Terahertz ionic Kerr effect: Two-phonon contribution to the nonlinear optical response in insulators

TL;DR

This paper demonstrates a significant ionic Kerr effect in insulators like SrTiO3, caused by two-phonon excitations, offering a new way to modulate refractive index ultrafast via infrared-active phonons.

Contribution

It provides experimental evidence and theoretical modeling of the ionic Kerr effect involving two-phonon processes in insulators, distinct from electronic excitations.

Findings

Sizable Kerr response observed in SrTiO3 due to lattice vibrations

Disentanglement of ionic Kerr effect from electronic contributions

Potential for ultrafast refractive index modulation using phonons

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 SrTiO3 due to infrared-active lattice vibrations. Such a signal, named 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 birefringence 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 time-scales via infra-red active phonons.