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The new phenomena induced by femtosecond lasers lead to the new area of ultrafast science.It is a significant challenge to explain the phenomena associated with complex non-equilibrium and non-linear processes.Although there is a growing body of experimental observation,a comprehensive model remains undeveloped.We review the challenges in understanding the photon absorption stage mainly for the femtosecond ablation of wide bandgap materials at the intensities of 1013～1014 W/cm2.Major opinions and challenges in ionization mechanisms are presented by primarily considering multiphoton ionization and avalanche ionization.