RIKEN scientists have actually established a portable gadget that efficiently utilizes the terahertz band of the electro-magnetic spectrum for X-raying things without damaging radiation. By optimizing particular methods and utilizing particular products, they significantly enhanced terahertz wave power output and miniaturized the device. The innovation assures numerous applications consisting of non-destructive imaging and quantum research study, with commercial partnerships underway.
Utilizing new palm-sized gadgets, RIKEN scientists may have lastly harnessed the terahertz band of the electromagnetic spectrum to efficiently X-ray things without utilizing damaging ionizing radiation.
Numerous innovations– from smart devices and TVs to infrared instruments on the James Webb Space Telescope and high-speed wireless telecommunication devices using microwaves– make use of sections of the electro-magnetic spectrum.
Someplace in between typically utilized microwaves and infrared light, lies a disregarded region called the terahertz band. Terahertz waves have various interesting potential usages, not least since they can be utilized to translucent or inside products in a similar way to X-rays. Unlike X-rays, nevertheless, terahertz waves dont provide damaging ionizing radiation.
Terahertz innovations have so far suffered due to the fact that it has actually been difficult to adapt microwave- or visible-light technologies to the terahertz range at beneficial sizes and power outputs.
A device developed by Hiroaki Minamide and his group, which efficiently transforms infrared radiation into terahertz waves. It can generate terahertz radiation over the entire variety of the terahertz band. Credit: © 2023 RIKEN
One technique to produce terahertz waves has actually been to develop electrical devices that produce greater frequency, ultrashort-wavelength microwaves. This has been tough in part due to the fact that these gadgets require extremely optimized criteria to produce greater electrical performance, which has actually shown difficult.
An alternative strategy is to produce terahertz waves by transforming shorter, higher-frequency waves of infrared light, utilizing products referred to as nonlinear crystals.
At the RIKEN Center for Advanced Photonics, we are exploring this second technique– producing terahertz waves by transforming the output from an infrared laser. This method has actually typically required enormous lasers to produce terahertz waves powerful enough for many practical applications. This has actually restricted the take-up of terahertz innovation for real-world applications– where portable devices for in situ analysis would be far more important.
In the Tera-Photonics Research Team, which I lead, we wish to develop palm-sized, powerful terahertz wave sources for applications in market and essential research. We have recently taken substantial strides towards this objective and have numerous commercial partnerships underway.
Palm-sized gadgets
We have concentrated on utilizing lithium niobate, a nonlinear crystal that produces a beam of terahertz waves when irradiated with near-infrared laser light. When I presumed management of the team in 2010, it was impossible to produce adequately effective terahertz waves utilizing this method, regardless of numerous years of work.
In 2011, we had to stop lab research for numerous months after a major earthquake struck Sendai, Japan, where our campus is. During that period, I kept in mind the result of a previous experiment that had actually caught my attention, and, I found an interesting tip of a possible course forward.
At that time, we used a near-infrared laser with pulse periods in the nanoseconds. The outcomes indicated that when shorter, sub-nanosecond laser pulses were utilized, terahertz-wave generation as a function of the input laser pulse was modified. I questioned why this was.
I then uncovered a 1993 paper [1] that reported the effects of laser pulse period in nonlinear crystals. The research study– evaluating noticeable light– suggested that utilizing shorter pulses lowered a light-scattering result called Brillouin scattering. I questioned whether, by decreasing our laser pulse duration, we could minimize Brillouin spreading from our lithium niobate crystals. This may allow us to transform more of the laser light into terahertz waves and increase the power output.
MIND THE GAP: Sandwiched in between microwaves and infrared radiation on the electromagnetic spectrum, the terahertz space has actually been underutilized in innovations till now. Like X-rays, terahertz waves have the capability to see through materials. However due to the fact that terahertz waves have much lower frequencies (and for this reason energies) than X-rays, they do not position the very same threat to health that ionizing radiation does. Credit: © 2023 RIKEN
When we went back to the lab and checked this theory, we were surprised by the outcome. Using sub-nanosecond laser pulses, we could leave Brillouin scattering to improve our terahertz wave power output by six orders of magnitude– from 200 milliwatts to 100 kilowatts [2] We finally had effective emission from a gadget just one meter square, much smaller than previous terahertz apparatuses, which filled whole rooms. But when we revealed this gadget to industry, they told us that it was still too huge for real-world applications.
