6G and Beyond: A Terahertz Research Roundup
Many researchers consider terahertz technology the key to the next wave of wireless communication. Here are a few recent R&D highlights.
Recent research into terahertz technologies could hold the key to next-generation wireless and advanced imaging technologies.
Terahertz technologies use the submillimeter band of the electromagnetic spectrum, located between infrared light and millimeter wavelengths. With a range between 30 micrometers and 3 millimeters, terahertz radiation offers better depth penetration than infrared light and higher resolution than microwaves. Terahertz radiation is also non-ionizing, making it safe for use around humans and animals.
THz band in the electromagnetic spectrum.
This article focuses on some of the uses of terahertz radiation. We'll also discuss how researchers are investigating terahertz signal generation and detection to overcome the challenges of wireless implementation.
The Uses (and Challenges) of Terahertz Technology
From the world's first terahertz IC to the most compact terahertz laser, the terahertz wavelength has been a popular research subject throughout the past century. In recent years, terahertz research has picked up steam with potential applications in everything from advanced sensing and spectroscopy to next-generation wireless communications.
Schematic of a terahertz spectroscopic instrument. Image courtesy of RSC Advances
Terahertz technologies are useful in medical applications, including imaging for skin and dental diagnosis. It is also commonly used for non-destructive security screening and detecting unwanted materials. Perhaps most commonly, terahertz technology is recognized for yielding low latency and fast wireless data transfer while decreasing congestion, making it a possible candidate for the sixth generation of wireless telecommunications (6G).
Researchers have already demonstrated that using terahertz wavelengths creates data transfer speeds that exceed those of the 5G network. Even so, these technologies are still in their early stages of development and face crucial challenges such as path loss—the reduced power density of EM waves as they propagate through a given medium. Terahertz technology is also expensive and lacks efficient source and detector designs, preventing it from being widely adopted.
Researchers Develop Highly-sensitive Terahertz Detector
Recently, a team of scientists from Cambridge University, the University of Augsburg, and the University of Lancaster published their findings on a new type of terahertz detector using two-dimensional (2D) electron gas. An electron gas is free to move in two axes but is tightly constrained in the third, thus appearing to exist as a 2D plane in a 3D environment.
By exposing their sensor to terahertz radiation, the researchers were able to read out a much stronger signal than previously theorized. They attributed these findings to the way that electromagnetic waves interact with matter at different frequencies—building upon the already-familiar photoelectric effect.
The terahertz detector developed by Cambridge researchers. Image used courtesy of Wladislaw Michailow and the University of Cambridge
Discovered by the German physicist Heinrich Rudolf Hertz, the photoelectric effect occurs when the light above a certain energy threshold hits the surface of select materials. Electrons that were previously bound to that material are then released. This is the basis of many vital modern technologies like solar power generation and optical imaging sensors.
Up until now, the photoelectric effect hasn’t been observed to work in the terahertz range. While this team of scientists still doesn’t fully understand how and why their discovery works, their experimental proof carries a lot of benefits for the future of terahertz technologies. This new property is named the “in-plane” photoelectric effect, derived from the 2D electron gas plane.
When it detects terahertz radiation, the team's sensor generates a magnitude of response much stronger than other methods. This gives the new detector a considerably higher sensitivity, thus mitigating path loss of depreciating signals.
Lithium Improves Terahertz Photonic Sources
Another recent development in terahertz technologies, this time in the area of signal generation, comes from a team of researchers from the Nankai University in China and their colleagues at the INRS-ENT in Canada. Led by professors Jiayi Wang, Shiqi Xia, and Ride Wang, a group of scientists developed a single lithium niobate photonic chip for use in a novel terahertz source module.
The material in question is a type of non-naturally occurring crystal with the chemical composition of lithium, niobium, and oxygen. This material is commonly used in engineering, particularly in telecommunications and nonlinear optics.
The nonlinear generation and confinement of terahertz waves in a Su–Schrieffer–Heeger-type microstructure. Image used courtesy of Wang et al
The team manufactured their sensor using a photonic microstructure that contained lithium niobate waveguide strips. These strips were capable of topically trivial and nontrivial transitions. Next, using a femtosecond laser writing method, they inserted a topological defect at the central interface of their photonic chip. The team directly mapped a terahertz field, demonstrating tunable confinement along their chip. Using this method, the scientists achieved wave confinement as a consequence of topological protection.
This research gives engineers a new platform to tune the confinement and topological properties of terahertz radiation, opening new possibilities for photonic circuits to be used for signal generation in advanced telecommunications and imaging applications.
Adopting Terahertz for 6G
One major obstacle to terahertz adoption is the challenge of designing and implementing transmitter and receiver modules that are efficient, affordable, and operable in a real-world environment. Solving these problems doesn’t only carry the weight of advanced medical and security terahertz sensors but also the development of other emerging technologies that are indirectly dependent on faster wireless protocols.
Current wireless technologies don't support holographics, artificial intelligence, and even 4K video streaming on a large-enough scale—even with the theoretical limits of the 5G standard. These two new discoveries by Cambridge University and Nankai University open up the possibility of electronics that use terahertz frequencies, pushing the future of a sixth-generation wireless network forward.