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Terahertz radiation (THz), electromagnetic radiation with frequencies ranging between 0.1 and 10 THz, could be leveraged to develop various new technologies, including imaging and communication systems. So far, however, a lack of fast and sensitive detectors that can detect radiation across a wide range of frequencies has limited the development of these THz-sensing technologies.

In a recent paper in Nature Electronics, researchers at the University of Wisconsin-Madison, the University of Tennessee and other institutes have introduced new photodetectors made of tantalum iridium telluride (TaIrTeâ‚„), a 2D-correlated topological semimetal that exhibits advantageous properties. Most notably, this material exhibits a strong nonlinear Hall effect, a physical effect that entails a transverse voltage in the absence of an external magnetic field, which is nonlinearly proportional to an applied electric field or current.

"THz technology is critical in quantum information technology and biomedical sensing because its frequency resonates with low-energy collective excitations in quantum materials and molecular vibrations in biological matters," Jun Xiao, senior author of the paper, told ÌÇÐÄÊÓƵ.

"Moreover, the ultra-high bandwidth of the THz band could enable desired high-speed wireless communication. However, the widespread application of THz technologies has been hindered due to the lack of simultaneous sensitive, broadband, and fast THz detection in state-of-the-art detectors such as thermal bolometers and electronic Schottky diodes."

Existing photodetectors capable of detecting THz radiation are either too slow, not sensitive enough, or only capable of detecting signals at some frequencies. Xiao and his colleagues thus set out to develop new photodetectors based on alternative materials, which could overcome the limitations of previously developed devices, exhibiting good sensitivity, fast speeds and broadband.

"We fabricated Har bar geometry sensing devices using atomically thin TaIrTeâ‚„, a 2D-correlated topological semimetal, and exposed them to terahertz (THz) radiation," explained Xiao. "We characterized this effect by measuring THz-induced photocurrent and evaluating device performance metrics such as responsivity and sensitivity. To assess the response speed, we conducted ultrafast autocorrelation measurements using femtosecond laser-generated THz pulses, revealing intrinsic picosecond-scale dynamics."

As part of their study, Xiao and his colleagues also probed the crystal symmetry of TaIrTeâ‚„ using a technique known as second-harmonic generation (SHG) spectroscopy. Using this technique, they observed the emergence of a correlated electronic phase at low temperatures that further enhanced their photodetectors' THz response, or in other words, improved their ability to quickly and precisely detect THz radiation.

"Additionally, we demonstrated that the sensing performance and electronic state could be tuned via electrostatic gating," said Xiao. "These combined methods revealed TaIrTeâ‚„'s promise for fast, broadband, and highly sensitive room-temperature THz sensing."

In initial tests, the photodetectors developed by this team of researchers were found to perform remarkably well. At room temperature, they attained a large zero-bias responsivity (~ 0.3 A/W), ultralow NEP (~ pW/Hz^1/2), broadband THz response (0.1 to 10 THz) and ultrafast intrinsic speed (~ ps).

"We also discovered that the zero-bias responsivity can be boosted by around 50 times (~ 18 A/W) when the topological semimetal transitions into a correlated charge ordering," said Xiao. "Thanks to the new topological physics and quantum properties, the demonstrated device metrics show tremendous advantages over the attainable THz detectors based on other 2D materials and conventional technology."

In the future, the photodetectors developed by Xiao and his colleagues could contribute to the advancement of THz sensing technologies, while also potentially inspiring other teams to create THz sensing devices using 2D-correlated topological semimetals. In their next studies, the researchers plan to evaluate the potential of their devices for imaging and other real-world applications.

"Our current demonstration focuses on a single THz sensing device," added Xiao. "Building on this, we aim to develop large-scale imaging arrays and intelligent THz sensing by integrating machine learning algorithms with the highly tunable sensing properties of the material."