The Third-order Nonlinear Optical Properties Of Cu₂Se,FAPbBr₃ And Their Composites With Graphene

Since the emergence of laser technology,nonlinear optics has developed rapidly.As an important branch of nonlinear optics,the study of third-order nonlinear optics has attracted more and more attention.Therefore,it is crucial to find third-order nonlinear optics materials with superior properties for the applications in photonics such as optical communication,optical signal processing and optical storage.The two-dimensional layered materials shined in the fields of nonlinear photonics owing to their unique physical and electronic structure.Graphene,a typical two-dimensional material,has been applied in numerous fields.Due to the high carrier mobility and broadband absorption properties of graphene,researchers are committed to the application of graphene in nonlinear optical devices.However,base graphene shows weak nonlinear absorption properties owing to its zero-bandgap structure,which is not suitable for practical applications.In this paper,to solve the problem,semiconductor materials were grown directly on the surface of graphene oxide by a defect-mediated crystal growth process to construct different graphene-semiconductor composite structures,and the third-order nonlinear optical properties of graphene were improved by the charge transfer effect in the composite structures.The internal mechanism of composite structure and charge transfer process on the nonlinear optical properties were investigated by the picosecond Z-scanning technique and pump detection technique.The specific research contents and achievements of this paper are as follows:Firstly,the factors affecting the third-order nonlinear optical properties of Cu₂Se/G composites were systematically studied.Cu₂Se nanocrystals were grown at the oxygen-containing defect sites on the graphene oxide surface,and the Cu₂Se/G composites with different Cu₂Se concentrations were successfully prepared via a solvothermal method.The characterization of structure and morphology indicated that Cu₂Se nanocrystals were uniformly dispersed on the surface of graphene nanosheets.The third-order nonlinear optical properties of the composites with different Cu₂Se contents were investigated via the picosecond Z-scan technique at 532 nm.The results showed that the Cu₂Se/G composites had the properties of reverse saturated absorption and positive nonlinear refraction,and the nonlinear optical properties of Cu₂Se/G composites could be effectively regulated by controlling the contents of Cu₂Se.The third-order nonlinear susceptibility of the Cu₂Se/G composite with a 1.5:1 mass ratio of Cu₂Se to graphene oxide reach 6.52×10^-13 esu,which increased five-fold times compared to graphene.The enhanced nonlinear optical properties are related to the increase of photoinduced transition dipole moment induced by the decrease of Cu₂Se size and the charge transfer process between the two components of the composites.The transient absorption spectrum and carrier dynamics analysis of Cu₂Se/G were obtained by pump-probe technique,and the charge transfer model of Cu₂Se/G composite was established to analyze the promotion of reverse saturated absorption by the interfacial charge transfer process between graphene and Cu₂Se.This study lays a foundation for the application of Cu₂Se/G composites in nonlinear optical devices.Secondly,in this study,G/FAPbBr₃ composites were synthesized by growing FAPb Br₃ nanocrystals directly from a graphene oxide lattice to improve the nonlinear optical properties of graphene.The analysis of structure and morphology showed that FAPb Br₃ crystals were successfully grown on the surface of graphene with covalent bond.The blue shift of the absorption peak and fluorescence peak of G/FAPb Br₃and the decrease of fluorescence lifetime indicated the presence of an ultrafast charge transfer process in the composites.Picosecond Z-scan results showed that the saturated absorption effect of G/FAPb B_r3composites was significantly stronger than that of monomers.The reasons for the saturated absorption enhancement of G/FAPb Br₃composites were studied by the transient absorption spectra and the carrier dynamics analysis.The ultrafast charge transfer process suppressed the excited state absorption,promoted the coupling between different states and thus enhanced the saturated absorption effect.Therefore,G/FAPb Br₃ composites exhibit excellent saturable absorption properties with large modulation depth（10.8%）and extremely low saturation intensity（0.1GW/cm²）,which demonstrate the potential applications as saturable absorbers in optoelectronic devices.Thirdly,the effect of size on the third-order nonlinear optical properties of G/FAPb Br₃ composites was systematically investigated.By increasing the amount of Oleylamine ligand,G/FAPb Br₃ samples with gradually decreasing size were synthesized and the fundamental characterization were carried out.The decrease in the size of the composite resulted in the blue shift of the absorption peaks and the increase of optical band gap.The charge transfer process of G/FAPb Br₃ enhanced with the decrease of the size,which also lead to the decrease of the fluorescence lifetime.The Z-scan results showed that the saturated absorption properties of G/FAPb Br₃ were improved with the decrease of the size.In addition,the nonlinear optical properties of the composites under high laser energies were also investigated in this study.With the increase of laser energy,there was a transition process from saturated absorption to two-photon absorption in G/FAPb Br₃,and the G/FAPb Br₃ composite samples with larger band gap exhibited stronger two-photon absorption properties.The regulation of size on the third-order nonlinear optical properties of composites provides a variety of ideas for the application of G/FAPbBr₃ in nonlinear optical devices.