Scientists discover way to guide bouncing light, sound waves in complex spaces

Scientists found a way to stop light and sound waves from scattering chaotically inside complex, oddly shaped spaces.

Shout into an irregularly shaped room, and your voice shatters into a chaotic mess of echoes. The incoming wave hits a slanted wall, bounces off at an angle, and rapidly spirals into complete unpredictability. Physicists call this non-linear mess a “dynamical billiard.” It is the bane of optical engineers and acoustic designers. Now, scientists have found a way to stop the madness.

Researchers at the Advanced Science Research Center at the CUNY Graduate Center (CUNY ASRC) have discovered that passing waves through a special class of synthetic media forces them to abandon chaotic scattering entirely. Instead of splintering, the waves naturally organize themselves into repeating geometric loops.

The discovery could reshape how engineers route light on computer chips, direct wireless signals, and build biological sensors.

“This work shows how geometry and the properties of a material can work together to produce wave behavior that is both surprising and useful,” said Andrea Alù, the study’s principal investigator.

“By understanding how waves organize themselves in these hyperbolic media, we can begin to explore new approaches to controlling energy, information, and communication signals in complex environments,” Alù added.

The geometry of order

The research team aimed to investigate how combining these non-standard reflection properties with an asymmetrical cavity reshapes wave dynamics.

In everyday materials, reflection obeys a simple rule: the angle of incidence equals the angle of reflection. Throw light into an oddly shaped box, and those reflections quickly turn messy.

Standard materials send reflecting waves outward at equal angles, creating unpredictable chaotic patterns inside irregular containers.

Hyperbolic materials overcome this limitation by restricting wave propagation along narrow, highly directed paths and altering reflection behavior off slanted surfaces. Inside these specialized cavities, waves progressively organize into stable, scale-invariant closed loops as broken mirror symmetry and shrinking wavelengths collapse chaotic scattering into structured trajectories.

This unique propagation gives rise to wave trajectories with directional rotation, or handedness, which dictates whether paths rotate clockwise or counterclockwise.

Researchers experimentally validated this self-organizing behavior by controlling vibrations within an engineered mechanical metamaterial.

“The most exciting aspect of these results is that they connect a simple geometric idea with a rich set of wave phenomena,” said Enrico Renzi, a doctoral student in Alù’s lab. “We can observe how the waves become organized, and we can connect that organization to properties such as stability and handedness. This gives us a framework for designing wave behavior rather than simply observing it.”

Ocean roots

The physics behind this tabletop development shares surprising roots with deep-sea oceanography.

In the ocean, underwater waves travel through layers of water with differing densities. When these internal waves hit underwater slopes, they reflect in unusual ways, often locking into closed trajectories that concentrate energy beneath the surface.

The CUNY team translated this fluid-dynamics concept into a solid-state platform using a mechanical metamaterial — a custom-designed structure with miniature pillars that guide elastic vibrations.

The usage could go beyond mechanical vibrations. The same mathematical framework applies to 2D materials like hexagonal boron nitride and molybdenum trioxide, which host hybrid light-matter excitations called hyperbolic phonon polaritons.

Researchers aim to harness wave attractors to construct low-loss nanophotonic circuits, engineer highly sensitive biosensors, manipulate microscopic particles using light, and create wave-based analog computing systems.

The study was published in the journal Nature Physics.