Temperature dependent linear and nonlinear optical properties of SrTiO₃ from first-principles

Strontium titanate (SrTiO₃) is one of the most studied perovskite type materials for its promising electronic and optical features. Due to the symmetry dependent nature, second harmonic generation (SHG) can be used as nondestructive way to identify structural transitions. SHG exists in the absence of centrosymmetry so it is forbidden for bulk cubic SrTiO₃. Factors breaking the inversion symmetry such as impurities, defects, strain, and temperature can give rise to SHG in centrosymmetric materials.

Here we investigated the temperature dependent linear and nonlinear optical properties of bulk SrTiO₃ through dielectric function and second order optical susceptibility using density functional theory. Dielectric function calculated with lattice temperatures 0K, 100K, 200K, and 300K did not show a significant temperature dependence, with the first prominent peak located between photon energy 4 - 5eV. On the other hand, the second order optical susceptibility vanishes at 0K, but finite values arise at temperatures as low as 10K. This signal increases at higher temperature of 100K, with the first prominent peak emerging between 1 - 2.5eV.

Our study shows that lattice temperature has a huge impact on SHG for SrTiO₃ compared to its rather negligible effect on linear optics. Further study including a larger span of temperatures would be useful to analyze the entangled factors on SHG signal detected in experiments.