Multicolor optical switching is an important phenomenon with potential applications in fields such as telecommunications and optical computing. However, most materials typically exhibit monochromatic 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 electrical voltage or current to operate, resulting in slow response times.

To address this gap, a group of researchers, led by Professor Junjun Jia of the School of Science and Engineering at Waseda University (Japan), in collaboration with Professor Hui Ye and Dr. Hossam A. Almossalami of the School of Optical Science and Engineering at Zhejiang University (China), Professor Naoomi Yamada of the Department of Applied Chemistry at Chubu University (Japan), and Dr. Takashi Yagi of the National Institute of Advanced Industrial Science and Technology (NIAIT) in Japan, investigated the phenomenon of multi-valley optical switching in germanium (Ge) films. They focused on how intense laser radiation induces ultrafast optical switching across multiple wavelengths in Ge, a multi-valley semiconductor. Their study demonstrated the efficiency of multi-color optical switching using a monochromatic pulsed laser, which could overcome the limitations of traditional monochromatic optical nonlinearities. Their research was published in Physical Review Applied on February 24, 2025.
By irradiating Ge with an intense pulsed laser, the team achieved ultrafast switching between transparency and opacity across a wide range of wavelengths. Time-resolved femtosecond transient transmission measurements revealed ultrafast optical switching in both the Γ and L valleys, due to the existence of intravalley and intervalle scattering. “Our results confirm that intense laser irradiation of Ge films enables ultrafast optical switching across multiple wavelengths, offering the possibility of controlling a material’s transparency and opening new doors for potential applications in optical communications, optical computing, and beyond,” explains Professor Jia.
This multi-valley optical switching has been found to be highly dependent 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, transient transmission spectra reveal a separation energy of 240 meV at the high-symmetry L point. “Careful selection of sounding energies, based on band scattering calculated with the HSE06 function and spin-orbit coupling effects, allowed us to accurately capture the transient electronic occupation in both the Γ and L valleys,” says Professor Jia. This makes it possible to extract scattering times between and within valleys in multi-valley 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 data transmission and high-speed computing. By enabling transparency control across multiple wavelengths using a monochromatic pulsed laser, it opens up exciting possibilities for the development of ultrafast optical switches. “This finding is expected to address the growing demand for higher data rates and enhanced security in the face of increasing internet traffic, representing a significant step forward in the advancement of ultrafast optical switching devices,” concludes Professor Jia.

Reference
Authors: Junjun Jia1, Hossam A. Almossalami2, Hui Ye2, Naoomi Yamada3, Takashi Yagi4
Original article title:Multivalley optical switching in germanium

Magazine: Physical Review Applied

DOI: 10.1103/PhysRevApplied.23.024060
Date of publication of the article: February 24, 2025

Affiliations
1Global Center for Science and Engineering (GCSE), College of Science and Engineering, Waseda University, Japan
2College of Optical Science and Engineering, Zhejiang University, China
3Department of Applied Chemistry, Chubu University, Japan
4National Institute of Metrology of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST), Japan

About Professor Junjun Jia:
Junjun Jia is a professor in the Faculty of Science and Engineering at Waseda University, Japan. He received his PhD from the University of Tokyo in 2011. His research focuses on the design and fabrication of functional solid-state materials, as well as the development of solid-state devices, including solid-state thermal circuit elements, acoustic wave-based devices, and unbalanced electronic devices. His interests include nonlinear optics, nonequilibrium physics, and excited electronic/phononic structure in solid materials. Dr. Jia has published numerous articles in peer-reviewed journals such as Advanced Functional Materials, Physical Review B, and Physical Review Applied. He has received several awards, including the Waseda e-Teaching Award in 2022. He is a member of several committees, including the Materials Research Society of Japan.