Breaking a century-old physics barrier: perfect wave trapping with simple cylinders

Many technologies around us -- from smartphones and ultrasound devices to radios -- rely on resonance, a phenomenon in which waves are amplified at specific frequencies. However, typical resonators gradually lose energy over time, requiring constant energy input to maintain their function.

Nearly a century ago, Nobel laureates John von Neumann and Eugene Wigner proposed a counterintuitive concept: under certain conditions, waves could be trapped indefinitely without any energy leakage. These so-called Bound States in the Continuum (BIC) are like whirlpools that remain in place even as a river flows around them. But for decades, scientists believed this phenomenon could not exist in a compact, single-particle system.

Now, the research team has broken this long-standing theoretical boundary by successfully realizing BIC in a single particle.

Using a system of cylindrical granular particles -- small solid rods made of quartz -- the researchers built a highly tunable mechanical platform. By precisely adjusting how the cylinders touch each other, they could control the way mechanical waves interact at the contact boundaries.

Under special alignment, a wave mode became fully confined within a single cylinder without any energy escaping into the surrounding structure. This so-called polarization-protected BIC was not just theoretical -- it was observed in real experiments. Even more remarkably, the system achieved quality factors (Q-factors) over 1,000, a measure of how efficiently a resonator stores energy with minimal loss.

What happens when many of these special cylinders are connected in a chain? The team discovered that the trapped wave modes could extend throughout the chain without dispersing -- a phenomenon known as a flat band.

"It's like tossing a stone into a still pond and seeing the ripples remain motionless, vibrating only in place," said lead author Dr. Yeongtae Jang. "Even though the system allows wave motion, the energy doesn't spread -- it stays perfectly confined." This behavior is described as a Bound Band in the Continuum (BBIC) and opens new possibilities for energy harvesting, ultra-sensitive sensors, and even advanced communications.

"We have broken a long-standing theoretical boundary," said Professor Junsuk Rho, who leads the research. "While this is still in the fundamental research phase, the implications are significant -- from low-loss energy devices to next-generation sensing and signal technologies."

This research was supported by the Mid-Career Research Program of the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT, as well as the POSCO-POSTECH-RIST Convergence Research Center.