Terahertz-Induced Second-Harmonic Generation in Quantum Paraelectrics: Hot-Phonon Effect

Recent THz-pump second-harmonic-generation (SHG) probe measurements of quantum paraelectrics observed a significant long-lived nonoscillatory SHG component following an ultrafast resonant excitation of the soft mode, which was interpreted as a signature of THz-induced transient ferroelectric order. We propose that the THz-induced modulation of the SHG signal can be attributed solely to the dynamic variation of the dielectric environment associated with the lattice background, which reflects the coherent response of soft mode under THz pumping. We develop a temperature-dependent dynamic model incorporating the hot-phonon effect to simulate the soft-mode behaviors under ultrafast THz excitation. Its application to paraelectric KTaO3 produces quantitatively most of the features exhibited in our time-resolved SHG measurements and those in existing literature, including a long-lived nonoscillatory SHG response, SHG oscillations at twice the soft-mode frequency, SHG dampings, and temperature and field-strength dependencies. We conclude that the observed THz-induced nonoscillatory SHG response in quantum paraelectrics is a consequence of the nonequilibrium hot-phonon effect, offering an alternative to its existing interpretation as a signature of transient ferroelectric order.