Efficient tailored nonlinear optical responses by nanoassemblies: Focus on spindle β-FeOOH nanorods

Assemblies of beta-iron oxyhydroxide (beta-FeOOH) nanostructures were synthesized by a facile, mild, and non-toxic ultrasonication method. Quasi-rectangular, chrysanthemum-like, and silk-like architectures, respectively, were obtained by introducing a urea surfactant, synthesized Fe3O4 template, and natural kapok seed template. This versatile and efficient synthetic method can be extended to the fabrication of a series of novel, multifunctional assembled architectures. Hierarchical structures with identical morphologies were constructed from porous beta-FeOOH nanorods with high surface areas. The effects of the assembly mode on the nonlinear optical (NLO) properties of the beta-FeOOH nanorod assemblies were explored using the open-aperture Z-scan technique. Individual beta-FeOOH nanorods and their analogous nanoassemblies exhibited excellent NLO performances upon excitation at 532 and 1064 nm in the nanosecond laser pulse regime, both in liquid and sol-gel solid-state matrices. These nanomaterials are promising candidates for broadband optical limiters; however, their abilities to attenuate intense and potentially dangerous laser beams differ. Our results confirm that nanoassembly formation can be used to effectively tailor the NLO activities of beta-FeOOH nanorods, and the possible mechanisms for these behaviors are discussed. The observed NLO effects originate from nonlinear scattering and free-carrier absorption at 532 and 1064 nm, respectively. This study provides new insights into the assembly of one-dimensional building blocks, which will be useful in the rational design and preparation of hierarchical materials and enable tailoring and improvement of the NLO properties of materials.