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, PhD 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. Their findings were published on July 15 in Physical Review Letters, one of the world's most prestigious physics journals.
Many of today's technologies rely on frequency conversion: green laser pointers, for instance, double the frequency of invisible infrared light to create visible green light, while directional speaker convert ultrasonic frequencies into audible sounds. These processes typically use nonlinear effects in which the system's response does not scale linearly with input intensity. However, such frequency conversion traditionally requires complex structures, fixed propagation directions, or external modulation methods.
To overcome these limitations, the team designed a granular phononic crystal structure consisting of connected cylindrical elements with locally varying stiffness. This structure enables the system to exhibit completely different responses depending on the wave's propagation direction.
In their system, weak waves are mostly blocked. However, when the intensity of the incoming wave grows stronger, asymmetric frequency conversion occurs: waves entering from one side are upconverted to higher frequencies, producing sharper sounds, while those entering from the opposite side are downconverted to lower frequencies, yielding deeper tones. It is as if the same doorway emits different sounds depending on whether one approaches it from the front or the back.
Importantly, by combining nonlinear effects with spatial asymmetry and local resonance—where specific vibrations strongly amplify within certain parts of the structure—the team achieved simultaneous bidirectional frequency conversion within a single system. This has never been demonstrated before in physical experiments.
This technology holds promising potential across various fields. It could enable selective suppression of vibrations from construction or seismic activities, enhance the resolution of medical ultrasound imaging, and lead to acoustic devices capable of detecting otherwise inaudible sounds from specific directions. Moreover, it offers new possibilities in analog signal processing and next-generation frequency conversion technologies.
Professors Rho and Kim commented, "What had been only theoretically envisioned has now been experimentally verified. This technology could find widespread applications in next-generation signal processing and frequency conversion systems."
