糖心原创出品

A paper by UAH physics professor Dr. Don Gregory and UAH Ph.D. student Seyed Sadreddin Mirshafieyan was recently published in Nature, Scientific Reports.

Michael Mercier | UAH

The technology for controlling light absorption at selected wavelengths in nanostructures has garnered much attention in recent years; however, dynamically tuning absorption wavelengths without also changing the geometry of their structure has been somewhat elusive. A recently published paper in Nature, Scientific Reports by Dr. Don Gregory, distinguished professor in the Department of Physics and Astronomy at 糖心原创出品 (UAH), and his Ph.D. student, Seyed Sadreddin Mirshafieyan, proposes a solution for doing just that.

Their paper, 鈥淓lectrically tunable perfect light absorbers as color filters and modulators,鈥� theorizes how voltage, when applied to a nanocavity structure made of an epsilon-near-zero (ENZ) material such as indium antimonide (InSb), allows for real-time manipulation of absorption wavelengths and device colors, which could lead to significant advances in displays, switching, sensors, and spectral analysis.

State-of-the-art technology in color filters uses what鈥檚 known as a Fabry-Perot nanocavity made up of thin semiconductor and metal films to absorb light at selected wavelengths. Dr. Gregory describes this nanocavity as analogous to having two mirrors, one highly reflective and the other partially transmitted, with light entering the partially transmitting mirror and bouncing off the perfectly reflecting mirror. 鈥淚f the mirror spacing is just right, you get constructive interference between light traveling in the two different directions,鈥� he says. 鈥淭hat means that you can pick what wavelength gets reflected from that surface.鈥� In other words, the absorption wavelength 鈥� or the color that gets reflected back to the eye 鈥� is controlled by the thickness of the nanocavity.

Until now, that thickness has been determined by fixed layers tuned for one particular color or another. 鈥淭hat means for a particular layer of thickness and a particular number of layers, you get a particular color reflected from that combination,鈥� Dr. Gregory explains. 鈥淵ou have to change the thickness of the layers to get a different color, but the idea in this paper is that we can build these different materials and electrically control the light that鈥檚 reflected back. So we could tune it for green light, blue light, red light by changing the voltage across the layers.鈥�

We can create very small pixels with this technology because it doesn鈥檛 have any nanopatterning that limits the fabrication process.

Under Dr. Gregory鈥檚 supervision, Mirshafieyan has modeled a structure capable of being electrically tuned for different absorption wavelengths and a first draft of his Ph.D. dissertation has been completed.

The structure comprises an ultrathin, nanometer-thick ENZ material called InSb and a titanium dioxide (TiO2) layer sandwiched between two silver mirrors. The total thickness of the device including the mirrors, InSb, and TiO2 is less than 200 nm, which is 500 times thinner than human hair. InSb is a III-V semiconductor whose carrier density (when it is doped) is ideal for electrically induced carrier modulation, making it behave more like a metal under the right applied voltage. Aware of several previous but often incomplete attempts to achieve electrically tunable perfect light absorbers, Mirshafieyan notes, that 鈥渞esearchers have already shown that if you change the thickness of the cavity, you can change the color, but that is difficult in real-time display applications because the thickness of each pixel is fixed. We want to change the color of each pixel dynamically without physically changing the thickness of that pixel.鈥�

With these materials, the index of refraction changes with the doping that鈥檚 used inside the material, which Dr. Gregory explains is how many electrons or holes you鈥檝e added to the basic semiconductor material. 鈥淪o, you can change its conductivity, its resistivity in the making of the material or you can do it with applied voltage,鈥� he says. 鈥淵ou don鈥檛 have to physically change the separation between mirrors.鈥� This can be more difficult than it sounds depending on the circumstances. 鈥淚t鈥檚 easy enough to do it in the lab with two mirrors. We can change the spacing between the mirrors and we can get different color light reflected,鈥� he says. 鈥淏ut to have two mirrors that are fixed and then changing the index of refraction of the material inside, electrically, in real time, that鈥檚 tough.鈥�

This doping also means there is no need for nanopatterning or the creation of additional exotic materials, and it鈥檚 this distinction that separates Mirshafieyan鈥檚 structure from previous iterations that called for changes in structural geometry 鈥� a distinction that also has implications for the telecommunications industry.

Schematic structure of an electrically tunable perfect light absorber.

Being able to change the index of refraction easily with a low applied voltage also helps explain why the use of InSb as opposed to say, silicon, may prove a better material option in the telecommunications or switching industry. Applying voltage to switches with an active layer of InSb increases the carrier density, and consequently, the permittivity, which leads to a greater change in refractive index. 鈥淚t鈥檚 the difference between off and on that really matters,鈥� says Dr. Gregory. 鈥淲e get much higher difference between off and on, which means that we can run with a much lower error rate. And error rate is everything in telecommunications.鈥� The result, therefore, is very high speed switching.

Silicon, on the other hand, does not produce much change in index with an applied voltage. Even with the addition of other materials designed to improve switching, silicon can鈥檛 currently match the fidelity of InSb.

Dr. Gregory also anticipates that this technology could replace silicon in switching altogether. And while the use of InSb isn鈥檛 necessarily cheaper, it could prove more cost effective in the long run because of improved bit error rates, which people would be willing to pay for.

As for display applications, this technology could generate even thinner and faster displays than are currently on the market, without the same quality control issues.

Current LCD and LED technology consists of several different components besides the liquid crystal itself. 鈥淎nd each stack has a thickness,鈥� says Mirshafieyan. 鈥淏ut with InSb technology, you can combine everything. It is itself a color filter.鈥� As a result, a much thinner, faster, higher-resolution display is possible.

鈥淚f you鈥檝e ever tried to watch a hockey game on a liquid crystal TV, you can鈥檛 follow the puck on the ice at all, and that鈥檚 because the TV can鈥檛 run at high-enough rates,鈥� says Dr. Gregory. This is because of the image distortions created by the variation in the layers of many liquid crystal displays and the basic reaction speed.

However, these quality control issues could be eliminated with the technology that Dr. Gregory and Mirshafieyan are proposing because it would allow for decreased pixel size. 鈥淲e can create very small pixels with this technology because it doesn鈥檛 have any nanopatterning that limits the fabrication process,鈥� Mirshafieyan says. 鈥淲e can make ultra-ultrasmall pixels with distinct colors and that will improve the quality of the display well beyond what鈥檚 available now.鈥�

Dr. Gregory and Mirshafieyan have filed for a patent on this new technology with the university and are now seeking funds to build and test their devices. While Dr. Gregory approximates the cost at about $500,000, he says that the campus already has the facilities for building them.