Utilizing new palm-sized gadgets, RIKEN researchers might have lastly harnessed the terahertz band of the electromagnetic spectrum to successfully ‘X-ray’ issues with out utilizing dangerous ionizing radiation.
Numerous applied sciences—from smartphones and TVs to infrared devices on the James Webb Space Telescope and high-speed wireless telecommunication devices using microwaves—exploit sections of the electromagnetic spectrum.
But somewhere between commonly used microwaves and infrared light, lies a neglected region called the terahertz band. Terahertz waves have numerous exciting potential uses, not least because they can be used to see through or inside materials in a similar way to X-rays. Unlike X-rays, however, terahertz waves don’t deliver damaging ionizing radiation.
But terahertz technologies have so far languished because it has been difficult to adapt microwave- or visible-light technologies to the terahertz range at useful sizes and power outputs.
For example, one approach to generate terahertz waves has been to develop electrical devices that produce higher frequency, ultrashort-wavelength microwaves. But this has been difficult in part because these devices need highly optimized parameters to produce greater electrical performance, which has proved challenging.
An alternative strategy is to produce terahertz waves by converting shorter, higher-frequency waves of infrared light, using materials known as nonlinear crystals.
At the RIKEN Center for Advanced Photonics, we are exploring this second strategy—producing terahertz waves by converting the output from an infrared laser. This method has traditionally required enormous lasers to generate terahertz waves powerful enough for most practical applications. But this has limited the take-up of terahertz technology for real-world applications—where portable devices for in situ analysis would be far more valuable.
In the Tera-Photonics Research Team, which I lead, we hope to develop palm-sized, powerful terahertz wave sources for applications in industry and fundamental research. We have recently taken huge strides toward this goal and have multiple industrial collaborations underway.
We have focused on using lithium niobate, a nonlinear crystal that produces a beam of terahertz waves when irradiated with near-infrared laser light. When I assumed leadership of the team in 2010, it was impossible to produce sufficiently powerful terahertz waves using this method, despite many years of work.
In 2011, we had to stop lab research for several months after a major earthquake struck Sendai, Japan, where our campus is. During that period, I remembered the result of a previous experiment that had caught my attention, and, I found an exciting hint of a possible path forward.
At that time, we used a near-infrared laser with pulse durations in the nanoseconds. The results indicated that when shorter, sub-nanosecond laser pulses were used, terahertz-wave generation as a function of the input laser pulse was altered. I wondered why this was.
I then uncovered a 1993 paper that reported the consequences of laser pulse length in nonlinear crystals. The examine—analyzing seen mild—implied that utilizing shorter pulses lowered a light-scattering impact known as Brillouin scattering. I questioned whether or not, by decreasing our laser pulse length, we may decrease Brillouin scattering from our lithium niobate crystals. This may enable us to transform extra of the laser mild into terahertz waves and enhance the ability output.
As soon as we returned to the lab and examined this idea, we had been amazed by the outcome. Utilizing sub-nanosecond laser pulses, we may escape Brillouin scattering to enhance our terahertz wave energy output by six orders of magnitude—from 200 milliwatts to 100 kilowatts. We lastly had highly effective emission from a tool just one meter sq., a lot smaller than earlier terahertz apparatuses, which crammed whole rooms. However after we confirmed this system to trade, they informed us that it was nonetheless too large for real-world functions.
To additional miniaturize our terahertz-wave supply, we changed the majority lithium niobate crystal ingot we had beforehand used with a skinny lithium niobate crystal with a synthetic polarization-modulated microstructure, which is known as a periodically poled lithium niobate (PPLN) crystal. Generally employed within the visible-light area, the PPLN crystal enabled us to develop a handheld system attributable to its larger mild conversion effectivity.
Firstly of our PPLN analysis, there was no identified method to effectively generate terahertz waves utilizing PPLN crystals. As we proceeded with our personal experiments, we had been initially very puzzled by the conduct of PPLN crystals. We noticed no terahertz waves, simply an sudden mild beam, produced from the crystal.
