Study On Optical Nonlinear Transformation And Ultrafast Dynamics Of Ag Nanoparticle

The unique surface plasmon resonance characteristics of noble metal nanoparticles can lead to the enhancement of local electromagnetic fields.The obvious separation of interband transition and surface plasmon resonance region of Ag nanoparticles make them exhibit special nonlinear optical properties under strong light.Therefore,Ag nanoparticles have extensive applications in biomedicine,biosensors,optoelectronics,chemical catalysis and other fields.The study of the nonlinear optical properties and ultrafast dynamics process of Ag nanoparticles is helpful to understand the physical process of the interaction between laser and Ag nanoparticles,to clarify the internal mechanism of the nonlinear process,and to provide a theoretical basis for its application in related fields.In this work,Z-scan technology and time-resolved pump-probe technology are used to study the optical nonlinear transformation and carrier ultrafast dynamics of Ag nanoparticles.The specific research contents are as follows:1.Transformation of nonlinear absorptionThe nonlinear absorption of Ag nanoparticles was studied by using the open aperture Z-scan technique at the off-resonance wavelength of 500～625 nm and the resonant wavelength of 400 nm,respectively.It is found that the nonlinear absorption of Ag nanoparticles is wavelength and energy dependent,and the increase of excitation energy can lead to the transformation of saturable absorption to reverse saturable absorption.The nonlinear absorption is significant and the saturation intensity decreases near the surface plasmon resonance region.Under the excitation of 532 nm wavelength,it is found that the nonlinear absorption of Ag nanoparticles has a double transformation of saturable absorption—reverse saturable absorption—saturable absorption.The experimental results are analyzed theoretically,and the single photon saturation intensity,two-photon saturation intensity and nonlinear absorption coefficient are obtained.2.Transformation of nonlinear refractionThe nonlinear refraction of Ag nanoparticles was studied by closed aperture Z-scan technique.It is found that with the increase of energy,the nonlinear refraction has a transformation process from self-focusing to self-defocusing.Through theoretical analysis,the physical mechanism of self-focusing and self-defocusing is explained from the perspective of hot electron and thermal accumulation,and the third-order and fifth order nonlinear refractive index coefficients of Ag nanoparticles are calculated.3.Size effect of nonlinear absorption and refractionThe size effect of optical nonlinear refraction and absorption of Ag nanoparticles was studied by Z-scan technique.The research shows that,in terms of nonlinear absorption,10 nm Ag nanoparticles show saturable absorption,and 20 nm and 40 nm Ag nanoparticles show the transformation of saturable absorption and reverse saturable absorption.In terms of nonlinear refraction,Ag nanoparticles at 10 nm show insignificant nonlinear refraction,while Ag nanoparticles at 20 nm and 40 nm show self-focusing or self-defocusing behavior.It is concluded that the nonlinear absorption and refraction of Ag nanoparticles have size effect,and the physical mechanism of size effect is explained.4.Carrier ultrafast dynamics processThe ultrafast dynamics of Ag nanoparticles was studied by white light pump-probe technology.The results show that the energy relaxation of Ag nanoparticles under different pump energies has gone through the process of electron-electron,electronphonon and phonon-phonon.The theoretical analysis of experimental data obtained the carrier relaxation lifetime.It was found that the electron-phonon and phonon-phonon relaxation lifetimes increased slightly with the increase of pump power.The detection wavelength had little effect on the electron phonon lifetimes,while the phonon phonon relaxation process increased gradually with the increase of detection wavelength.The dynamic process of carriers of Ag nanoparticles with different sizes is the same and the energy relaxation lifetime is not different.The physical mechanism of ultrafast dynamic process independent of size is explained from the point of view of phonon density of states and particle population distribution.In addition,the ultrafast dynamics of Ag nanoparticles at 800 nm single detection wavelength was also studied.It is found that the process of carrier dynamics of Ag nanoparticles at different pump energies is the same as that of white light pumping detection,and the relaxation lifetime at different pump energies has little difference.