image: Professor Nicholas X. Fang, Jockey Club STEM Professor in the Department of Mechanical Engineering and Professor Xiang Zhang, HKU President and Vice-Chancellor, have been jointly awarded the 2025 Brillouin Medal.
Credit: The University of Hong Kong
Professor Nicholas X. Fang, Jockey Club STEM Professor in the Department of Mechanical Engineering, Faculty of Engineering, and Professor Xiang Zhang, HKU President and Vice-Chancellor, and Chair of Physics in the Department of Physics, Faculty of Science, have been jointly awarded the 2025 Brillouin Medal by the International Phononics Society (IPS). The honour recognises their pioneering discovery of ultrasonic metamaterials with negative elastic modulus.
Their research advances the foundational principles of acoustic metamaterials, a field dedicated to understanding how sound waves interact with intricately engineered materials. Central to this are two critical properties: mass density and bulk modulus, which govern these interactions. Mass density determines a material’s response to vibrations, while bulk modulus reflects its ability to withstand compression. By successfully achieving a negative bulk modulus, Professors Fang and Professor Zhang have pioneered groundbreaking methods to manipulate sound waves with unparalleled precision. This milestone builds upon earlier progress in controlling mass density, opening the door to transformative applications in acoustic technology. Their work represents a significant leap forward in the development of advanced materials capable of shaping and controlling sound in ways that were once beyond imagination.
Reflecting on this breakthrough, Professor Fang remarked, “While our discovery of a negative modulus was entirely accidental, I’m thrilled to see this early work inspire breakthroughs in acoustic, electromagnetic, and seismic metamaterials—driving innovation across engineering and applied sciences.”
Professor Zhang shared his perspective on the broader significance of this research, "Scientific progress begins with the exploration of the unknown, and our research embodies this spirit of discovery. This honour reminds us that innovation knows no limits, and our mission is to continue expanding the frontiers of human knowledge, laying a solid foundation for the future of technology," Professor Zhang remarked.
The Brillouin Medal, awarded biennially by the IPS, celebrates seminal contributions in phononics, including phononic crystals, acoustic/elastic metamaterials, nanoscale phonon transport, wave propagation in periodic structures, coupled phenomena involving phonons, topological phononics, and related areas.