Terahertz electro-optic Kerr effect in LaAlO₃

In this Letter, we investigate the terahertz (THz) electro-optic Kerr effect (KE) dynamics in LaAlO₃ (LAO), a widely used substrate for thin-film preparation. We show that the KE dynamics strongly depend on the material anisotropy due to interference between THz field-induced and strain-induced optical birefringence. Such interference leads to quasi-phase matching conditions of the KE, which becomes strongly frequency dependent. Depending on the THz frequency, the KE exhibits a uni- and bipolar shape of the quadratic response. The demonstrated effects will be present in a wide variety of materials used as substrates in different THz pump laser-probe experiments and need to be considered in order to disentangle the different contributions to the measured ultrafast dynamic signals.