Enhancement of anisotropic ultrafast optical nonlinearity in Fe decorated SrTiO3 single crystal

Abstract

We report on the anisotropic ultrafast optical nonlinearity of Fe-doped SrTiO₃ composites (FSTO) using the femtosecond (532 nm, 100 fs) polarization-resolved Z-scan method. The SrTiO₃ (STO) and FSTO are first grown via the flame fusion method, and their fundamental optical constants, namely the linear optical absorption and optical band gap value, are then accessed through the optical transmittance measurement. Furthermore, the Z-scan results show that the FSTO achieves an enhanced reverse saturation absorption and a significant nonlinear refraction signal compared to the pristine one. More importantly, the ΔT and ΔT_P−V of FSTO as a function of the polarization angle θ exhibit oscillation curves with a period of π/2, which means that the realization of highly polarized optical nonlinearities. In particular, with a further reduction in the repetition frequency of the laser, both the nonlinear absorption and refraction remain unchanged, indicating that the thermal effect is negligible. The maximum nonlinear absorption coefficient β and nonlinear refractive index n₂ of FSTO are determined to be 4.5 × 10⁻¹¹ m/W and 6.4 × 10⁻¹⁸ m²/W, respectively, which is a boost of around 7 times compared to that of STO (6.1 × 10⁻¹² m/W / NA). Such enhanced and anisotropic optical nonlinearities are due to the fact that Fe-related defect states give rise to excited state absorption and the crystal orientation is optimized by the Fe element substrate. In addition, the FSTO with enhanced optical nonlinearity has also been demonstrated to exhibit excellent optical limiting performance. The findings presented here indicate that the FSTO can be regarded as a potential candidate for versatile nonlinear optical and photonic devices.