Terahertz Kerr effect in a methylammonium lead bromide perovskite crystal

We have observed short-lived optical birefringence in a ${\mathrm{CH}}₃{\mathrm{NH}}₃{\mathrm{PbBr}}₃$ single crystal induced by a powerful nearly single-cycle terahertz pulse. Apart from the instantaneous contribution that follows the profile of the squared pump electric field, the recorded anisotropy signal contains an exponential component decaying in $\ensuremath{\sim}350\phantom{\rule{0.28em}{0ex}}\mathrm{fs}$, underdamped oscillations at the frequency of $\ensuremath{\sim}0.16\phantom{\rule{0.28em}{0ex}}\mathrm{THz}$ and an intermediate picosecond relaxation process with a Gaussian tail. We associate these three nontrivial features with, respectively, Kerr effect in the inorganic lattice, terahertz-induced transient alignment of ${\mathrm{CH}}₃{\mathrm{NH}}₃⁺$ cations, and their coherent rotation excited by the terahertz pulse in a Raman process.