After fastidiously analyzing the properties of this mild, we ultimately realized that terahertz waves had been being produced, however in an sudden path. The interplay between the sunshine and the PPLN’s polarization-modulated construction precipitated terahertz waves to be generated on the rear of the crystal. Once we moved our detector behind it, we discovered the terahertz wave. We may lastly make a palm-sized prototype with a excessive conversion effectivity and ample energy.
Remarkably, we additionally found that just by rotating the crystal, we may tune the frequency of the terahertz waves produced. Our gadgets can fully cowl the crucial sub-terahertz area of the spectrum, which is very essential for non-destructive imaging functions.
Our analysis relies on photon conversion between light waves and terahertz waves by nonlinear optical effects based on mature photonic and laser technologies. We’ve achieved cascade oscillation in backward terahertz-wave parametric oscillation by using optical injection to lower the threshold and stabilize the output power—achieving a peak terahertz output power of 200 watts at a frequency of 0.3 terahertz; converted terahertz waves to light waves in a backward optical quantum conversion process; and succeeded in detecting ultraweak terahertz waves of approximately 50 attojoules, which is 1,000 times more sensitive than a 4 kelvin bolometer. These results provide new quantum research based on terahertz-to-light quantum photon conversion. Our most recent results are based on incorporating quantum theory into our work. And our future work will explore quantum entanglement—where one quantum particle mysteriously mirrors another far away—to improve the sensitivity of terahertz detectors.
Furthermore, our highly miniaturized, high-power terahertz waves systems are complemented by recent developments in compact, powerful photonic lasers. Our devices use a new microchip laser that produces far-infrared laser pulses at sub-nanosecond speeds and high powers.
We’re now at the point where industrial collaborations form a key part of our work. The strong sub-terahertz emissions that our devices can generate are highly suited to imaging and analytical work. We’re conducting joint research with Japanese companies specializing in electronics, optical and photonics—such as Ricoh, Topcon, Mitsubishi Electric and Hamamatsu Photonics—to develop non-destructive testing applications and terahertz-wave spectroscopy equipment.
To demonstrate the potential of our technology for security purposes, we have assembled a prototype terahertz imaging device. With it we showed that a plastic gun, which can fire plastic bullets, could be clearly detected when concealed behind bumpy glass that scatters light a lot. We could also clearly image a pair of scissors hidden in a thick leather bag.
Terahertz waves can also reveal the chemical composition of substances, due to characteristic ‘fingerprint’ absorption patterns. Different colorless liquids—such as kerosene and acetone—that look identical to the naked eye can be readily identified by this method, for example. Thus, applications under consideration for terahertz waves range from airport security scanners to the analysis of historic artworks.
Industrial paint and outer coatings can also be analyzed, from things as varied as new cars and pharmaceutical tablets—and non-destructively, unlike current methods. In the future, we could mount our devices onto robots to crawl along industrial pipework to inspect for corrosion or on drones to inspect paint on power transmission towers.
These and other uses could give us a better understanding of how materials interact and degrade in situ. If we can better understand these issues using non-destructive technologies, we can more easily tweak production processes in real time to improve efficiencies and make patches to extend the life span of structures, for example. The economic and environmental benefits should be exponential.
- “High-resolution stimulated Brillouin gain spectroscopy in glasses and crystals” by Gregory W. Faris, Leonard E. Jusinski and A. Peet Hickman, 1 April 1993, Journal of the Optical Society of America B.
- “Kilowatt-peak Terahertz-wave Generation and Sub-femtojoule Terahertz-wave Pulse Detection Based on Nonlinear Optical Wavelength-conversion at Room Temperature” by Hiroaki Minamide, Shin’ichiro Hayashi, Koji Nawata, Takunori Taira, Jun-ichi Shikata and Kodo Kawase, 19 December 2013, Journal of Infrared, Millimeter, and Terahertz Waves.
- “Tunable Backward Terahertz-wave Parametric Oscillation” by Kouji Nawata, Yu Tokizane, Yuma Takida and Hiroaki Minamide, 24 January 2019, Scientific Reports.
- Minamide, H. et al. in 2021 Conference on Lasers and Electro-Optics (CLEO) (IEEE, 2021).