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A German-French team of physicists from TU Dortmund University, University of Würzburg, and Le Mans Université has succeeded in launching shear hypersound pulses with exceptionally large amplitudes in metal halide perovskites using pulsed optical excitation. This discovery was published in the journal Science Advances. Whereas the material has been of high interest for photovoltaics so far, the new results turn it into a candidate to be used for optically driven devices capable of generating and detecting sound waves at sub-terahertz frequencies, with potential applications across electronic, photonic, magnetic, and biomedical devices.
We all experience sound every day — we communicate, enjoy music, and recognize countless noises around us. All these phenomena are related to longitudinal sound waves, which travel through air as vibrations of molecules. In crystals, however, other types of sound waves can exist: Shear waves, where atoms shift sideways like the sliding motion in a deck of cards or S waves in earthquakes. As a result, shear waves provide a new tool to explore the internal structure and dynamics of crystalline materials, beyond the reach of conventional acoustic techniques such as ultrasound. In particular, shear sound waves have a vector nature that allows control of their polarization. By combining orthogonal polarizations, one can create circularly polarized, or chiral, acoustic waves capable of coupling to the spin and thus the magnetic degrees of freedom in materials. Moreover, because shear waves travel slower than longitudinal waves, their wavelengths are shorter at the same frequency, enabling higher spatial resolution in acoustic imaging and nanoscale probing. However, generation of shear sound waves is challenging, particularly in ultrafast acoustics at sub-terahertz frequencies, as required for next-generation electronic and optoelectronic devices. Among the various methods, the usage of ultrashort femtosecond light pulses to generate hypersound stands out as one of the most promising approaches.
Motivated by this challenge, the authors have explored a double-perovskite semiconductor for its potential in ultrafast acoustics. The choice is well founded, given the remarkable optical and structural properties of these materials. On one hand, perovskites possess excellent optical properties and therefore, have attracted broad attention due to their success in photovoltaic applications. In particular, inorganic lead-free double perovskites are appealing as a nontoxic and stable material platform. On the other hand, a key feature of these materials is their structural phase transitions (from cubic to tetragonal) and strong electron-lattice interactions.
Hypersound waves in the lead-free double perovskite Cs₂BiAgBr₆ were investigated using pump-probe Brillouin spectroscopy. In this technique, a 100-femtosecond laser pulse with a photon energy above the band gap, where the light absorption is strong, generates an acoustic pulse, while a second laser pulse probes its action in the transparency window of the material. The propagating strain pulse modifies the dielectric constant, and its motion from the surface into the crystal is detected as oscillations in the reflection signal. The experiments revealed a distinct shear strain pulse propagating together with the longitudinal one — a clear signature of efficient transverse hypersound generation.
The team found that strong shear hypersound waves appear only when the crystal enters its tetragonal phase, a state in which the atomic lattice becomes slightly distorted along one of the directions. In this phase, light excitation produces an unusual anisotropic expansion of atoms, where the crystal expands in one direction while contracting in another. Importantly this effect has non-thermal origin: It is not caused by heating of the lattice but by the directional pressure exerted by photo-generated charge carriers created by the laser pulse. These findings mark a significant step toward precise control of optically generated hypersound paving the way for next-generation perovskite-based optoacoustic devices operating in the sub-THz frequency range.
Dr. Dmytro Horiachyi
dmytro.horiachyi@tu-dortmund.de
Dr. Mikhail Nestoklon
mikhail.nestoklon@tu-dortmund.de
https://doi.org/10.1126/sciadv.adw9172
A crystal of the lead-free double perovskite Cs₂BiAgBr₆, for which the physicists observed the gener ...
Quelle: Dirk Schemionek
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