The terahertz range includes the high frequency wavelengths that lie between microwaves and infrared light on the electromagnetic spectrum. Because terahertz waves can pass through most non-conductive materials non-destructively, the frequency range lends itself to multiple applications, making it a promising tool for future research. However, hurdles remain. For one, terahertz radiation is difficult to produce efficiently, and the materials necessary to receive and adjust transmission of the waves are largely unknown. Now, researchers at Linköping University have developed an aerogel — one of the world's lightest materials — made of cellulose and a conducting polymer enabling the tuning of terahertz waves. Its absorption of terahertz signals can be switched on and off through a redox reaction. The researchers test the aerogel's ability to absorb terahertz signals using an optical measuring instrument. Courtesy of Thor Balkhed. “It’s like an adjustable filter for terahertz waves. In one position, the electromagnetic signal will not be absorbed and in the other position, it can. That property can be useful for long-range signals such as from space or radar signals,” said Shangzhi Chen, postdoctoral fellow at the Laboratory of Organic Electronics (LOE) at Linköping University. The Linköping researchers used a conducting polymer, PEDOT:PSS, and cellulose to create their aerogel. They also designed the aerogel with outdoor applications in mind, because it is both hydrophobic and can be frosted off with the help of the sun's heat. “The transmittance of terahertz waves within a wide frequency range could be regulated from between about 13% and 91% with our material, which is a very high tunability,” said Chaoyang Kuang, postdoctoral researcher at LOE. According to the researchers, this is the largest achieved modulation capability for broadband terahertz transmission materials reported to date. Researchers show off the hydrophobic nature of the terahertz permeable aerogel. Courtesy of Thor Balkhed. Conductive polymers have many advantages compared to other materials that have been used to create tunable materials. Among other things, they are biocompatible, durable, and have a great ability to be adjusted. The controllability comes from the ability to change the charge density in the material. The big advantage of cellulose is the relatively cheap production cost compared to other similar materials and it is a renewable material, which is a key for sustainable applications. Future applications, the researchers said, include tunable attenuators for terahertz light sources or dynamic protecting shields for wireless electronic devices, which can selectively prevent unwanted electromagnetic interference. Conducting polymer-cellulose aerogels presents various possibilities for further interdisciplinary studies; for example, in combination with their existing use in wearable bioelectronics, they pave the way for on-body terahertz communication systems. The research was published in Advanced Science (www.doi.org/10.1002/advs.202305898).