New Ultrafast Optical and Structural Probes in Material Science

New methods for optical control and observation of electronic, spin, and lattice degrees of freedom have enabled previously inaccessible study of light-induced phase transitions and other transformations. Several such methods will be discussed.<br/>Strong terahertz-frequency light fields have recently been used to drive far-from-equilibrium excursions and material transformations. Detailed understanding of the underlying mode-mode interactions involved can be obtained through 2-dimensional THz spectroscopy. Off-diagonal peaks in 2D THz spectra, as in other 2D spectra, directly reveal mode-mode couplings that are invisible to linear spectroscopy. Until recently, a single 2D spectrum typically required several days of data acquisition time because two time variables, the (interpulse) time between two THz pulses and the temporal profile of the nonlinear THz signal field induced by the incident pulses, needed to be scanned point-by-point on the two time axes. Instead, the entire time-dependent THz signal profile can be measured in a single laser shot using several hundred optical “readout” pulses with different time delays that are all overlapped spatially with the THz signal field in an electro-optic crystal and then directed to different regions of a multi-pixel detector. In recent work, systematically incremented THz magnetic field polarizations relative to the crystallographic axes of canted antiferromagnets were used to record 144 2D spectra in less than 24 hours, introducing <i>2D THz polarimetry</i> which revealed a previously unknown but likely ubiquitous mechanism for coupling between different magnon modes in this material class (<i>arXiv</i>:2207.07103; <i>arXiv</i>:2301.12555). Similar methodology has been used for true single-shot THz and optical measurements of a photoinduced transition into a metastable phase of TaS₂ at low temperatures (<i>Sci. Adv. </i><b>8</b>, eabp9076).<br/>New advances and results in x-ray diffraction and spectroscopy at x-ray free-electron lasers (XFELs) will be discussed very briefly. A THz-induced transient ferroelectric (FE) structure in low-temperature SrTiO₃ has been studied using two inverted THz generation crystals to drive the crystal with single-cycle THz fields of inverted polarity. The resulting inverted x-ray scattering signals recorded at wavevectors near but not on Bragg diffraction peaks revealed that the THz-induced FE rearrangements occur predominantly in pre-existing polar nanoregions, a key mechanistic insight not accessible through earlier optical measurements (<i>Science</i> <b>364</b>, 1079). In separate measurements in the extreme UV (EUV) transient grating facility at the FERMI beamline (<i>Sci. Adv.</i> <b>5</b>, eaaw5805), crossed EUV pulses generated coherent magnons with 50-100-nm wavelengths whose time-dependent oscillations revealed magnon dispersion in this often inaccessible wavevector range. Coherent driving of high-wavevector phonons, magnons, and other excitations with spatial periods &lt;10 nm is anticipated.<br/>Finally, very recent development (<i>arXiv</i>:2209.13897) of a method for <i>laser generation of shock waves without damage to the optically irradiated region of the sample</i> permits significant structural control to be incorporated into ultrafast measurements conducted at high repetition rates. A shock wave can be generated nondestructively and propagated into a pristine sample region where THz, optical, and/or x-ray pulses are used for ultrafast measurements. The shock-induced strain duration is ~1 ns, so ultrafast measurements can be conducted with quasi-DC strains whose magnitudes (up to 3%, ~10 GPa pressure) can be specified. Multimodal control using strain plus THz and/or optical excitation pulses can be used to guide materials into new phases or other states of interest. Repeated shock delivery to the same sample region also permits detailed study of fatigue, where macroscopic material failure occurs only after many shocks. X-ray imaging may permit characterization of the nanoscale defects that accumulate prior to failure.