyan proposed a solution.

Their paper "Electrically adjustable perfect light absorber as a color filter and modulator" theorized the voltage when applied to a nanocavity made of epsilon-near-zero (ENZ) materials such as indium antimonide (InSb) How to allow time control of the true absorption wavelength and device color when structured, this may lead to major advancements in displays, switches, sensors, and spectrum analysis.

The most advanced color filter technology uses a Fabry-Perot nanocavity composed of thin semiconductors and metal films to absorb light of selected wavelengths. Dr. Gregory described this nanocavity as similar to having two mirrors, one highly reflective and the other partially transmitting. Light enters a partially transparent mirror and reflects from a perfect mirror. "If the mirror spacing is just right, then constructive interference will occur between light propagating in two different directions," he said. "This means you can choose the wavelength reflected from that surface." In other words, the absorption wavelength—or the color reflected back to the eye—is controlled by the thickness of the nanocavity.

So far, the thickness is determined by a fixed layer adjusted for a specific color or another color. "This means that for a specific layer of a specific thickness and a specific number of layers, you will reflect a specific color from that combination," explained Dr. Gregory. "You have to change the thickness of the layers to obtain different colors, but the idea of ​​this article is that we can build these different materials and electrically control the reflected light. So we can adjust the green, blue, and blue light by changing the voltage between the layers. Red light."

Under the supervision of Dr. Gregory, Mirshafieyan established a structural model that can be electrically tuned to different absorption wavelengths, as well as the first draft of his doctoral thesis. The thesis has been completed.

The structure includes an ultra-thin nanoscale ENZ material called InSb and a layer of titanium dioxide (TiO2) sandwiched between two silver mirrors. The total thickness of the device including the mirror, InSb and TiO2 is less than 200 nm, which is 500 times thinner than a human hair. InSb is a III-V semiconductor whose carrier density (when doped) is very suitable for inductive carrier modulation, making it behave more like metal under the correct applied voltage. Mirshafieyan noticed that several previous attempts to achieve an electrically adjustable perfect absorber were often incomplete. He pointed out, “Researchers have shown that if the thickness of the cavity is changed, the color can be changed, but it is difficult to display in real time because each The thickness of each pixel is fixed. We want to dynamically change the color of each pixel without physically changing the thickness of that pixel."

For these materials, the refractive index changes with the doping used inside the material, Dr. Gregory explained, this is how many electrons or holes you add to the basic semiconductor material. "So you can change the conductivity and resistivity of the material when manufacturing it, or you can do it by applying a voltage," he said. "You don't have to physically change the spacing between the mirrors." Depending on the circumstances, this may be more difficult than it sounds. "It's easy to do this with two mirrors in the laboratory. We can change the distance between the mirrors, and we can reflect different colors of light," he said. "But it is very difficult to have two fixed mirrors and then change the refractive index of the internal material in real time."

This doping also means that there is no need for nano-patterning or the creation of additional exotic materials. It is this difference that distinguishes Mirshafieyan's structure from previous iterations that require changes in the geometry of the structure-this difference also has a bearing on the telecommunications industry. The impact.

Being able to easily change the refractive index with low voltage applied also helps explain why InSb is used instead of silicon, which may prove to be a better material choice for the telecommunications or switching industries. Applying a voltage to a switch with an InSb active layer will increase the carrier density, thereby increasing the dielectric constant, resulting in a greater refractive index change. "What really matters is the difference between closed and open," Dr. Gregory said. "We get a bigger difference between off and on, which means we can operate with a lower error rate. In the telecommunications field, error rate is everything." Therefore, the result is very high-speed switching.

On the other hand, silicon does not produce much exponential change when voltage is applied. Even with the addition of other materials designed to improve the switch, silicon still cannot match the fidelity of InSb.

Dr. Gregory also predicts that this technology can completely replace silicon switches. Although using InSb is not necessarily cheaper, it can prove to be more cost-effective in the long run because people are willing to pay for it.

As for display applications, this technology can produce thinner and faster displays than currently on the market without the same quality control issues.

Current LCD and LED technologies include several different components in addition to the liquid crystal itself. "And each stack has a certain thickness," Mirshafieyan said. "But with InSb technology, you can combine everything. It is a color filter in itself." As a result, thinner, faster, and higher-resolution displays become possible.

"If you have ever tried to watch a hockey game on an LCD TV, you can't follow the puck on the ice at all. That's because the TV cannot run at a high enough speed," the doctor said. Gregory. This is because of image distortion caused by changes in the layers of many liquid crystal displays and basic reaction speeds.

However, using the technique proposed by Dr. Gregory and Mirshafieyan can eliminate these quality control problems because it can reduce the pixel size. "We can use this technology to create very small pixels because it does not have any nano patterns that restrict the manufacturing process," Mirshafieyan said. "We can make ultra-small pixels with vivid colors, which will greatly improve the quality of the display, far exceeding the quality currently available."

Dr. Gregory and Mirshafieyan have applied to the university for a patent for this new technology and are currently seeking funding to build and test their equipment.

Caption: Dr. Don Gregory and Seyed Sadreddin Mirshafieyan

Picture description: Schematic diagram of the electrically adjustable perfect absorber.

Scientific Reports Volume 8, Article Number: 2635 (2018); Nature

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