Effect of optical pump on the dielectric properties of SrTiO₃ in terahertz range

Tuning the dielectric permittivity spectra of strontium titanate (SrTiO₃) single crystals in an external optical field is investigated at room temperature by means of terahertz time-domain spectroscopy. The application of the optical field leads to an appreciable tuning of the permittivity, reaching up to 2.8%, with the dielectric loss changing about 3%. The observed behavior is interpreted in terms of soft-mode hardening due to the anharmonic character of its potential. We also find that the change of the refractive index responds linearly to the applied light power. These findings are attributed to a linear electro-optical effect of the internal space charge field of the crystal.