Simultaneous Achievement of Enhanced Nonlinear Optical Absorption and Nonlinear Refraction in Highly Crystalline 2D Covalent Organic Frameworks Ultrathin Films

Abstract Large enhancement of nonlinear absorption and nonlinear refraction are achieved simultaneously in highly ordered two dimensional (2D) covalent organic framework (COF) films prepared by solidliquid interface one‐step method to overcome the weakness of COF powders in solubility. In the intrinsic nonlinear optical response obtained at 532 nm with 5 ns pulse, the nonlinear absorption coefficients (β) of two COF films are −4.87 × 10⁻⁵ and −1.29 × 10⁻⁵ m W⁻¹, respectively. Simultaneously, the fitted closed‐aperture curves also show large nonlinear refractive indexes (n₂), −5.62 × 10⁻¹² m² W⁻¹ and −0.76 × 10⁻¹² m² W⁻¹. The 4f coherent imaging performed at the same condition with a single‐shot pulse further verifies the outstanding nonlinear optical response without any damage probably experienced in the Z‐scan technique. Moreover, the differences in framework electronic structure and photoexcited states between two COF films are compared to explain the difference in nonlinear optical response. All the results indicate that two COF crystalline films with intrinsic giant nonlinear optical response can be capable of modulating both amplitude and phase of light, providing huge potential in all‐optical manipulating and switching at the nanoscale as outstanding nonlinear optical materials.