Investigation of the ultrafast phase evolution of the ejected material generated during femtosecond laser ablation of aluminum by the coherent light illuminated schlieren apparatus

Abstract

A novel phase measurement method based on the schlieren apparatus is proposed, and the ultrafast phase evolution of the ejected material generated during the femtosecond laser ablation of aluminum is experimentally studied by this method. Different from the conventional schlieren technique, the phase measurement method presented in this work uses coherent light as the illuminating light. The specimen's phase under-test is derived with the help of the interference between the light which irradiates the surroundings of the specimen and the light which transmits through the specimen and diffracts on the razor edge of the schlieren apparatus. One remarkable merit of this method is that it can clearly exhibit the specimen's phase variation of mπ or 2mπ (m is an integer). The ultrafast process of the ejected material generated during the 5.4 J/cm2, 50 fs laser pulses ablation of the aluminum target is investigated by this novel phase measurement method and the pump-probe technique. Results show that the ejected material is composed of three sequentially appearing regions with different phase evolving processes, which are respectively corresponding to the ejected plasma-state material, the successively ejected material normal to the target surface and the shock wave. It is also found that during the time interval of 0–9.0 ns after the femtosecond pulse strikes the target, the phase of the ejected plasma-state material varies beyond π due to the expansion and recombination, but the phase variation of the successively ejected material does not exceed π.