Origin of Terahertz Soft-Mode Nonlinearities in Ferroelectric Perovskites

Soft modes are intimately linked to structural instabilities and key for the understanding of phase transitions. The soft in ferroelectrics, example, map directly polar order parameter a crystal lattice. Driving these into nonlinear, frequency-changing regime with intense terahertz (THz) light fields is an efficient way alter lattice and, it, physical properties. However, recent studies show that THz electric-field amplitudes triggering nonlinear soft-mode response surprisingly low, which raises question on microscopic picture behind origin this response. Here, we use linear two-dimensional (2D THz) spectroscopy unravel nonlinearities strain-engineered epitaxial ferroelectric SrTiO3 thin film. We find dielectric function mode quantitatively incompatible pure ionic or electronic motions. Instead, 2D reveals pronounced coupling ionic-displacement dipoles. Hence, hybrid (ionic) motions interband confirm conclusion model calculations based simplified pseudopotential concept band structure. It entire nonlinearity caused by off-resonant transitions lattice-electronic mode. With work, provide fundamental insights processes govern softness any material assumes near ferroic transition. This knowledge will allow us gain all-optical control over associated large effects.Received 14 September 2020Revised 20 January 2021Accepted 12 March 2021DOI:https://doi.org/10.1103/PhysRevX.11.021023Published American Physical Society under terms Creative Commons Attribution 4.0 International license. Further distribution work must maintain attribution author(s) published article’s title, journal citation, DOI.Published SocietyPhysics Subject Headings (PhySH)Research AreasDielectric propertiesElectric polarizationElectronic excitation & ionizationElectronic transitionsFerroelectricityLight-matter interactionNonlinear opticsPhononsStrong electromagnetic field effectsUltrafast phenomenaTechniquesTerahertz time-domain spectroscopyUltrafast pump-probe spectroscopyCondensed Matter, Materials Applied PhysicsNonlinear DynamicsAtomic, Molecular Optical