The Synthesis And Nonlinear Optical Properties Of Graphene-like Nanomaterials

Since the advent of laser,the studies of the third-order nonlinear optical(NLO)materials and related applications have attracted researchers’ s attention.With the development of powerful-pulsed lasers with tunable wavelength and short pulse width,the third-order NLO materials are put forward: fast response,wide bandwidth,high modulation depth,high damage threshold and low cost,since the traditional NLO materials have been unable to meet the demand.Due to the unique optoelectronic properties,the third-order NLO responses of graphene have been widely studies in recent years.However,its zero band gap structure and low modulation depth limit its practical applications in powerful-pulsed lasers.Hence,the graphene-like nanomaterials in which layered structure and band gap exist have been prepared,such as transition metal sulfides(TMDs)and topological insulators(TIs).Recently,these materials have been proven to own excellent NLO properties.However,the investigation on the NLO responses of this kind of materials and related utilizations are just beginning,lacking of systematic and deep research,the dependence of the performance and the improvement of the NLO properties are still to be studied.In addition,most of the graphene-like nanomaterials based on the resreaches of NLO responses were synthesised by liquid-phase exliation,there are many disadvantages,which is not conducive to the performance exploration and the large-scale device fabrication.Based on the above key factors,we carried out the following works:Ⅰ.Study on the preparation and nonlinear saturable absorption properties of TMDs and related applicationsWe successfully synthesized two-dimensional(2D)molybdenum disulfide(MoS2)nanosheets by the hydrothermal method.Significant NLO responses in the broad nearinfrared range(800-1550 nm)were observed in the obtained MoS2 nanosheets,identified by the Z-scan experiment under femtosecond pulses at different wavelengths.The nonlinear saturable absorption mechanism of the MoS2 nanosheets has been explained detailedly when it is irradiated by light with photon energy smaller than the gap energy.The sample has high modulation depth and large saturation absorption intensity.Using the obtained MoS2 nanosheets as a saturable absorber,a stable passively Q-switched fiber laser pulse was achieved.Compared to recently reported passively Q-switched fiber lasers utilizing exfoliated MoS2 nanosheets,the efficiency of the laser for our passive Q-switching operation is larger and reaches to 3.50 %.The experimental results show that the 2D MoS2 nanosheets prepared by hydrothermal method not only do not have the disadvantages of the nanosheets synthesized by liquid-phase exfoliation,but also can be a saturable absorber to realize tunable,short pulse and powerful-pulsed lasers.In order to further enhance the nonlinear saturable absorption of 2D MoS2 nanosheets and realize excellent ultrafast pulse generation,we used the method of introducing dopant,which can not only distort and break the structure,inducing more defects and edge-states,but also improve the electrical conductivity of materials,thus leading to rapidly absorb the incident photons,improving the NLO property.Using the doped-material as a saturable absorber,we realized a stable passively Q-switched fiber laser pulse,which is superior to that of the pristine MoS2 nanosheets-based passively Q-switching operation.The results show that we provide a feasible method for improving the NLO property and obtaining excellent ultrafast pulse emission of MoS2,which can be expanded to the other 2D nanomaterials.At present,based on the studies of nonlinear saturable absorption of TMDs and related applications amost concentrate on the 2D nanosheets,it is not known whether the TMDs of other dimensions have excellent NLO properties.Firstly,we successfully synthesized threedimensional(3D)MoS2 nanoflowers with high-yield and uniform size through hydrothermal method.Using the open Z-scan measurement,we observed strong NLO performances under femtosecond pulses at 800 nm.The fitting calculation shows that the obtaind material has high modulation depth and large saturation absorption intensity.Incorporating with polyvinyl alcohol(PVA)aqueous solution,we fabricated the flexible 3D MoS2 nanoflowers/PVA film.Stable Q-switched pulses were attained when the film was embedded between two fiber connectors.The output performance is superior to that of the above 2D MoS2 nanosheetsbased passively Q-switching operation.The efficiency of the laser reaches 4.71 %.Our experimental results suggest that the saturable absorption performance of TMDs does not critically depend on their structural dimensionality,indicating the great potential of non-2Dstructured TMDs in the development of passively Q-switched ultrafast lasers in the future.Ⅱ.Study on the preparation and nonlinear saturable absorption properties of TIs and related applicationsFirstly,we successfully fabricated 2D hexagonal Bi2Te3 nanosheets with high-yield,uniform size and thickness via a facile solvothermal method with the assistance of poly(vinyl pyrrolidone)(PVP).We illustrated the effects of PVP molecular weight and concentration on the morphology and size distribution of the products.Using the open Z-scan measurement,we confirmed that the as-prepared Bi2Te3 nanosheets show excellent nonlinear saturable absorption,having high modulation depth and large saturation absorption intensity.Using the Bi2Te3/(polymethyl methacrylate)PMMA film as a saturable absorber,we obtained a stable passively Q-switched fiber laser pulse with a relatively low threshold power and a quite output power by tuning the cavity parameters.The results show that the 2D Bi2Te3 sample prepared by the solvothermal method is also an excellent Q-switched device.At present,the study of NLO properties on TIs is focused on binary TIs,it is not clear whether the ternary TIs have excellent NLO responses.We fabricated the ternary Bi2SeTe2 nanosheets,and demonstrated that it shows excellent nonlinear saturable absorption,the modulation depth can reach to 61.9 %.The ultrafast intraband scattering processes in Bi2SeTe2 nanosheets were theoretical simulated.The experiment proves that the ternary Bi2SeTe2 nanosheets is a good candidate material for future photonic devices.Ⅲ.Study on the preparation and nonlinear saturable absorption properties of optical limiting materialsWith the output of the powerful femtosecond pulse laser,laser causes a serious threat for the precision instruments and human eyes.Hence,it is of great significance to develop the optical limiting materials with ultra-fast response,ultra-low optical limiting threshold,damage threshold and low cost.Such materials can effectively block high intensity light but allow low intensity pass.Firstly,we successfully synthesized high-yielded ultrathin 2D MoS2 and WS2 via a simple,cost-effective and reproducible solid-state reaction method.Using the Z-scan experiment under femtosecond pulses at 800 nm,we observed strong optical limiting effect of MoS2 dispersions.Combined with the above experiments,we confirmed that the NLO properties of MoS2 nanosheets depend on the thickness.The fitting calculation shows that MoS2 dispersions have high nonlinear figure of merit,large nonlinear refractive index and low optical limiting threshold.Incorporating with PMMA,we successfully fabricated the WS2/PMMA composite solid materials,which can be stored for a long time and is conducive to practical application.The nonlinear figure of merit and nonlinear refractive index of WS2/PMMA composite is larger than that of other NLO materials by one order of magnitude.The optical limiting threshold is 2.4 mJ/cm2,which is very low.The results show that the obtained ultrathin TMDs can be the optical limiting materials with large nonlinear refractive index,ultra-fast response and ultra-low optical limiting threshold,indicating that we provide a feasible method to prepare this kind of materials.In order to improve the optical limiting effect of ultrathin 2D MoS2 nanosheets,we prepared MoS2/TiO2 nanocomposite with vertical layered MoS2 nanosheets rooting into TiO2 nanofibers through a facile two-step method: prefabrication of porous TiO2 nanofibers based on an electrospinning technique,and assembly of MoS2 ultrathin nanosheets through a simple hydrothermal reaction.The unique structure promotes the two-photon absorptions of MoS2 and TiO2,and induces the excited-state absorption,then enhances the nonlinear reverse saturable absorption responses.Such material can effectively block the powerful femtosecond pulse laser.The results show that we provide a feasible method for improving the optical limiting effect of material.