Temporal Shape Effects On Strong Hyper-Gaussian Laser Pulse Induced Two-photon Processes

The propagation of strong femtosecond hyper-Gaussian pulses in a cascade three-level molecular medium is studied by solving numerically the Maxwell-Bloch equations using an iterative predictor-corrector finite-difference time-domain technique.Strong two-photon absorption induced optical power limiting behavior is observed for the hyper-Gaussian pulses of different orders.For the ultrashort hyper-Gaussian pulses,it is found that the “two-photon area” is no further the deciding quantity of the two-photon induced dynamics,and pulses with different temporal profiles will induce different two-photon absorption dynamics and optical limiting processes.With the same pulse duration and field amplitude,pulses of a lower order is found to have a larger input“two-photon area”but a smaller output area,and therefore show a better optical limiting behavior.The population distribution and the generation of new fields caused by high order nonlinear optical effects during pulse propagation also depend on the shape of the incident field,the field strength and the pulse duration.Besides,we have simulated the evolution of the output peak amplitude as a function of the input peak amplitude with the same FWHM of the incident pulse.It is found that with the increase of the input electric field intensity,the optical limiting effect breaks down sooner when n is smaller.