To further miniaturize our terahertz-wave source, we replaced the bulk lithium niobate crystal ingot we had actually formerly used with a thin lithium niobate crystal with an artificial polarization-modulated microstructure, which is called a periodically poled lithium niobate (PPLN) crystal. Frequently utilized in the visible-light region, the PPLN crystal enabled us to establish a portable device due to its higher light conversion efficiency.
At the beginning of our PPLN research, there was no recognized way to efficiently generate terahertz waves using PPLN crystals. As we continued with our own experiments, we were at first extremely puzzled by the habits of PPLN crystals. We saw no terahertz waves, just an unanticipated beam, produced from the crystal.
After thoroughly analyzing the residential or commercial properties of this light, we ultimately realized that terahertz waves were being produced, however in an unanticipated direction. The interaction between the light and the PPLNs polarization-modulated structure caused terahertz waves to be generated at the back of the crystal.
Extremely, we likewise found that merely by rotating the crystal, we might tune the frequency of the terahertz waves produced [4] Our gadgets can completely cover the vital sub-terahertz region of the spectrum, which is especially essential for non-destructive imaging applications.
Breakthrough
Our research study is based on photon conversion in between light waves and terahertz waves by nonlinear optical effects based on fully grown photonic and laser technologies. Weve achieved waterfall oscillation in backward terahertz-wave parametric oscillation by using optical injection to decrease the threshold and support the output power– accomplishing 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 prospered in finding ultraweak terahertz waves of roughly 50 attojoules, which is 1,000 times more sensitive than a 4 kelvin bolometer.
Our extremely miniaturized, high-power terahertz waves systems are complemented by current advancements in compact, powerful photonic lasers. Our devices utilize a brand-new microchip laser that produces far-infrared laser pulses at sub-nanosecond speeds and high powers.
Were now at the point where industrial partnerships form a crucial part of our work. The strong sub-terahertz emissions that our devices can generate are extremely suited to imaging and analytical work. Were conducting joint research with Japanese business specializing in electronic devices, optical and photonics– such as Ricoh, Topcon, Mitsubishi Electric and Hamamatsu Photonics– to develop non-destructive screening applications and terahertz-wave spectroscopy equipment.
To demonstrate the potential of our innovation for security purposes, we have assembled a model terahertz imaging device. With it we showed that a plastic weapon, which can fire plastic bullets, might be plainly identified when hidden behind rough glass that scatters light a lot. We could likewise clearly image a set of scissors concealed 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 appearance identical to the naked eye can be easily recognized by this technique, for instance. Thus, applications under factor to consider for terahertz waves range from airport security scanners to the analysis of historic artworks.
Industrial paint and outer coatings can likewise be analyzed, from things as varied as brand-new cars and trucks and pharmaceutical tablets– and non-destructively, unlike present methods. In the future, we could install our gadgets onto robotics to crawl along commercial pipework to examine for rust or on drones to inspect paint on power transmission towers.
These and other usages could offer us a better understanding of how materials degrade and engage in situ. If we can better understand these concerns using non-destructive innovations, we can more quickly fine-tune production procedures in genuine time to improve performances and make patches to extend the life span of structures, for instance. The financial and environmental benefits ought to be rapid.
References:
Terahertz waves have many amazing prospective usages, not least since they can be used to see through or inside materials in a comparable way to X-rays. It can create terahertz radiation over the whole range of the terahertz band. At the beginning of our PPLN research study, there was no recognized method to effectively produce terahertz waves utilizing PPLN crystals. Our research is based on photon conversion between light waves and terahertz waves by nonlinear optical results based on mature photonic and laser technologies. Weve attained cascade oscillation in backwards terahertz-wave parametric oscillation by utilizing optical injection to decrease the threshold and stabilize the output power– accomplishing a peak terahertz output power of 200 watts at a frequency of 0.3 terahertz; converted terahertz waves to light waves in a backwards optical quantum conversion procedure; and prospered in discovering ultraweak terahertz waves of around 50 attojoules, which is 1,000 times more sensitive than a 4 kelvin bolometer.
” High-resolution stimulated Brillouin get 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.DOI: 10.1364/ JOSAB.10.000587.
” 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.DOI: 10.1007/ s10762-013-0041-0.
” Tunable Backward Terahertz-wave Parametric Oscillation” by Kouji Nawata, Yu Tokizane, Yuma Takida and Hiroaki Minamide, 24 January 2019, Scientific Reports.DOI: 10.1038/ s41598-018-37068-7.
Minamide, H. et al. in 2021 Conference on Lasers and Electro-Optics (CLEO) (IEEE, 2021).
