Reviewed by Lexie CornerMar 25 2025
Researchers from Beihang University in China have developed a spintronic THz emitter featuring a microscale stripe pattern that enables the modulation of chirality during THz wave generation. This emitter integrates polarization tuning directly into its design, offering a more streamlined and efficient technology compared to conventional THz sources that rely on external optical components.
Schematic of chiral terahertz generation and control: A femtosecond laser interacts with a patterned spintronic emitter, producing elliptically or circularly polarized terahertz waves. Rotating the emitter adjusts the polarization, 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.
Terahertz (THz) waves occupy a position between microwaves and infrared light in the electromagnetic spectrum. They are useful for high-speed wireless communication, medical imaging, and security scanning because they penetrate various materials without causing damage.
Unlike radio waves or visible light, THz waves can pass through nonmetallic materials like paper and clothing, revealing structural details of biological molecules.
To fully utilize THz waves, controlling their polarization—the direction in which the waves vibrate—is essential. Polarization control is crucial for optimizing THz applications, which span from enhancing imaging and sensing to improving data transmission.
Current methods for controlling THz polarization often rely on large external components like metamaterials or wave plates. These solutions are limited, typically work within specific frequency ranges, and are unsuitable for small devices. Researchers have been investigating ways to control THz polarization directly at the source to overcome these challenges.
The emitter consists of thin-film layers of platinum, cobalt-iron-boron, and tungsten. When exposed to ultrafast laser pulses, the material generates a spin current, which the inverse spin Hall effect then converts into an electrical charge.
The emitter's microscale stripe pattern modifies charge distribution, creating an inherent electric field that influences the amplitude and phase of the emitted THz waves. By adjusting the stripe arrangements, the researchers achieved precise polarization tuning without relying on external optical components.
By simply rotating the emitter, it is possible to switch between linear, elliptical, and circular polarization states with both flexibility and efficiency. Notably, the device demonstrates broadband polarization control by maintaining high-quality circular polarization with an ellipticity greater than 0.85 across a wide frequency range of 0.74–1.66 THz.
To validate their patterned emitter, the research team designed and tested seven distinct configurations, each with a different stripe aspect ratio. They evaluated the effect of various patterns on emitted THz polarization using THz time-domain spectroscopy. The results indicated that larger stripe aspect ratios generated stronger built-in electric fields, leading to greater polarization control.
The configurations with larger aspect ratios produced THz waves with tunable polarization, allowing precise switching between left- and right-handed circular polarization by adjusting the azimuth angles of the stripe pattern. This level of integrated control within a single device represents a significant improvement over conventional THz sources.
This technology could impact various fields, including biomedical imaging, by enabling more accurate biomolecule detection and earlier disease diagnosis, and wireless communication, where it may enhance data transmission rates through polarization multiplexing. The increased measurement sensitivity could also advance precision sensing and quantum optics.
The compact and efficient design of the spintronic emitter makes it suitable for on-chip integration, which is essential for developing scalable, cost-effective THz devices for practical applications. Future research will focus on improving frequency-selective control, opening up further possibilities for advanced photonic and wireless systems.
This innovation brings the potential of THz technology closer to real-world applications and represents a significant step forward in the development of this area of the electromagnetic spectrum.
Journal Reference:
Yang, Q., et al. (2025) Broadband polarization spectrum tuning enabled by the built-in electric field of patterned spintronic terahertz emitters. Advanced Photonics. doi.org/10.1117/1.AP.7.2.026007.