Terahertz wave control for enhanced wireless technology

Terahertz (THz) waves are located between microwaves and infrared light in the electromagnetic spectrum. They can pass through many materials without causing damage, making them useful for security scanning, medical imaging and high-speed wireless communication. They can also penetrate non-metallic objects like clothing and paper.

To harness terahertz waves effectively, their polarisation (the direction in which the waves vibrate) must be controlled. Polarisation control is necessary for enhancing data transmission and improving imaging and sending. Unfortunately, existing THz polarisation methods rely on external components like wave plates or metamaterials, which are often inefficient or unsuitable for compact devices.

Researchers from Beihang University in China have developed a spintronic THz emitter with a microscale strip pattern that enables the modulation of chirality during THz wave generation. Unlike traditional THz sources that rely on external optical components, this emitter incorporates polarisation tuning directly onto its design, streamlining the technology and enhancing its capabilities.

The emitter comprises thin-film layers of tungsten, cobalt-iron-boron and platinum. When exposed to ultra-fast laser pulses, the material generates a spin current, which is converted into an electrical charge through the inverse spin Hall effect. The emitter’s microscale strip pattern alters charge distribution, forming a build-in electric field that influences the amplitude and phase of emitted THz waves. By designing different strip arrangements, the researchers achieved precise polarisation tuning without external optical components.

Rotating the emitter enables flexible and efficient switching between linear, elliptical and circular polarisation states. The device also maintains high-quality circular polarisation with an ellipticity greater than 0.85 across a broad frequency range of 0.74–1.66 THz, demonstrating its efficiency in broadband polarisation control.

To validate the effectiveness of their patterned emitter, the researchers fabricated and tested seven different designs, each with a unique stripe aspect ratio. Using THz time-domain spectroscopy, they measured the impact of different patterns on the emitted THz polarisation. The results confirmed that larger strip aspect ratios produced stronger built-in electric fields, providing greater control over polarisation. Emitter configurations with large aspect ratios also generated THz waves with tuneable polarisation; by adjusting the azimuth angles of stripe pattern, the researchers were able to achieve precise switching between left- and right-handed circular polarisation. This level of integrated control within a single device represents an advancement over traditional THz sources.

This innovation could enhance wireless communication by doubling data transmission rates through polarisation multiplexing. The enhanced measurement sensitivity afforded by this technology could also lead to developments in fundamental research in fields like quantum optics and precision sensing.

The compact and efficient design of the spintronic emitter is suited for on-chip integration, to help realise scalable and cost-effective THz devices for real-world applications. Future research will focus on refining the emitter’s frequency-selective control, opening up further possibilities for advanced photonic and wireless systems.

Image caption: Schematic of chiral terahertz generation and control — A femtosecond laser interacts with a patterned spintronic emitter, producing elliptically or circularly polarised terahertz waves. Rotating the emitter adjusts the polarisation, while built-in electric fields — formed by charge accumulation at the pattern's edges — control the amplitude and phase differences. Image credit: Q. Yang et al.