The dynamics of femtosecond pulsed laser removal of 20 nm Ni films from an interface

摘要

The dynamics of femtosecond laser removal of 20 nm Ni films on glass substrates was studied using time-resolved pump-probe microscopy. 20 nm thin films exhibit removal at two distinct threshold fluences, removal of the top 7nm of Ni above 0.14 J/cm~2, and removal of the entire 20 nm film above 0.36 J/cm~2. Previous work shows the top 7nm is removed through liquid spalla-tion, after irradiation the Ni melts and rapidly expands leading to tensile stress and cavitation within the Ni film. This work shows that above 0.36 J/cm~2 the 20 nm film is removed in two distinct layers, 7nm and 13nm thick. The top 7nm layer reaches a speed 500% faster than the bottom 13nm layer at the same absorbed fluence, 500-2000 m/s and 300-700 m/s in the fluence ranges studied. Significantly different velocities for the top 7 nm layer and bottom 13 nm layer indicate removal from an interface occurs by a different physical mechanism. The method of measuring film displacement from the development of Newton's rings was refined so it could be shown that the 13nm layer separates from the substrate within 70 ps and accelerates to its final velocity within several hundred picoseconds. We propose that removal of the bottom 13 nm is consistent with heterogeneous nucleation and growth of vapor at the Ni-glass interface, but that the rapid separation and acceleration of the 13nm layer from the Ni-glass interface requires consideration of exotic phases of Ni after excitation.