Acoustic frequency combs serve to organise sound or mechanical vibrations into a sequence of evenly spaced frequencies, analogous to the arrangement of teeth on a comb. As the acoustic counterparts of optical frequency combs, which consist of equally spaced spectral lines, these systems function as exceptionally precise tools for measuring light. While optical frequency combs have transformed various fields such as precision metrology, spectroscopy, and astronomy, acoustic frequency combs leverage sound waves, which interact with materials in fundamentally distinct ways, making them particularly suitable for a range of sensing and imaging applications.

Historically, existing acoustic frequency combs have operated solely at very high, inaudible frequencies exceeding 100 kHz and typically generated only a few hundred comb teeth, thus constraining their practical applications. However, recent research published in Advanced Photonics has revealed an acoustic frequency comb capable of containing up to 6000 teeth, with spacing that can be adjusted across a broad spectrum from approximately 10 Hz to 100 kHz. This research was conducted in collaboration with scholars from China, Japan, India, Singapore, the USA, and the United Arab Emirates, demonstrating the highest tooth count and the widest tunable bandwidth achieved in an acoustic frequency comb to date.

This significant advancement is facilitated by a novel method of generating the mechanical vibrations essential for the comb, employing phonon lasers. In this innovative approach, an ultra-thin silicon nitride membrane, measuring around 100 nanometers in thickness, functions as a miniature mechanical drum. This membrane is strategically positioned within an optical cavity, wherein laser light circulates multiple times, and the entire system is maintained in a low-pressure vacuum to minimise air interference.

As the laser power increases, the circulating light applies a stronger radiation pressure on the membrane, tightly coupling the intracavity light to its motion. Upon the laser power surpassing a critical threshold, the membrane begins to vibrate at specific, well-defined frequencies, alongside their harmonics. This phenomenon signifies the onset of phonon lasing, where mechanical vibrations attain a level of order and intensity comparable to that of optical laser light, but manifest as sound.

These coherent vibrations modulate the laser light within the cavity, resulting in the formation of an intermediate optomechanical frequency comb. As the interaction intensifies, nonlinear wave mixing between distinct vibrational modes leads to the emergence of a fully developed phonon-laser frequency comb, characterised by thousands of evenly spaced acoustic frequencies. Notably, this comb exists concurrently in both mechanical and optical domains, facilitating acoustic and optical output channels simultaneously—a capability that has not been previously demonstrated in acoustic frequency comb systems.

This research establishes new performance benchmarks for acoustic frequency combs and paves the way for applications in underwater sensing, structural flaw detection, and biomedical ultrasonics. Prof. Franco Nori remarked, “Compared to previous acoustic frequency combs, our phonon-laser comb features phonon lasing, a record-high number of teeth, tunable spacing, and a broad bandwidth spanning the low-frequency audible region.”

Currently, the phonon-laser frequency comb operates effectively at pressures up to 1 kPa. The forthcoming objective is to adapt this system for operation at normal atmospheric pressure, a critical requirement for numerous real-world applications. Prof. Nori concluded, “This could be accomplished through advanced nanofabrication techniques such as dissipation dilution and metasurface engineering, which would enhance the mechanical quality of the membrane while reducing air damping, thereby extending the practical impact of the technology.”

For further information, please refer to the original Gold Open Access article by G. Xiao et al., titled “Ultrabroadband phonon laser frequency comb,” published in Adv. Photon. 8(2), 026004 (2026), doi: 10.1117/1.AP.8.2.026004.