Tunable materials clear the way for advanced optics

In books and movies, wizards use magic spells to easily change things from a solid to see-through state. However, in reality, so-called phase-transition materials can pull off a similar trick, changing from clear to cloudy depending on the temperature or an applied electric field.

Recently, an international team of researchers, including physicists from the Friedrich Schiller University Jena in Germany, developed a way to engineer the transition point for the phase-transition material vanadium dioxide, allowing them to control the temperature at which the transition occurs. The research, published in Nano Letters, could lead to new types of tunable materials for optics and thermal regulation.

“Essentially, any optical component would be better if it were tunable,” says Mikhail Kats, an assistant professor of electrical and computer engineering at the University of Wisconsin-Madison and senior author of the paper.

Rather than relying on mechanical components to focus an object such as a camera lens or a telescope eyepiece, a tunable material changes its innate optical properties on demand. Scientists have known for more than 50 years that substances like vanadium dioxide can transition between opaque and transparent. However, these materials typically only switch between the two states under one particular set of conditions, limiting their applicability. “In most phase-transition materials, the change occurs at conditions that are far from room temperature, and thus are difficult to incorporate into useful devices,” explains Kats.

In this latest study, the researchers have not only changed vanadium dioxide’s intrinsic shift point from 68°C to below room temperature, they have also found a way to tune the transition point for the material to any temperature. “This finding is going to open new frontiers in photonic devices,” says Shriram Ramanathan, a professor of materials engineering at Purdue University in West Lafayette, Indiana, who also contributed to the research.

Because optical and physical properties arise from the same underlying physical principles, vanadium dioxide’s thermal and electrical conductivities also shift with the transition. Such phase-transition materials could thus be used in homes as ‘smart’ walls or windows that respond to the environment. “Objects designed to emit light efficiently at high temperatures but not at low temperatures could be used as purely passive temperature regulators that do not require external circuitry or power sources,” Kats says.

Previously, researchers attempting to change the transition temperatures of vanadium dioxide did so by introducing impurities into the material, but the methods they used uniformly altered the material’s entire surface. So in order to gain more control over the process, the German-American team of researchers bombarded specific regions of the vanadium dioxide with energetic ions. Such ion irradiation is known to create defects in materials, usually as an unintended side effect

According to Carsten Ronning of the Friedrich Schiller University Jena, the researchers’ advance capitalizes on those defects. “The beauty in our approach is that we take advantage of the ‘unwanted’ defects,” he says. Directing the ion-beam at specific regions on the surface of vanadium dioxide allowed the researchers to make nanoscale modifications to the material.

“We can precisely control the transition temperature everywhere on the sample, with roughly 20nm precision,” Ronning states. “We have been able to use this method to create highly effective meta-surface areas which have multiple phase transitions at the same time.” This technique allowed the researchers to design and create a novel optical polarizer that changes selectivity based on temperature.

This story is adapted from material from the Friedrich Schiller University Jena, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.