RIKEN scientists have succeeded in harnessing the terahertz band of the electromagnetic spectrum with new hand-held devices that can perform “X-ray imaging” without the use of ionizing radiation.
Several technologies—from smartphones to infrared telescopes in the James Webb Space Telescope to high-speed wireless microwave telecommunications devices—use portions of the electromagnetic spectrum.
However, between commonly used microwaves and infrared light lies a neglected band called the terahertz band. Terahertz waves have many potentially exciting applications, such as They can be used to see through or into X-ray-like materials. However, unlike X-rays, terahertz waves do not emit ionizing radiation, which can be harmful in the long term.
Technological challenges of the terahertz spectrum
The implementation of terahertz technologies has been hampered by the difficulty of adapting microwave or visible light technologies to the terahertz range in useful formats and with sufficient output powers. For example, one approach to generating terahertz waves has been to develop electrical devices that generate high-frequency, very short-wavelength microwaves.
However, this has been difficult in part because these devices require highly optimized parameters to achieve better electrical performance, which has proven to be a challenge.
Another strategy is to create terahertz waves by converting shorter, higher-frequency waves of infrared light using materials known as nonlinear crystals. At the RIKEN Center for Advanced Photonics, this second strategy is being researched – the generation of terahertz waves by converting the power of an infrared laser.
THE AWAKENING SPIRIT: Located in the electromagnetic spectrum between microwaves and infrared radiation, the terahertz interval has been underexploited in technology. Like X-rays, terahertz waves have the ability to see through materials. However, because terahertz waves have much lower frequencies (and therefore energies) than x-rays, they do not pose the same health risk as ionizing radiation. 2023 RIKEN
The crucial role of handheld devices
RIKEN focused on using lithium niobate, a nonlinear crystal that produces a beam of terahertz waves when irradiated with near-infrared laser light. These researchers have recently made significant strides towards this goal and have several ongoing industrial collaborations.
They managed to further miniaturize their terahertz wave source by replacing the previously used lithium niobate crystal with a thin lithium niobate crystal with an artificially polarized modulated microstructure, called a periodically polarized lithium niobate (PPLN) crystal.
Impact on industry and research
Furthermore, these highly miniaturized, high-power terahertz wave systems are complemented by recent developments in compact and powerful photonic lasers. These devices use a new on-chip laser that produces far-infrared laser pulses at subnanosecond speeds and high power.
Industry cooperations are currently a central part of their work. The strong sub-terahertz emissions that their devices can generate are particularly suitable for imaging and analytical work. They conduct joint research with Japanese companies specializing in electronics, optics and photonics to develop non-destructive testing applications and equipment for terahertz wave spectroscopy.
For better understanding
1. What is the terahertz spectrum?
The terahertz spectrum describes the part of the electromagnetic spectrum that lies between microwaves and infrared light. Terahertz waves have many potential applications because they can be used in a manner similar to x-rays to see through or into materials without emitting harmful ionizing radiation.
2. What are the challenges in using the terahertz spectrum?
One of the biggest challenges is the difficulty of adapting microwave or visible light technologies to the terahertz band in useful sizes and powers. Furthermore, terahertz wave generation requires highly optimized parameters to achieve superior electrical performance, which has proven to be a challenge.
3. How did the RIKEN research team overcome these challenges?
The RIKEN research team explored a strategy to generate terahertz waves by converting infrared waves using materials called nonlinear crystals. They managed to create hand-sized devices capable of generating terahertz waves powerful enough for most practical applications.
4. What possible uses are there for terahertz waves?
Terahertz waves can be used in a wide variety of fields, from medical imaging to security to the analysis of ancient materials. They can use specific absorption patterns to reveal the chemical composition of substances and, for example, easily identify colorless liquids that look identical to the naked eye. In addition, they can be used for the non-destructive analysis of industrial paints and exterior coatings.
5. What distinguishes RIKEN’s work in the field of terahertz waves?
RIKEN’s work is unique in that it has developed hand-sized devices capable of generating powerful terahertz waves, paving the way for practical and portable applications of terahertz technology. In addition, their research is based on a photonic conversion between light waves and terahertz waves, which opens up new possibilities for quantum research.
Caption for main illustration: Device developed by Hiroaki Minamide and his team that efficiently converts infrared radiation into terahertz waves. It can generate terahertz radiation in the entire terahertz band. Credit 2023 RIKEN
About the researcher: Hiroaki Minamide is the leader of the tera-photonics research team and leader of the terahertz research group at RIKEN. He is also a visiting professor at the University of Chiba. He joined RIKEN in 1999 and after working as a researcher and deputy team leader, he has been a team leader since 2010 and a group leader since 2020. He completed his undergraduate studies in communications engineering, as well as his masters and doctorate degrees in 1993, 1996 and 1999 in electrical engineering from Tohoku University, Japan. His research focuses on high power terahertz wave generation and highly sensitive terahertz wave detection using nonlinear optics and their unique terahertz applications.
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