Optical Limiting And Corresponding Ultrafast Dynamics Research On CS₂

Nonlinear absorption is a major mechanism for optical limiting and has been reported a lot in recent years.Excited state absorption?ESA?and multi-photon absorption?MPA?are basically the most important parts of it.Since Z-scan technology is developed,it has been widely applied to extract the non-linear absorption coefficient or an effective one.A spectrally resolved Z-scan system is developed based on white light continuum Z-scan system,and an energy-calibration system is raised based on black-body irradiation theory.De-pendency of chirp of incident pulses on the 2PA coefficient of ZnSe is inves-tigated,and its 2PA coefficients are determined at continuous spectral range form 550 nm to 750 nm.The?extracted by this method is consistent with that obtained by the previous work.Based on this work,it is proved that the nonde-generate absorption effect enhances with the increase of nonlinear absorption within respective spectral wavelength;therefore materials with a broad nonlin-ear absorption bandwidth is impacted more by the nondegenerate absorption compared to those with a narrow one.Besides that,it is also proved that nonde-generate contribution can be increased by nonlinear-effect-caused broadening,but decreased by the broadening caused by group velocity dispersion?GVD?.To better comprehend Z-scan theory and investigate its application on both ESA and MPA,pieces of theoretical work are performed.A deduction has been carried out for ESA model compared to 2PA model.It turns out that the format of the attenuation equation for these two models are the same.Despite that,effective TPA coefficients increase proportionally with the increase of the pulse widths,and they increase with the increase of the lifetime of the intermediate state?i but converge to a certain limit when?i is over?80 times of the pulse width.Further simulation has been applied in which the time-dependent term is introduced to the attenuation equation at sub-picosecond scale.The propagation of femtosecond pulses and picosecond pulses in the material are contrasted.The simulation result infers a mechanism of the Transient Coherent Effect?TCE?,which temporally broadens the pulse and attenuates it rapidly.Therefore,in the cases of thick sample or intensive pulse intensity,the transmittance change caused by short-pulse effect should be taken into consideration.Since spectrally resolved Z-scan system is built,the nonlinear absorption in CS₂when filamentation takes place is further studied by this system.More-over,stimulated scattering is also observed using spot photographs.Nonlinear absorption around 800 nm is not affected by filamentation or plasma generat-ed.The reason for that is that MPA process and plasma generation take place successively and�re-absorption�of the excited molecules of CS₂may reduce the emission around 800 nm from the plasma.Furthermore,it is found that the length of filaments affects the symmetry location of Z-scan trace on�z�axis.To demonstrate the 3PA model of CS₂at 800 nma and interpret�re-absorption�as well as filamentation generation and dissociation at relatively low pulse in-tensity,we further applied time-resolved research on dynamics for pump at 800nm.A switched pump-probe system,which use both single and multiple photon pumps,is employed.Besides global fitting,frequency analysis and principal component analysis?PCA?are applied to process transient absorption data.We have proposed a model,in which case the triplet dissociation-ionization channel is proved to play the major part in plasma generation at relatively low intensity.Meanwhile,inter-system crossing?ISC?and quasi-bounded vibrational motions are demonstrated to be responsible for�re-absorption�.In addition,3PA nature of CS₂at 800 nm is determined by PCA.Hence,the whole process and dynam-ics after 3PA is pictured and interpreted.