Strong Super-Gaussian Laser Pulse Induced Reverse Saturable Absorption And Optical Power Limiting Processes

In this thesis,the propagation of strong nanosecond super-Gaussian laser pulse in the fullerene carbon 60 molecular medium is studied by solving numerical the paraxial wave equation and rate equations.Due to the large time span in the five-level model of C60,the system is divided into two subsystems:fast change and slow change subsystems.In this case,the rate equations of the five-level system are simplified into one rate equation describing the evolution of the ground state population.Considering the transverse distribution of the incident laser pulse,we use the Crank-Nicholson finite difference scheme to solve numerically the rate equation of the ground state population and the paraxial wave equation of the light field.The optical limiting behavior caused by reverse saturable absorption is observed for different orders of the strong super-Gaussian pulses.It is found that the spatio-temporal shape of the super-Gaussian pulse is obviously reshaped in the propagation process,and the pulse width is strongly compressed due to the strong reverse saturable absorption of C60.During pulse propagation,the leading edge of the pulse is slightly weakened due to the linear absorption by the ground state,while the main pulse is significantly absorbed because of the strong nonlinear sequential(singlet ? singlet)×(triplet triplet)two-photon absorption,and the trailing edge of the pulse is also obviously depleted by the strong triplet absorption.During pulse propagation,the incident flat-top super-Gaussian pulse is gradually reshaped into a Gaussian-type pulse with a narrow top,and the pulse duration is reduced by one order of magnitude from 10.0ns to 1.0ns.The higher the order of the super-Gaussian pulse,the stronger the reverse saturable absorption and the narrower the width of the output pulse.For the intensities considered here,pulses with a larger input field intensity has instead has a smaller output field intensity and shows a better optical power limiting behavior.The super-Gaussian laser pulse with a wavelength of 650nm shows a better optical limiting property than the same type of super-Gaussian laser pulse with a wavelength of 532nm.The population distribution near the input interface of the C60 medium follows the spatio-temporal shape of the incident super-Gaussian laser pulse,therefore,one can coherently control the population distribution of the system by use of pulses with different spatio-temporal shapes.The rate of population transfer from the ground state to the lowest triplet state depends on the field intensity.By increase of the incident field intensity,more populations are transferred to the triplet state by intersystem crossing,which in turn enhances the reverse saturable absorption and the optical limiting performance.