Hybrid x-ray laser-plasma/laser-synchrotron facility for pump-probe studies of the extreme state of matter at NRC "Kurchatov Institute"

Dynamics of ultrafast phase transitions in MgF2 triggered by laser-induced THz coherent phonons

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 MgF₂ 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.

Dynamics of Ultrafast Phase Transitions in (001) Si on the Shock-Wave Front

We demonstrate an ultrafast (<0.1 ps) reversible phase transition in silicon (Si) under ultrafast pressure loading using molecular dynamics. Si changes its structure from cubic diamond to β-Sn on the shock-wave front. The phase transition occurs when the shock-wave pressure exceeds 11 GPa. Atomic volume, centrosymmetry, and the X-ray-diffraction spectrum were revealed as effective indicators of phase-transition dynamics. The latter, being registered in actual experimental conditions, constitutes a breakthrough in the path towards simple X-ray optical cross-correlation and pump-probe experiments.

High-Power Solid-State Near- and Mid-IR Ultrafast Laser Sources for Strong-Field Science

This review highlights the development of ultrafast sources in the near- and middle-IR range, developed in the laboratory of Nonlinear Optics and Superstrong Laser Fields at Lomonosov Moscow State University. The design of laser systems is based on a powerful ultrafast Cr:Forsterite system as a front-end and the subsequent nonlinear conversion of radiation into the mid-IR, THz, and UV spectral range. Various schemes of optical parametric amplifiers based on oxide and non-oxide crystals pumped with Cr:Forsterite laser can receive pulses in the range of 4–6 µm with gigawatt peak power. Alternative sources of mid-IR ultrashort laser pulses at a relatively high (MHz) repetition rate are also proposed as difference frequency generators and as a femtosecond mode-locked oscillator based on an Fe:ZnSe crystal. Iron ion-doped chalcogenides (Fe:ZnSe and Fe:CdSe) are shown to be effective gain media for broadband high-peak power mid-IR pulses in this spectral range. The developed sources pave the way for advanced research in strong-field science.