Ultafast Nonlinear Opical Properties Of Graphene Oxide And Its Hybrid Materials

Graphene is a planar layer of carbon atoms in a hexagonal lattice, with a linear energy spectrum near the intersection of the electron and hole cones in the band structure (the Dirac point). Graphite shows a variety of interesting structural and electronic properties which have recently captured a lot of attention both for their fundamental description. Owing to its unique electronic and optical attributes, graphene have great potential application in photonics and electric devices.However, graphene could not be dissolved in normal solvents. Thus, the application of graphene would be limited in optical region. A reliable way to dissolve the graphene is to oxide or hybrids covalently functionalized by other materials. In order to study the nonlinear optical properties and ultrafast carrier dynamics of these graphene based materials and understand the relationship between nonlinear optical properties and materials structure, we should study on the nonlinear optical properties and carrier dynamics in detail for these novel graphene based materials, produced in different ways. Based on these results, we could further master the method of modifying the nonlinear optical properties of graphene materials.In this thesis, we mainly study the nonlinear optical properties of graphene oxide and graphene hybrid materials by using time-resolved pump-probe and Z-scan measurements, and compare the nonlinear optical properties and carrier dynamics in these materials. Through these works, we could know the nonlinear optical properties better and understand the dynamics for these properties. The contents are shown following:1, We study nonlinear optical response and ultrafast carrier relaxation dynamics in single and few-layered graphene oxide (GO) by using transient differential transmission spectroscopy and Z-scan technique under various pump intensities. It is the first time of observing a heterogeneous nonlinear optical response contribution to the transient differential transmission in GO. Results show that charge carriers with subpicosecond-to-picosecond dynamics from sp2-hybridized domains dominate the ultrafast response at low pump intensities, like graphene. And, the influence of two-photon absorption from sp3-hybridized domains on the transient absorption signal becomes increasingly strong with pump intensities. The unique atomic and electronic structure of GO leads to the emergence of variable absorption processes under different input intensities, and the different absorption processes are distributed in the sp2cluster and the sp3domains of GO. On the basis of heterogeneous ultrafast dynamics of GO with saturable absorption in sp2domains and two-photon absorption in sp3domains, the nonlinear optical response can be tailored by manipulation of the degree and location of oxidation on GO sheets; this unravels the important role of sp3domains in graphene optics and will facilitate the potential applications of GO in optoelectronics.2, The ultrafast carrier dynamics and nonlinear optical properties in graphene oxide and reduced graphene oxides are studied by using time-resolved pump-probe and Z-scan techniques with femtosecond laser pulses at800nm. Partially reduced graphene oxides are produced by expositing graphene oxide to the hydrazine vapor with different time. We measured the extent of oxidation and contents of different functional groups in materials by X-ray photoelectron spectroscopy. Results of pump-probe experiments show that the decay time of carrier relaxation of graphene oxide become shorter after being reduced. And, the amount of functional group in reduced graphene oxide also affects the ultrafast dynamics process. For nonlinear optical properties, Z-scan measurements show that the saturable absorption property of graphene oxide was enhanced after being reduced. However, after reduction for a certain time, the enhancement of saturable absorption will not continue visibly. It is greatly related to the extent of reduction. The absorption properties of graphene oxide could not be further modified by excess reduction.3, Ultrafast carrier dynamics and saturable absorption of solution-processable few-layered graphene oxide are studied using femtosecond pump-probe and Z-scan techniques at800nm. The results show that few-layered graphene oxide has a fast energy relaxation of hot carriers and strong saturable absorption, which is comparable with that of reduced graphene oxide. Fast carrier relaxation arises from the large fraction of sp2carbon atom inside the few-layered graphene oxide sheet together with oxidation mainly existing at the edge areas. This superiority of few-layered graphene oxide will facilitate potential applications of graphene for ultrafast photonics. So, few-layered graphene oxide is a good candidate for uses in applications of ultrafast photonics and optical switching.4, Nonlinear optical and optical limiting properties of three graphene materials (pure graphene, graphene oxide, reduced graphene oxide) covalently functionalized by zinc phthalocyanine were studied in nanosecond and femtosecond regimes. In nanosecond regime, reduced graphene oxide-phthalocyanine hybrid shows better optical limiting effect than the other two hybrids. In femtosecond regime, graphene oxide-phthalocyanine shows the best nonlinear optical properties among the three hybrid materials. Based on Z-scan and transient absorption measurements, the mechanisms for nonlinear optical property were analyzed for these materials. Among the three hybrid materials, RGO-ZnPc shows the best optical limiting effect in ns regime. Together with its larger nonlinear attenuation coefficient (i.e.,βeff) and higher linear transmittance than the other two materials. Compared with the other two hybrids, reduced graphene oxide-phthalocyanine hybrid is a better candidate in the application of optical limiter.