Lattice temperature induced second harmonic generation in SrTiO₃ from first-principles

Second Harmonic Generation (SHG), which is the simplest case of nonlinear optics has been widely studied due to its applicational advantages with non-destructive nature and higher sensitivity than linear optics. SHG is symmetry-forbidden for centrosymmetric materials but experimental observations have been made.

Here we study SrTiO₃ (STO), which has high potential for optoelectronics with its rich physical properties such as high dielectric constant and structural transitions, to understand this unexpected SHG. Structures with lattice temperatures generate nonvanishing second order optical susceptibility. SHG magnitude increased at higher temperature, with the prominent peaks of χ⁽²⁾_xyz at 100K and 300K ranging 0.26 - 1.12 (10^-7 esu)/0.11 - 0.42 (10^-7 esu) before and after scissor shift.

We compared our result with SHG in several non-centrosymmetric materials, which proved that temperature induced SHG in STO is comparable to the materials exhibiting intrinsically small SHG, such as α-quartz SiO₂, having prominent peak 0.66 (10^-7 esu) within 8 eV photon energy range.

We expect our findings to expand the understanding of SHG in centrosymmetric materials, and combining further symmetry breaking factors into the study would pave a way to analyze and control SHG to discover new centrosymmetric materials that enables wider materials selection to reach full potential for SHG applications.