Investigations On Optical Nonlinear Absorption And Carrier Dynamics Of Several 2D Materials

Compared with their bulk forms,materials with low-dimensional structure have specialphysical and chemical properties.Materials with three dimensions at nanometer size are usually named as zero-dimensional quantum dots.The material with only one dimension at nanometer size is called two-dimensional(2D)material,and its structure is characterized with single atomic layer or multi-atomic layers nanosheets.The atomic interaction of 2D materials can be demonstrated by chemical bonding in atomic layer and by the van der Waals force between the layers,which the 2D layer structure leads to physical and chemical properties of materials with anisotropy.Compared with bulk forms,the small size effect and quantum confinement effect show the interesting and different properties,which have important research value and wide application prospect in the field of nonlinear optics,broadband ultrafast laser,and other fields.Studing and understanding the relevant nonlinear optical properties of 2D materials,including broadband saturable absorption and carrier dynamics mechanism will helps us to reveal the mechanism on the interactions between light and novel material and provide experimental/theoretical basis for their application in the photoelectric field.In this thesis,the nonlinear optical absorption and carrier dynamics of 2D black phosphorus(BP),tungsten disulfide(WS₂),and Ti₃C₂ nanosheets are investigated by ultrafast spectroscopic techniques,concrete research contents are as follows.1.Nonlinear absorption and carrier dynamics of BP nanosheetsZ-scan experiment is used to study BP nanosheets in the nonlinear absorption characteristics at visible spectral range,with the wavelength increasing,saturable intensity becomes stronger,which shows the enhancement of absorption in visible light region.From saturable absorption to reverse saturable absorption are observed under different nanosecond laser pulses excitation,which shows the linear absorption with two-photon absorption of physical process.The influence of different pulses width and different solvent conditions on the saturable absorption intensity of BP is explained.Based on the transient absorption spectra at different wavelengths,the carrier dynamics of the BP nanosheets on the pump fluence-dependent and wavelength-dependent is studied.For 400 nm excitation,an additional decaying channel is observed at a larger pump fluence,which is explained by an effective subband structure.For 800 nm excitation,pump fluence-dependent lifetime shows complex behaviors,which might be due to the competition between the linear absorption and two photon absorption.The saturable intensity shows an increasing trend with increase in the wavelength.Nonliear optical parameters are extract by bi-exponential fitting dynamics curves.2.Nonlinear absorption and carrier dynamics of WS₂ nanosheetsNonlinear optical response of WS₂ is investigated via broadband(450-700 nm)Z-scanmeasurement,with lower excited energy,the wavelength-dependent saturable absorption is observed,which originates from the ground state bleaching mechanism.Resonance enhancement effect of WS₂ nanosheets is investigated.It is found that with the increasing excitation at the resonance peak of 500 nm,a switch from saturable absorption to reverse saturated absorption are observed that could be according to the competition between the linear absorption and two photon absorption.Nonlinear optical paraments are obtained by fitting Z-scan curve under different extractions at 532 nm,compared with that of 500nm,an enhancement of absorption is observed,which may be according to the enhancement resonance absorption in saturable absorption.Nonlinear absorption of the WS₂ doped with silver nanoparticles is studied,an enhancement effect is observed,which could be arising from Ag nanoparticles surface plasmon resonance.The carrier dynamics of WS₂ nanosheets is studied by femtosecond transient absorptionspectroscopy.It is found that the energy transfer consisted of a fast and a slow decay component,which may be arising from electron-phonon and phonon-phonon coupling,and nonlinear optical parameters are extracted by using bi-exponential energy decay model.3.Nonlinear absorption and photophysical mechanism of Ti₃C₂ nanosheetsNonlinear optical response of Ti₃C₂ nanosheets is studied by Z-scan experiment,and reverse saturable absorption is observed.A systematic investigation on the nonlinear absorption of Ti₃C₂ nanosheets in visible light region is established,and the physical process of two-photon absorption of Ti₃C₂ nanosheets is illustrated.In order to study the carrier dynamics of Ti₃C₂ nanosheets,the ultrafast dynamics and spectrum is invenstigated under 400 nm laser excitaion.The decay lifetime of carrier relaxation is obtained.The results show that the energy decay lifetime of Ti₃C₂ nanosheets becomes longer with the increasing excitations.Transient absorption spectroscopy is performed through Ti₃C₂ doped with silver nanoparticles,it is found that decay lifetime of Ti₃C₂/Ag nanosheets is longer than pure Ti₃C₂ nanosheets.The intermediate levels due to the doping Ag nanoparticles are produced and then trapped the carrier.