Direct Observation Of Phase Transitions By Time-resolved Pyro/reflectometry Of Krf Laser-irradiated Metal Oxides And Metals

摘要

New experimental results are obtained by coupling both time-resolved reflectivity and rapid infrared pyrometry under a hemispherical reactor. The heating source KrF laser beam (28 ns, 248 nm) is homogenized and as for probing, a CW He-Ne laser beam (10 mW, 633 nm) is used. Using both methods infrared pyrometry with an IR detector cooled with liquid nitrogen and sensitive in the spectral range 1-12 μm, and time-resolved reflectivity with a rapid photodiode, we were able to study complex thermodynamic transitions with nanosecond time resolution. Three different materials are studied by varying the KrF fluence (energy/surface) from 100 to 2000 mJ/cm～2: thin films melting (Au/ Ni), the threshold of plasma formation (Ti), and complex liquid phase segregation under semi-conductor state (ZnO). The formation of a liquid Zn film induced by temperature gradient is well evidenced by our signals. Also melting of thin films irradiated by low laser fluences (less than 500 mJ/cm～2) translates the typical thermodynamic behavior. Finally, wide fluence dynamic (400-2000 mJ/cm～2) is analyzed in the case of Ti surface, and results show two distinguished regimes: first one bellow 1000mJ/cm～2 corresponding to the early stage plasma initiation, and second one over 1000 mJ/cm～2 to the dynamics of plasma expansion.