Terahertz-induced polar state in quantum paraelectric SrTiO₃

The second harmonic generation from strontium titanate crystal under the terahertz excitation has studied at room temperature. The obtained anisotropic dependences show the presence of a terahertz electric field induced polar phase. By analyzing the polarization dependencies of the optical second harmonic intensity, the relative values of the third-order nonlinear susceptibility tensor components were obtained.