Optical technique enables generation of hypersound waves in perovskite crystals

by Robert Schreiber
Berlin, Germany (SPX) Nov 25, 2025
Researchers from TU Dortmund University, University of Wurzburg, and Le Mans Universite have demonstrated the optical generation of shear hypersound pulses with large amplitudes in metal halide perovskites, published in Science Advances. While these materials have previously drawn interest for their role in photovoltaics, the new results indicate suitability for optically driven devices that generate and detect sound waves at sub-terahertz frequencies. Potential applications extend across electronic, photonic, magnetic, and biomedical devices.

Sound, as encountered in everyday life, corresponds to longitudinal waves traveling through air. In crystalline materials, however, shear waves can emerge, characterized by atoms shifting in a sideways motion similar to S waves in earthquakes. Shear waves offer a means to probe the internal dynamics and structure of crystals beyond the reach of standard ultrasound techniques. Their vector nature allows for polarization control.

The combination of orthogonally polarized shear waves facilitates creation of circularly polarized, chiral acoustic waves that can couple to the spin and magnetic properties of materials. Because shear waves propagate more slowly than longitudinal waves, they offer shorter wavelengths - enabling improved spatial resolution in acoustic imaging and nanoscale investigations. Yet, producing shear sound waves, particularly in the sub-terahertz range necessary for advanced electronics and optoelectronics, remains challenging. Femtosecond light pulses present a promising method for hypersound generation in this regime.

The collaborative team investigated the double-perovskite semiconductor Cs2BiAgBr6, known for its appealing optical qualities and phase transition behaviors. Lead-free double perovskites are both stable and nontoxic, making them attractive for technological applications. Significant in these materials are the structural phase transitions, especially from cubic to tetragonal, and pronounced electron-lattice interactions.

Using pump-probe Brillouin spectroscopy, the team produced hypersound waves in Cs2BiAgBr6. A 100-femtosecond laser pulse above the material's band gap generates an acoustic pulse, while a secondary probe pulse tracks its progress. The strain pulse alters the crystal's dielectric constant, and its migration inside the crystal was monitored as oscillations in reflected light signals. Experimental results disclosed a pronounced shear strain pulse moving alongside the longitudinal pulse, evidencing efficient transverse hypersound generation.

The strongest shear hypersound effects appeared when the perovskite entered its tetragonal phase - when the atomic lattice is slightly distorted in one direction. Here, optical excitation causes anisotropic atomic expansion: the lattice expands along one axis and contracts along another.

This is a non-thermal phenomenon, driven by the directional pressure of photo-generated charge carriers from the laser pulse, rather than heat. The findings advance prospects for precise control of optically generated hypersound, potentially guiding development of next-generation perovskite-based optoacoustic devices in the sub-terahertz frequency domain.

Research Report: Efficient launching of shear phonons in photostrictive halide perovskites