Full rainbows made of sound have been caught and imaged for the first time. Harnessing the techniques behind this could lead to new sound-based devices for information processing or energy harvesting.
To create a rainbow from any source of light, you must sort that light by frequency. For instance, a patch of red light consists of light waves oscillating at one frequency, and waves that make the orange patch next to it oscillate at another. Similar sorting ought to be possible for other types of waves, including sound or vibrations within materials, but researchers have struggled to make such 鈥渆lastic rainbows鈥� other than across tiny areas in the corners or edges of materials.
Now, Riyi Zheng at the South China University of Technology and his colleagues have trapped such a rainbow on a silicon chip, while at Tongji University in China and his team have created one in an aluminium device 鈥� presenting the most complete elastic rainbows to date.
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Making an elastic rainbow requires corralling particle-like quantum excitations called phonons, which make up vibrations, into groups by frequency. Next, each group must be made to stay put at a different location. To achieve this, both teams drew inspiration from how the motion of electrons, another quantum particle, in thin materials can be constrained by immersing them into electromagnetic fields.
The researchers engineered their devices to have microscopic patterns, such as tiny triangular pillars. Typically, vibrations spread through solids in a largely disordered, undirected way. But these patterns were designed so that when phonons moved through, their speed and direction of motion changed, as if nudged by an electromagnetic field.
鈥淭he researchers have turned the geometry of an ordinary solid into an effective landscape for steering vibrations, almost as if the chip contained magnetic and electric fields designed specifically for sound. That is both conceptually beautiful and potentially very useful,鈥� says at the National and Kapodistrian University of Athens in Greece, who was on using patterned materials to trap waves.
Once the phonons formed a rainbow in this way, Zheng and his colleagues found it could be used to route an incoming elastic wave, similar to guiding it through only one patch of colour.
Chen and his colleagues also imaged their rainbow. They made it from ultrasonic sound beyond human hearing by illuminating it with a laser and recording the subsequent vibrations. 鈥淥bserving the ultrasonic waves smoothly separate and localise at their exactly predicted locations was an incredible experience,鈥� says Chen.
The two experiments are technically impressive and offer convincing proof that an elastic rainbow is something that can be practically made and manipulated, says at the Eastern Institute of Technology in China.
The connection to electron physics means that these results could open the door for exploring quantum effects, as well as phenomena from Albert Einstein鈥檚 special relativity in new materials settings, says at Imperial College London. The two experiments could also lead to the creation of new devices like vibration filters, ultrasonic routers, sensitive mechanical sensors or devices that harvest energy from elastic waves, he says.
But these applications are unlikely to be immediate, says at Lancaster University in the UK. This is because the existing devices would have to be made smaller and the amount of vibrational energy that is trapped within the rainbow would have to be increased, for example. Chen says his team is already working in this direction. 鈥淥ur methodology holds promise for the ultra鈥慶ompact on-chip acoustic devices鈥� he says.
Physical Review Letters
Physical Review Letters
