Make waves with terahertz technology

(Nanowerk News) Countless technologies – from smartphones and TVs to the infrared instruments on the James Webb Space Telescope and high-speed wireless telecommunication devices that use microwaves – exploit this part of the electromagnetic spectrum.

But somewhere between commonly used microwaves and infrared light, there is a neglected region called the terahertz band. Terahertz waves have many interesting potential uses, not least because they can be used to see through or inside materials in a similar way to x-rays. However, unlike x-rays, terahertz waves do not produce damaging ionizing radiation.

But terahertz technology has so far languished because it has been difficult to adapt microwave or visible light technology to the terahertz range of useful size and power output.

For example, one approach to generating terahertz waves is to develop electrical devices that generate microwaves with very short wavelengths and higher frequencies. But this is partly difficult because these devices require highly optimized parameters to deliver greater electrical performance, which can prove challenging.

An alternative strategy is to generate terahertz waves by converting shorter, higher-frequency infrared light waves, using materials known as nonlinear crystals.

At the RIKEN Center for Advanced Photonics, we are exploring this second strategy—generating terahertz waves by changing the output of an infrared laser. This method traditionally required very large lasers to produce terahertz waves that were strong enough for most practical applications. However this limits the use of terahertz technology for real world applications – where portable devices are used in place analysis would be much more valuable.

Within the Tera-Photonics Research Team, which I lead, we hope to develop a powerful palm-sized terahertz wave source for applications in industry and fundamental research. We recently took a major step towards this goal and have many industry collaborations ongoing. A device created by Hiroaki Minamide and his team, which efficiently converts infrared radiation into terahertz waves. It can generate terahertz radiation over the entire terahertz band range. (Image: RIKEN)

Palm-sized device

We’ve focused on using lithium niobate, a nonlinear crystal that produces a beam of terahertz waves when irradiated with a near-infrared laser. When I took over the leadership of the team in 2010, it was impossible to produce strong enough terahertz waves using this method, despite years of work.

In 2011, we had to stop lab research for several months after a major earthquake hit Sendai, Japan, where our campus is located. During that period, I recalled the results of previous experiments that caught my attention, and, I found interesting clues about possible ways forward.

Back then, we used a near-infrared laser with a pulse duration in nanoseconds. The results show that when shorter sub-nanosecond laser pulses are used, the generation of terahertz waves as a function of the input laser pulse is altered. I’m wondering why this is happening.

I then came across a 1993 paper (Faris, GW, Jusinski, LE & Hickman, AP J. Opt. Soc. I. 8, 87–99 (1993)) who reported the effect of laser pulse duration on nonlinear crystals. The study — analyzing visible light — implies that using shorter pulses reduces the effect of light scattering called Brillouin scattering. I was wondering if, by reducing the duration of our laser pulses, we could minimize the Brillouin scattering from our lithium niobate crystals. This allows us to convert more of the laser beam into terahertz waves and increases the power output.

Once we returned to the lab and tested this theory, we were amazed by the results. Using a sub-nanosecond laser pulse, we were able to avoid Brillouin scattering to increase the power output of our terahertz waves by sixfold—from 200 milliwatts to 100 kilowatts (Minamide, H. et al. J. Infrared, Millimeter, and Terahertz Waves 35, 25 (2014)). We ended up getting a powerful beam from a device that was only one square meter in size, much smaller than previous terahertz equipment, which filled an entire room. But when we showed these devices to the industry, they told us that they were too big for real-world applications.

To further minify our terahertz wave source, we replaced the bulk lithium niobate crystal ingots we previously used with thin lithium niobate crystals with artificial polarization modulated microstructures, called periodically blown lithium niobate crystals (PPLN). Typically used in the visible light region, PPLN crystals allow us to develop hand-held devices due to their higher light conversion efficiency.

At the start of our PPLN research, there was no known way to efficiently generate terahertz waves using PPLN crystals. As we continued our own experiments, we were initially very confused by the behavior of PPLN crystals. We don’t see terahertz waves, just unexpected streaks of light, generated from crystals.

After carefully analyzing the properties of this light, we finally realized that terahertz waves are generated, but in an unexpected direction. The interaction between light and the polarization modulated structure of the PPLN causes terahertz waves to be generated at the back of the crystal. When we moved our detector behind it, we encountered terahertz waves (Nawata, K. et al. Sci Rep-UK 9, 726 (2019)). We were finally able to create a palm-sized prototype with high conversion efficiency and sufficient power.

Remarkably, we also found that simply by rotating the crystal, we can tune the frequency of the resulting terahertz waves (Minamide, H. et al. in Laser and Electro-Optics Conference (CLEO) 2021 (IEEE, 2021)). Our devices can completely cover the sub-terahertz critical region of the spectrum, which is very important for non-destructive imaging applications. Sandwiched between microwave and infrared radiation on the electromagnetic spectrum, the terahertz gap was underutilized in technology until recently. Like x-rays, terahertz waves have the ability to see through matter. But because terahertz waves have a much lower frequency (and therefore energy) than x-rays, they don’t pose the same health risks as ionizing radiation. (Image: RIKEN)

What a jump

Our research is based on the conversion of photons between light waves and terahertz waves with nonlinear optical effects based on mature photonics and laser technologies. We have achieved cascade oscillations in parametric oscillations of backward terahertz waves 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; convert terahertz waves into light waves in the backward optical quantum conversion process; and successfully detected ultra-weak terahertz waves of about 50 attojoules, which is 1,000 times more sensitive than the 4 kelvin bolometer.

These results provide new quantum research based on the conversion of terahertz quantum photons to light. Our latest results are based on the incorporation of quantum theory into our work. And our future work will explore quantum entanglement – ​​where one quantum particle mysteriously reflects far away from another – to increase the sensitivity of terahertz detectors.

In addition, our ultra-miniature high-power terahertz wave system is equipped with the latest developments in compact and powerful photonic lasers. Our device uses a new laser microchip that generates far infrared laser pulses at sub-nanosecond speed and high power.

We are now at a point where industry collaboration is becoming an essential part of our work. The strong sub-terahertz emission that our device can produce is perfect for imaging and analytical work. We are conducting joint research with Japanese companies specializing in electronics, optics 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 show that a plastic gun, which can fire plastic bullets, can be clearly detected when hidden behind corrugated glass that scatters light a lot. We can also clearly imagine a pair of scissors hidden in a thick leather bag.

Terahertz waves can also reveal the chemical composition of substances, due to their characteristic ‘fingerprint’ absorption patterns. Different colorless liquids—such as kerosene and acetone—that look identical to the naked eye can be easily identified by this method, for example. As such, the applications under consideration for terahertz waves range from airport security scanners to the analysis of historic works of art.

Industrial paints and coatings can also be analyzed, from things like new cars and pharmaceutical tablets—and non-destructively, unlike current methods. In the future, we could attach our devices to robots to crawl along industrial pipes to check corrosion or to drones to check paint on power transmission towers.

These and other uses can give us a better understanding of how materials interact and degrade in place. If we can better understand this problem using non-destructive technologies, we can more easily change production processes in real time to increase efficiency and create patches to extend structure life, for example. The economic and environmental benefits must be exponential.