A new dimension in picosecond acoustics

Picosecond acoustics in a transparent thin film: a metal film is excited by a femtosecond laser pulse, an ultra-high-frequency acoustic pulse passes through the film and an oscillation is detected. The two signals are obtained with a standard film (black curve) and a tilted film (red curve). The Fourier transform visible on the right confirms that two acoustic frequencies are detected with the tilted film: transverse and longitudinal waves.
A team from the Institute of Electronics, Microelectronics and Nanotechnology in collaboration with FEMTO-ST in Besançon and LMI in Lyon (Laboratoire des multimatériaux et interfaces) has extended the metrological capabilities of picosecond acoustics. This laser technique makes it possible to measure the elastic properties of thin films, but until now only in the direction perpendicular to the layer. Thanks to a metal layer made of inclined columns, in-plane elasticity also becomes accessible. The results have been published in the journal Applied Physics Letters.
Picosecond acoustics refers to ultra-high-frequency acoustics that produce hypersound (far beyond ultrasound), which is of course no longer heard, but which is very useful for measuring the properties of thin films and other nanostructures. The technique first saw the light of day in the 1980s, and since then has become as popular in the academic world as it is in industry, where it is used to control microprocessors on production lines. To access the world of hyper or “nanosounds”, there are no microphones or transducers, just laser light delivered in extremely brief flashes, femtosecond pulses.
But this technique has an intrinsic limitation: only certain acoustic waves are accessible, namely longitudinal waves. And this is a problem, because we learn much more about the state of matter when we measure the speed of several types of wave (longitudinal and transverse). Previous work has shown that it is sometimes possible to get around this limitation, but always in specific sample configurations. In particular, it was impossible to generalize to thin-film samples on silicon, THE basic geometry for applications, so these attempts were in vain.
Using a thin metallic layer deposited in inclined columns, we have shown that any transparent layer on silicon can be characterized in terms of longitudinal and transverse waves. The inclined layer acts as a mixed longitudinal/transverse emitter when subjected to the laser, and picosecond acoustics gain a new dimension. Although the study was initially dedicated to transparent thin films, such as silica, aluminum nitride and silicon carbide, the process is now being extended to non-transparent layers, such as metallic layers. The applications are numerous, since measuring the elastic properties of thin films is essential in the design of technological objects as varied as filters for mobile telephony, space mirrors, intelligent glass…
Référence
Tilted columnar metal film as transducer of transverse coherent acoustic phonons in Picosecond Acoustics.
Asma Chargui, Nicolas Martin, Gabriel Ferro, et Arnaud Devos.
Applied Physics Letters, 8 novembre 2024.
https://doi.org/10.1063/5.0228331