Third-Order Optical Nonlinearities in Organometallic Methylammonium Lead Iodide Perovskite Thin Films

With solar conversion efficiencies surpassing 20%, organometallic perovskites show tremendous promise for solar cell technology. Their high brightness has also led to demonstrations of lasing and power-efficient electroluminescence. Here we show that thin films of methylammonium lead iodide, prepared by solution processing at temperatures not exceeding 100°C, exhibit a highly nonlinear intensity-dependent refractive index due to changes in the free-carrier concentration and for femtosecond excitation at higher intensities undergo saturation that can be attributed to the Pauli blocking effect. Nonlinear refractive index and nonlinear absorption coefficients were obtained by the Z-scan technique, performed simultaneously in open- A nd closed-aperture configurations. Both nanosecond- A nd femtosecond-pulsed lasers at multiple wavelengths were used in order to distinguish between the mechanisms inducing the nonlinearities. The magnitude and sign of the nonlinear refractive index n₂ were determined. For resonant excitation, free carrier generation is the dominant contribution to the nonlinear refractive index, with a large nonlinear refractive index of n₂ = 69 × 10^-12 cm²/W being observed for resonant femtosecond pumping and n₂ = 34.4 × 10^-9 cm²/W for resonant nanosecond pumping. For nonresonant femtosecond excitation, bound-charge-induced nonlinearity leads to n₂ = 36 × 10^-12 cm²/W. These values are equivalent to the best reported metrics for conventional semiconductors, suggesting that organometallic perovskites are promising materials for optical switching and bistability applications.