Exploring Fast and Ultrafast Dynamics of Matter with Magnons and Phonons

We have an ever-expanding repertoire of methods that enable control over elementary excitations, in some cases launching excursions into far-from-equilibrium regimes that may lead to phase transitions or other collective transformations. We also have a widening range of probes that allow incisive observation of the excursions along degrees of freedom that are excited directly or are coupled to the driven modes. Here we will review recent experiments involving coherent excitation of magnons and phonons and observations of the material behavior near and far from equilibrium.<br/><br/>In canted antiferromagnetic materials such as YFeO₃, we are able to drive nonlinear responses of magnon modes. They are measured using 2-dimensional THz spectroscopy in which two THz excitation pulses separated by variable delay times drive the system and time-dependent coherent THz signal fields are read out by variably delayed optical pulses that overlap the THz fields in an electro-optic crystal (<i>Nat. Phys</i>. 2024 DOI:10.1038/s41567-024-02386-3; DOI:10.1038/s41567-023-02350-7). The use of 500 readout pulses with variable delays on each laser shot reduces the data acquisition time for a single 2D spectrum from several days to minutes, enabling systematic study (including 2D THz polarimetry in which THz polarizations relative to crystallographic axes are incremented in small steps) that would otherwise be impossible. The use of a recently developed method for THz field enhancement throughout a small macroscopic volume (<i>Sci Rep</i> <b>13</b>, 15228 (2023)) enables generation of far higher-order magnonic responses up to 9-quantum coherences, well outside the perturbative limit. The results reveal magnon self-interaction at large excursions and suggest approaches for tailoring THz magnetic fields in order to drive magnetization reorientation transitions or domain reversal.<br/><br/>THz driving of “soft” optical phonons associated with the suppressed ferroelectric phase transition in the quantum paraelectric phase of SrTiO₃ were reported to have induced a transient ferroelectric configuration in the crystal (<i>Science</i> <b>364</b>, 1079 (2019)). It was assumed based on tabletop optical measurements that the induced changes were essentially inform in the irradiated sample. However, diffuse x-ray scattering measurements revealed singular responses at off-Bragg wavevectors corresponding to polar nanoregions of ~ 10-nm dimensions, suggesting that the quantum paraelectric material is near an instability with respect to a phase with spatially modulated ferroelectricity, stabilized by strong gradients in the strain and the polarization (i.e. flexoelectric effect), as well as a phase with spatially uniform ferroelectricity (<i>arXiv</i>:2403.17203 (2024)).<br/><br/>Recent advances in direct generation of high-wavevector coherent magnons and phonons through excitation with crossed beams (transient grating excitation; <i>Photoacoustics</i> <b>29</b>, 100453 (2023)) at extreme UV or hard x-ray wavelengths will be discussed. In very recent experiments, hard x-ray transient grating excitation with an interference period of 12 nm generated coherent acoustic phonons at that wavelength. The approach may enable transient grating measurements with periods as short as 1 nm or less.<br/><br/>Finally, a key degree of freedom for coherent control of material behavior is strain. We have recently used focusing nonlinear surface acoustic waves (surface acoustic shocks) to induce the insulator-to-metal phase transition in V₂O₃. The transition is irreversible, driven and observed on a single-shot basis. More generally, we have developed methods for nondestructive optical excitation of focusing and non-focusing shocks that reach pressures on the order of 10 GPa (<i>Phys. Rev. Appl</i>. <b>20</b>, 044044 (2023)). The approach enables high repetition rate pump-probe measurements with the shock playing the role of the pump, either by itself or supplemented by additional excitation using THz or optical pulses for multimodal collective coherent control.