An “acoustic frequency comb”, which produces sound at a precise set of frequencies, has been made by physicists at the University of Cambridge in the UK. The device, which is an acoustic analogue of an optical frequency comb, works at ultrasonic frequencies. With further improvements, the device could be used for imaging, metrology and materials testing.

Conventional optical frequency combs emit a spectrum of light made of thousands of discrete peaks at evenly spaced frequencies, like the teeth of a comb. Developed in the 1990s, such combs have been used in a range of applications such as comparing different atomic clocks.

One way of creating an optical frequency comb is to combine laser light of several different frequencies in a nonlinear optical medium. But in the new work, Adarsh Ganesan, Cuong Do and Ashwin Seshia have discovered that a similar effect occurs when ultrasound waves interact in a silicon wafer covered by a thin layer of aluminium nitride, which vibrates when driven by an electrical signal.

Surprising discovery

The three researchers were initially investigating if such a wafer could be used for sensing applications when they were surprised to see it vibrate at a number of different frequencies when a megahertz signal is applied to it. The gaps between the frequencies all had the same value (about 2 kHz) and the spectrum looked much like a frequency comb. The teeth of the comb extended over a frequency range of about 100 kHz, says Ganesan.

Puzzled by their discovery, the trio soon realized that their system is like a theoretical proposal for an acoustic frequency comb made in 2014 by Peter Schmelcher of the University of Hamburg and colleagues. Schmelcher’s group modelled the atoms in a solid material as a collection of masses connected by springs that have a restoring force with a nonlinear component.

In such a material, sound waves can interact with each other to create waves at several different frequencies. Ganesan told Physics World that while the Schmelcher model does describe some aspects of their acoustic comb, it does not capture the full complexity of the device.

The team is now making more frequency combs and is also thinking about possible applications, which include boosting the accuracy of sensors that operate using mechanical vibrations. Other possible uses include phonon lasers that create phase-coherent sound signals and ultrasonic imaging.

Follow-up studies

Ultrasound expert Bruce Drinkwater at the University of Bristol says the research is “fascinating” but warns that while there could be applications, “until follow-up studies are performed it’s hard to be sure”. If further research succeeds, the phenomenon could be used, says Drinkwater, to create sensitive new sensors that could be used for, say, gas and chemical monitoring. “There is also the intriguing possibility of using it to monitor the degradation of metallic structures, which are known to become increasingly nonlinear as they age,” he adds.

The frequency comb is described in Physical Review Letters.