Terahertz ionic Kerr effect: Two-phonon contribution to the nonlinear optical response in insulators
Martina Basini, Mattia Udina, Matteo Pancaldi, Vivek Unikandanunni, Stefano Bonetti, Lara Benfatto
Abstract
The THz Kerr effect measures the birefringence induced in an otherwise isotropic material by a strong THz pulse driving the Raman-active excitations of the systems. Here we provide experimental evidence of a sizable Kerr response in insulating ${\mathrm{SrTiO}}_{3}$ due to infrared-active lattice vibrations. Such a signal, named the ionic Kerr effect, is associated with the simultaneous excitation of multiple phonons. Thanks to a theoretical modeling of the time, polarization, and temperature dependence of the birefrengence, we can disentangle the ionic Kerr effect from the off-resonant electronic excitations, providing an alternative tunable mechanism to modulate the refractive index on ultrashort timescales via infrared active phonons.