How to probe lattice dynamics using tandem Fabry–Pérot Brillouin spectroscopy

Some changes in a material are first heard rather than seen. Before a structural phase transition becomes obvious in diffraction, a crystal may soften, an acoustic phonon may slow down, or a relaxation process may begin to broaden the phonon spectrum. Tandem Fabry–Pérot Brillouin spectroscopy is an established light-scattering method that can capture these early dynamical signatures by probing how a material carries hypersonic waves. This makes it especially useful for studying lattice dynamics, elastic anomalies, phase transitions and relaxation phenomena in condensed matter systems, because these processes directly alter the sound velocity and damping of the acoustic phonons measured by Brillouin spectroscopy.

The tandem Fabry–Pérot interferometer is what makes this measurement possible. A Fabry–Pérot interferometer consists of two highly reflective, parallel mirrors separated by a small distance. The light undergoes multiple reflections between them and only frequencies for which the reflected waves add constructively are transmitted efficiently. A single Fabry–Pérot interferometer provides high spectral resolution, but it transmits repeated spectral orders with a low contrast. In a tandem system, the two interferometers have slightly different free spectral ranges, so their repeated transmission orders do not coincide throughout the scan. Only frequencies that are transmitted by both interferometers pass efficiently. This Vernier effect suppresses the unwanted orders that would otherwise overlap with the Brillouin peaks, thereby extending the usable, order-free spectral window to more than 1,000 GHz. Multipass operation further improves the spectral contrast up to the order of 1015, which is essential because Brillouin signals are weak and lie close to the much stronger elastic scattering peak.

This is a preview of subscription content, access via your institution

Prices may be subject to local taxes which are calculated during checkout

The author thanks J.-H. Ko and S. H. Kim for their suggestions and support.

Department of Physics, Research Institute for Materials and Energy Sciences, Jeonbuk National University, Jeonju, Republic of Korea

Naqvi, F.H. How to probe lattice dynamics using tandem Fabry–Pérot Brillouin spectroscopy. Nat Rev Phys (2026). https://doi.org/10.1038/s42254-026-00961-6

Anyone you share the following link with will be able to read this content:

Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative