Reverse Saturable Absorption Induced Optical Power Limiting Behavior Of Laguerre-Gaussian Laser Pulses

Optical limiting during the laser-matter interaction is a typical nonlinear optical effect.Optical limiting mechanism and optical limiting materials are one of the most active research topics in the field of nonlinear optics,and have widely been used in the protection of photosensitive devices,laser weapons and other fields.In this thesis,we investigated the interaction process between a nanosecond Laguerre-Gaussian(LG)beams of different radial orders and C60 molecules by solving the two-dimensional paraxial wave equation numerically coupled with the five-level rate equations under the Crank-Nicholson scheme.The evolution of the light field,the evolution of the population and the distribution of the pulse energy transmittance of the strong field Laguerre-Gaussian beams with different radial orders are simulated.The propagation of the strong nanosecond Laguerre-Gaussian beams in C60 molecular medium exhibits obvious optical power limiting behavior,and the peak intensity of the light field is reduced by nearly two orders of magnitude at the end of the medium,which is attributed primarily to the reverse saturable absorption.The temporal shape of the pulse is strongly deformed during pulse propagation.The centrosymmetrical profile of the incident Laguerre-Gaussian pulses gradually evolves into a non-centrosymmetrical field distribution with crescent,and the pulse duration near the optical axis of the beam becomes shorter than that far away from the axis.The corresponding time point of the peak intensity of each ring of the outgoing pulse shifts obviously to the left relative to the incident pulse,and the higher the initial peak intensity of the field domain,the bigger the time shift.When the ground state effective populations transfer rate is equal to the rate of intersystem crossing transition,the characteristic transfer time of the effective population reaches the saturation limit value,and the incident light intensity approaches the optical power limiting damage thresholds at this point;after that,the optical limiting effect gradually fails,and as the light intensity continues to increase,the pulse energy transmittance will increase accompanied with the increase of the light intensity.By studying the propagation process of LG beams with different radial orders,it is found that the lower the radial order of the pulse is,the more concentrated the intensity distribution of the ring at the optical axis is,the more severe the pulse shaping is during propagation,and the higher the relative intensity between the outer ring and the inner ring is.Furthermore,the curvature of time distribution of crescent shaped light field increases along with the increase of the radial orders.Before the pulse energy transmittance reaches the saturation value,the lower the radial order of the incident pulse is,the more concentrated the energy distribution is,the lower the pulse transmittance is after passing through C60 molecular medium,and the better the optical power limiting effect is.After the pulse energy transmittance attains the saturation value,the higher the radial order of the pulse is,the more dispersed the energy distribution is,and the smaller the energy transmittance is.