A research team in Korea has experimentally demonstrated, for the first time in the world, a nonlinear wave phenomenon that changes its frequency—either rising or falling—depending on which direction the waves come from.

Much like Janus, the Roman god with two faces looking in opposite directions, the system exhibits different responses depending on the direction of the incoming wave. This groundbreaking work opens new horizons for technologies ranging from medical ultrasound imaging to advanced noise control.

The joint research team, led by Professor Junsuk Rho of POSTECH’s Departments of Mechanical Engineering, Chemical Engineering, Electrical Engineering, and the Graduate School of Convergence Science and Technology, along with Dr. Yeongtae Jang, Ph.D. candidate Beomseok Oh, and Professor Eunho Kim of Jeonbuk National University, has experimentally demonstrated a phenomenon of bidirectional asymmetric frequency conversion within a granular phononic crystal system.