How sound and light both interact, but not on a smaller scale

A world-famous light experiment from 1801 has now been performed using sound for the first time. Research by physicists in Leiden has produced new insights that can be applied in 5G devices and the emerging field of quantum acoustics. The study is published In the magazine Optics Letters.

Ph.D. “We have seen that sound waves in materials behave in the same way as light, but also in a slightly different way. Using a mathematical model, we can now explain and predict this behavior,” says student Thomas Steenbergen.

Thomas Young’s famous double slit experiment

Young’s famous double-slit experiment was the first to show that light sometimes behaves as a particle, and sometimes as a wave. In the experiment, light was shined through two narrow slits. Behind the slits, light waves either enhanced or canceled each other out due to interference, creating a pattern of bright and dark lines – an interference pattern.

The same experiment was later performed on particles, and showed that all particles can behave as a particle and as a wave. Over time, the double-slit experiment has been performed with all kinds of quantum objects, from electrons and neutrons to buckyballs, molecules made of 60 carbon atoms.

Now with sound instead of light

Steenbergen and his colleague Loeffler wanted to understand exactly how sound behaves on the smallest scales. The double-slit experiment provides valuable insight here. In preparing his experiment, Steenbergen relied on a research project initiated by physics undergraduate student Christian Czerniak.

In the experiment, the researchers used gigahertz sound waves, vibrating a billion times per second, much louder than humans can hear.

Experience

The sound waves were directed at a small piece of material: the semiconductor gallium arsenide, which is often used in electronic devices. Colleague Matthias Rogge from the Café La Hape research group carved two small grooves (slits) in the material using an ion beam.

“We then measure the sound using a very precise scanner,” Steenbergen explains. “This device can measure sound literally everywhere, including inside and in front of the cracks. We can measure the height of the sound waves with picometer precision, a millionth of a micrometer.”

Similarities and differences

Just as in double-slit experiments with light, the interference pattern appears at the back: you can clearly see where the sound is enhanced and where it is cancelled.

“But if you look closely, you also see that the pattern is not exactly symmetrical,” Steenbergen adds. “Sound waves do not move in the same way in all directions. The speed of the waves depends on the angle at which they pass through the material.” By developing a mathematical model, the team was able to accurately explain and predict these differences.

An old experience opens new doors

Gigahertz sound waves are widely used in communications, especially in 5G devices such as mobile phones. This research provides new knowledge that can be applied in these technologies, as well as in other microelectronic devices and sensors that use sound.

It also provides insights into the emerging field of quantum acoustics, where sound waves on the smallest (quantum) scale are used to transmit information. In this way, an experience dating back centuries opens new doors once again.