Mareev Evgenii, Potemkin Fedor
Faculty of Physics, M. V. Lomonosov Moscow State University, Moscow, Russia, 119991.
Institute of Photon Technologies of Federal Scientific Research Centre "Crystallography and Photonics" of Russian Academy of Sciences, Troitsk, Russia, 108840.
Sci Rep. 2022 Apr 22;12(1):6621. doi: 10.1038/s41598-022-09815-4.
The advent of free-electron lasers opens new routes for experimental high-pressure physics, which allows studying dynamics of condensed matter with femtosecond resolution. A rapid compression, that can be caused by laser-induced shock impact, leads to the cascade of high-pressure phase transitions. Despite many decades of study, a complete understanding of the lattice response to such a compression remains elusive. Moreover, in the dynamical case (in contrast to quasi-static loading) the thresholds of phase transitions can change significantly. Using the third harmonic pump-probe technique combined with molecular dynamics to simulate the terahertz (THz) spectrum, we revealed the dynamics of ultrafast laser-induced phase transitions in MgF in all-optical experiment. Tight focusing of femtosecond laser pulse into the transparent medium leads to the generation of sub-TPa shock waves and THz coherent phonons. The laser-induced shock wave propagation drastically displaces atoms in the lattice, which leads to phase transitions. We registered a cascade of ultrafast laser-induced phase transitions (P42/mnm ⇒ Pa-3 ⇒ Pnam) in magnesium fluoride as a change in the spectrum of coherent phonons. The phase transition has the characteristic time of 5-10 ps, and the lifetime of each phase is on the order of 40-60 ps. In addition, phonon density of states, simulated by molecular dynamics, together with third-harmonic time-resolved spectra prove that laser-excited phonons in a bulk of dielectrics are generated by displacive excitation (DECP) mechanism in plasma mediated conditions.
自由电子激光器的出现为实验高压物理学开辟了新途径，它能以飞秒分辨率研究凝聚态物质的动力学。由激光诱导的冲击产生的快速压缩会引发一系列高压相变。尽管经过了数十年的研究，但对晶格对这种压缩的响应仍未完全理解。此外，在动态情况下（与准静态加载相反），相变阈值可能会发生显著变化。我们使用三次谐波泵浦 - 探测技术结合分子动力学来模拟太赫兹（THz）光谱，在全光学实验中揭示了超快激光诱导的MgF中的相变动力学。飞秒激光脉冲紧密聚焦到透明介质中会导致产生亚太帕斯卡冲击波和太赫兹相干声子。激光诱导的冲击波传播会极大地使晶格中的原子发生位移，从而导致相变。我们记录到了氟化镁中一系列超快激光诱导的相变（P42/mnm ⇒ Pa - 3 ⇒ Pnam），表现为相干声子光谱的变化。相变的特征时间为5 - 10皮秒，每个相的寿命约为40 - 60皮秒。此外，通过分子动力学模拟的声子态密度，与三次谐波时间分辨光谱一起证明，在等离子体介导条件下，体介质中的激光激发声子是由位移激发（DECP）机制产生的。
