The understanding of the lattice dynamics in ferroic compounds driven by an ultrashort light pulse is an exciting research direction due to the exceptional non-linear properties (optical, elastic, electric and magnetic) of ferroic and multiferroic materials. Photo-induced strain in ferroic materials is driven by a complex interplay between charge, phonon and spin dynamics with microscopic mechanisms that still need to be elucidated. We present recent experiments where ultrafast photoinduced strain is evaluated in BiFeO3-based multiferroic materials, with a focus on the description of the ultrafast symmetry change of the unit-cell that appears after an ultrashort laser pulse. A combination of optical and X-ray time-resolved techniques will be presented. We show that it is possible to modulate at the picosecond time scale the ferroic order by playing with the out-of-plane and in-plane light-induced strains. These new results provide new insights for the understanding of the physics of photo-induced strain, in relation with the light-induced ferroelectric modulation in nanostructured ferroic compounds and could be the first step towards their use as on-purpose ferroic architectures in devices like actuators or modulators with ultra-short light pulses.
Over the last decade, the development of high-power ultrafast laser systems led to the emergence of intense pioseconds terahertz (THz) pulses, which provide a new tool for studying fundamental aspects of light-matter interactions by driving out-of-equilibrium electrons, phonons or magnons at ultrafast time scale. Thanks to spectral weight in the THz frequency range, it is possible to directly couple light to infrared-active optical phonon mode in solid and it has been widely demonstrated and studied in various materials. However, only sparse and incomplete reports are available on THz-induced coherent acoustics phonons and none of them clearly demonstrate the origin of coherent acoustics phonons generation. Here, we report on the generation of coherent acoustic phonons in materials with terahertz ultrashort pulses. This is demonstrated in metals and topological insulators by exciting acoustic eigenmode in nanometric sized thin films.
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