Crystals Convert Terahertz Radiation to Visible Light

Representation of the crystal structure used for visualization of T-rays: A schematic representation of the nanoscale cages with oxygen anions randomly occupy one sixth of the cages. (Source: Titech)

Scientists of Materials Research Center for Element Strategy, Tokyo Tech,  have success­fully visualized terahertz radiation using a crystal of mayenite (Ca12Al14O33). Their method cleverly utilizes the rattling motion caused by the vibra­tion of oxygen ions inside the cage-like structures of the crystal. In recent years, there has been growing interest in deve­loping practical devices based on terahertz technology. With wave­lengths longer than infrared light, T-rays are considered safer than conven­tional imaging systems. They are already used, for example, at airport security checkpoints, and are starting to be used more widely in areas such as medical screening, food inspec­tion and analysis of artworks. The visuali­zation of terahertz light itself, however, has so far proved chal­lenging.

Now, Hideo Hosono and co-workers in Japan, Ukraine and the US have devised a simple approach to convert T-rays to bright, visible light. First, the study involved beaming T-rays onto the mayenite crystal using a gyrotron. This led to the vibra­tion of oxygen anions, which collide with the inside walls of the cages within the crystal. Each cage has an inner diameter of 0.4 nano­meters and an outer diameter of 0.7 nano­meters.

“The rattling of oxygen ions within the cages promotes upward energy conver­sion,” Hosono explains. “Strong and frequent collisions of the oxygen ions induce electron transfer to neigh­boring empty cages. The excitation of the oxygen ions is key to the emission of visible light.” Spectro­scopy measure­ments confirmed that the visible light origi­nated from vibrations caused by the free-moving oxygen anions. The researchers took care to rule out the possi­bility of other sources such as black body radiation and surface polari­zation as reasons behind the pro­duction of visible light.

“The crystal in our study is just composed of calcium, aluminium and oxygen, all of which are in the top five of the most abundant elements,” says Hosono. “So, it’s one of the most inex­pensive materials, at around 15 cents per kilogram.” Despite its simpli­city, Hosono says that the crystal has many exciting proper­ties due to its nano­structure. Drawing on 20 years of research, his group has already succeeded in demonstrating that the material has excellent catalytic proper­ties for ammonia synthesis and supercon­ductivity.

Best known for his pio­neering work on iron-based super­conductors, Hosono says that the current study marks a new research direction. “Our group has been concen­trating on the cultivation of new func­tionalities using abundant elements, but it’s the first time for me to focus on ionic motion – this is completely new,” he says. The findings could lead to the develop­ment of a T-ray detector, as no such conven­tional detector has yet been designed.

Hosono adds: “Right now, our material is good at detecting strong tera­hertz radia­tion. The challenge will be how to adjust the sensi­tivity.” His group has also reported that the oxygen anions can be substi­tuted with gold or hydrogen anions inside the cages. By making use of these different anions, it may be possible to develop detectors that emit dif­ferent-colored light in future. (Source: Titech)

Reference: Y. Toda et al.: Rattling of Oxygen Ions in a Sub-Nanometer-Sized Cage Converts Terahertz Radiation to Visible Light, ACS Nano, online 3 November 2017; DOI: 10.1021/acsnano.7b06277

Link: Lab. for Materials and Structures, Tokyo Institute of Technology, Yokohama, Japan

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