Efficient launch of shear phonons in photostrictive halide perovskites

TL;DR

This study demonstrates that anisotropic photostriction in tetragonal halide perovskites enables efficient, non-thermal generation of transverse coherent phonons, with potential for high-speed active control of material properties.

Contribution

It reveals a novel mechanism for generating transverse coherent phonons in halide perovskites via anisotropic photostriction, expanding the understanding of photoinduced strain effects.

Findings

Coherent transverse and longitudinal phonons observed below 122 K.

Only longitudinal phonons generated in the cubic phase.

Transverse phonon polarization depends on c-axis projection.

Abstract

Optical generation of transverse coherent phonons by femtosecond light pulses is appealing for high-speed sub-THz active control of material properties. Lead-free double perovskite semiconductors, such as Cs2AgBiBr6, attract particular interest due to their cubic to tetragonal phase transition below room temperature and strong polaron effects from carrier-lattice coupling. Here, we reveal that the anisotropic photostriction in halide perovskites with tetragonal crystal structure represents an efficient non-thermal tool for generating transverse coherent phonons. In particular, we demonstrate that along with compressive strain, optical generation of photoexcited carriers leads to strong shear strain in Cs2AgBiBr6 below the phase transition temperature of 122 K. Using time-domain Brillouin spectroscopy, we observe coherent transverse and longitudinal acoustic phonons with comparable amplitudes in the tetragonal phase, while in the cubic phase only longitudinal phonons are generated. The polarization of the photoinduced transverse phonons is dictated by the projection of the c-axis on the surface plane, which leads to a prominent anisotropic polarization response in the detection. The generated strain pulses correspond to transverse acoustic soft eigenmodes with a strong temperature dependence of dispersion, which provides an additional degree of freedom for active hypersonic control.