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. Unlike visible light or radio waves, THz waves can reveal structural details of biological molecules and penetrate nonmetallic objects like clothing and paper.
THz waves hold great promise, but to harness them effectively, their polarization (the direction in which the waves vibrate) must be controlled. Polarization control is crucial for optimizing THz applications, from enhancing data transmission to improving imaging and sensing. Unfortunately, existing THz polarization control methods rely on bulky external components like wave plates or metamaterials. These solutions are often inefficient, limited to narrow frequency ranges, and unsuitable for compact devices. To overcome these limitations, researchers have been exploring approaches to control THz polarization directly at the source.
As reported in Advanced Photonics , researchers from Beihang University, China, recently developed a spintronic THz emitter with a microscale stripe pattern that enables the modulation of chirality during THz wave generation. Unlike traditional THz sources that rely on external optical components, this emitter incorporates polarization tuning directly into 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 ultrafast 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 stripe pattern alters charge distribution, forming a built-in electric field that influences the amplitude and phase of emitted THz waves. By designing different stripe arrangements, the researchers achieved precise polarization tuning without external optical components.
Simply rotating the emitter allows for flexible and efficient switching between linear, elliptical, and circular polarization states. Critically, the device maintains high-quality circular polarization with an ellipticity greater than 0.85 across a broad frequency range of 0.74–1.66 THz, demonstrating its efficiency in broadband polarization control.
To validate the effectiveness of their patterned emitter, the research team 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 polarization. The results confirmed that larger stripe aspect ratios produced stronger built-in electric fields, resulting in greater control over polarization. Emitter configurations with large aspect ratio successfully generated THz waves with tunable polarization, and by adjusting the azimuth angles of stripe pattern, the researchers achieved precise switching between left- and right-handed circular polarization. This level of integrated control within a single device represents a significant advancement over traditional THz sources.
This innovation promises to revolutionize fields from wireless communication, where it can double data transmission rates through polarization multiplexing, to biomedical imaging, where it can enable earlier disease diagnosis through more accurate biomolecule detection. Furthermore, the enhanced measurement sensitivity afforded by this technology could lead to breakthroughs in fundamental research within fields like quantum optics and precision sensing.
The compact and efficient design of this spintronic emitter is ideally suited for on-chip integration, a crucial step towards realizing 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.
This breakthrough represents a significant leap forward for THz technology, bringing the transformative potential of this underutilized region of the electromagnetic spectrum closer to reality.
For details, see the original Gold Open Access article by Q. Yang et al., " Broadband polarization spectrum tuning enabled by the built-in electric field of patterned spintronic terahertz emitters ," Adv. Photon. 7(2), 026007 (2025), doi: 10.1117/1.AP.7.2.026007 .
