Long-lived THz-induced birefringent state in quantum paraelectric KTaO₃

Ultrafast optical control in perovskites and their relatives has been of great interest in studying laser-induced phase transitions. We studied the response of quantum paraelectric KTaO₃ to intense THz pulses using the complimentary measurements of THz field-induced second harmonic generation (TFISH) and THz Kerr effect (TKE) and with effective Hamiltonian simulations. We observed the coherent excitation of the soft phonon mode superimposed on an exponential decay. The most astonishing result in the experiments is the dramatically different relaxation times for TFISH and TKE responses. The relaxation times for TKE is more than 20 times larger than TFISH from close to room temperature down to 8 K. Such difference in relaxation times implies that the two measurements interrogate entirely different excited states of the crystal. TFISH responds to the presence of broken inversion symmetry due to dipolar polarization induced by the THz pump. TKE results from the THz-induced birefringence. The long relaxation time of the TKE response has a two-fold origin. First, TKE scales as the square of THz electric field and is not accompanied by an electric dipole, which reduces its coupling to the soft optical phonon mode. Second, the proximity of KTO to the ferroelectric phase transition makes the material more soft and susceptible to long-lived deformations along the soft mode coordinate. The long-lived birefringent state can be classified as a hidden THz-induced phase in the material.