After almost 100 years, scientists achieve elusive bound states in continuum
About 96 years ago, physicists John von Neumann and Eugene Wigner proposed a strange wave behavior that is bewildering scientists even today. They named it bound state in the continuum (BIC).
BIC is a strange wave behavior where energy stays trapped in a system, even though it seems like it should escape. Imagine sound or light waves stuck in one spot indefinitely without leaking out, even when it’s surrounded by space where it normally would spread.
Until now, it’s been considered a theoretical concept that could never occur in reality. However, for the first time, a team of Korean researchers has successfully realized BIC in a particle.
They performed an interesting experiment during which they trapped mechanical waves inside a single resonator. Here’s how they achieved this nearly impossible feat.
Trapping waves in a cylinder
The researchers used small cylinder-shaped quartz rods to build a system that could control how mechanical waves move. They figured out a way to manipulate the way these waves interacted by carefully adjusting how the rods touched each other.
They noticed that when they aligned the rods in a particular way, a mechanical wave was found to be trapped completely inside a single rod without any energy leaking out. This entrapment is well-known among physicists as polarization-protected BIC.
The quartz rod system achieved this BIC with a quality factor (Q-factor) of more than 1,000, meaning it could store energy with very little loss. Next, the researchers connected several rods in a row and saw that the trapped waves could stretch along the whole chain without spreading out or losing energy. This unusual behavior is 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. Even though the system allows wave motion, the energy doesn’t spread—it stays perfectly confined,” Yeongtae Jang, lead researcher and a PhD student at Pohang University of Science & Technology (POSTECH), said.
The individually confined wave modes (BIC and flat band) collectively form a non-spreading band across a system, called Bound Band in the Continuum (BBIC). This special wave behavior keeps the energy trapped even when the wave moves through a connected system.
An important step towards super-efficient devices
Many of the machines we use today, such as microwave ovens, smartphones, speakers, smart watches, work using resonators that amplify different waves (ex., sound waves, electromagnetic waves, etc).
However, the currently used resonators come with a big limitation. They continuously lose energy and need a constant power supply to remain active. Bound State in the Continuum (BIC) and Bound Band in the Continuum (BBIC) can change this.
With BIC inside resonators, a system can store energy without losing it, which could help in making more efficient devices that don’t need constant energy input. With BBIC, when multiple systems are connected, the trapped waves can still move through the system without losing energy. This could lead to devices that work longer and more efficiently.
“We have broken a long-standing theoretical boundary. 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,” Junsuk Rho, one of the study authors, said.
Source: Interesting Engineering
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After almost 100 years, scientists achieve elusive bound states in continuum