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Imagine having a toolbox full of small- and large-sized screwdrivers and attempting to tighten a medium-sized screw. This is the current state of research into terahertz radiation, which is squeezed between infrared light and microwaves in the electromagnetic spectrum.
Terahertz radiation consists of submillimeter-long waves with frequencies around one terahertz. These waves can travel through soft materials like wood and plastic, penetrate the body without damaging tissue or DNA, and sense bonds between molecules. Terahertz radiation has huge potential for a wide range of applications, including airport security, telecommunications, tumor detection and remote sensing from space. But there is a lack of efficient and practical technologies for generating, detecting and visualizing these invisible waves, since they are too long to produce using conventional, high-frequency optical devices and too short to generate using electronic transmitters. The RIKEN Terahertz-wave Research Group was established in 2008 with the goal of plugging this ‘terahertz gap’.
RIKEN has been researching the interaction between light, materials and living organisms for the last three decades. It dedicated an entire program to the study of terahertz waves in 2005 and set up the Terahertz-wave Research Group as part of the RIKEN Advanced Science Institute (later reorganized into the RIKEN Center for Advanced Photonics). Based in Sendai, the group has an average annual budget of 400 million yen and employs 42 personnel. Research is divided between three key laboratories, each employing novel techniques to generate, sense and image terahertz waves.
Terahertz sources must be practical as well as powerful. Consequently, Hiroaki Minamide, who leads the Tera-Photonics Research Team, has used nonlinear optical crystals to produce intense terahertz laser beams at room temperature. The system can generate narrow waves with frequencies between 0.7 and 3 terahertz that are as strong as those produced by larger, costlier and more complex free-electron lasers. Meanwhile, Hideki Hirayama’s Terahertz Quantum Device Research Team is extending the reach of more ubiquitous quantum cascade lasers down to the terahertz region by fabricating them, for the first time in the world, using gallium nitride, instead of the traditional aluminum gallium arsenide. Hirayama’s team is also reducing the operating temperature of these lasers for use in everyday working environments.
The detection of terahertz waves is as crucial as their generation. Chiko Otani and the Terahertz Sensing and Imaging Research Team have developed a spectroscopic system that covers a broad frequency range of 0.5 to 30 terahertz to probe materials and measure the reflected wave characteristics. The team has also contributed to the development of highly sensitive superconducting detectors for millimeter and terahertz waves. Since 2008, Minamide’s team has been amassing absorption and emission profiles of hundreds of medically and industrially relevant materials in a free online database, accessed by researchers from nearly 80 countries. A similar database of potentially dangerous drugs and explosives is being used by RIKEN to test the suitability of terahertz spectroscopy for screening the 100,000 international packages that pass through Japanese post offices every day.
Researchers could also use these submillimeter waves to control the activity of soft materials and biomolecules, but not until they fill their toolbox with the right tools.