Efficient launching of shear phonons in photostrictive halide perovskites

Optical generation of coherent transverse phonons by femtosecond light pulses is appealing for sub-terahertz high-speed active control of material properties. Lead-free double-perovskite semiconductors, such as Cs₂AgBiBr₆, attract particular interest in this respect due to their structural phase transition and strong carrier-lattice coupling. Here, we reveal that the giant anisotropic photostriction in halide perovskites with tetragonal crystal structure provides an efficient nonthermal tool for generating coherent transverse phonons. Using time-domain Brillouin spectroscopy, we observe transverse and longitudinal acoustic phonons with comparable amplitudes in the tetragonal phase of Cs₂AgBiBr₆ below the temperature of 122 kelvins, while, in the cubic phase, only longitudinal phonons are generated. The polarization of the transverse phonons is dictated by the projection of the crystal c axis on the surface plane, which leads to a prominent anisotropic polarization response in the detection. The generated strain pulses correspond to soft transverse acoustic eigenmodes with a strong temperature dependence of dispersion, providing an additional degree of freedom for hypersonic manipulation.