Ultrafast Multivalley Optical Switching in Germanium for High-Speed Computing and Communications

Multicolored optical switching is essential for potential advancements in telecommunication and optical computing. However, most materials typically exhibit only single-colored optical nonlinearity under intense laser illumination. To address this, researchers have demonstrated that exciting the multivalley semiconductor germanium with a single-color pulse laser enables ultrafast transparency switching across multiple wavelengths. This breakthrough could drive the development of ultrafast optical switches for future multiband communication and optical computing.

Opaque materials can transmit light when excited by a high-intensity laser beam. This process, known as optical bleaching, induces a nonlinear effect that temporarily alters the properties of a material. Remarkably, when the laser is switched on and off at ultrahigh speeds, the effect can be dynamically controlled, opening new possibilities for advanced optical technologies.

Multicolored optical switching is an important phenomenon with potential applications in fields such as telecommunications and optical computing. However, most materials typically exhibit single-color optical nonlinearity under intense laser illumination, limiting their use in systems requiring multicolor or multiband switching capabilities. Currently, most optical switches are based on microelectromechanical systems, which require an electric voltage or current to operate, resulting in slow response times.

To address this gap, a group of researchers, led by Professor Junjun Jia from the Faculty of Science and Engineering at Waseda University, Japan, in collaboration with Professor Hui Ye and Dr. Hossam A. Almossalami from the College of Optical Science and Engineering at Zhejiang University, China, Professor Naoomi Yamada from the Department of Applied Chemistry at Chubu University, Japan, and Dr. Takashi Yagi from the National Institute of Advanced Industrial Science and Technology, Japan, investigated the multivalley optical switching phenomenon in germanium (Ge) films. They focused on how intense laser irradiation induces ultrafast optical switching across multiple wavelengths in Ge, a multivalley semiconductor. Their study demonstrated efficient multicolored optical switching using a single-color pulse laser, potentially overcoming the limitations of traditional single-color optical nonlinearities. Their research was published in Physical Review Applied on February 24, 2025.

By irradiating Ge with an intense pulse laser, the team achieved ultrafast switching between transparency and opacity across a wide wavelength range. Femtosecond time-resolved transient transmission measurements revealed ultrafast optical switching in both the Γ and L valleys, due to the existence of intravalley and intervalley scattering. "Our results confirm that intense laser irradiation in Ge films allows for ultrafast optical switching across multiple wavelengths, offering the possibility of controlling a material’s transparency and opening new doors for possible applications in optical communications, optical computing, and beyond,” explains Prof. Jia.

Such multivalley optical switching is found to strongly depend on the band structure of Ge. Experimental measurements suggest that the transient signal is highly dependent on the specific region of the band structure involved. For example, the transient transmission spectra reveal a split-off energy of 240 meV at the L high symmetric point. “Careful selection of probing energies, based on the band dispersion calculated with the HSE06 functional and spin-orbit coupling effects, allowed us to accurately capture the transient electronic occupation in both the Γ and L valleys,” says Prof. Jia. This allows the extraction of intervalley and intravalley scattering times in multivalley materials from transient measurements.

Overall, this study highlights the significant potential of Ge as a key material for advanced optical switching, with promising applications in high-speed data transmission and computing. By enabling control over transparency at multiple wavelengths using a single-color pulse laser, exciting possibilities open up for the development of ultrafast optical switches. “This finding is expected to address the growing demand for higher data rates and security in the face of increasing internet traffic, marking a key step forward in the advancement of ultrafast optical switching devices,” concludes Prof. Jia.