Direct observation of phase transitions by time-resolved pyro/reflectometry of KrF laser-irradiated metal oxides and metals

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/cm2: 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/cm2) translates the typical thermodynamic behavior. Finally, wide fluence dynamic (400–2000 mJ/cm2) is analyzed in the case of Ti surface, and results show two distinguished regimes: first one bellow 1000 mJ/cm2 corresponding to the early stage plasma initiation, and second one over 1000 mJ/cm2 to the dynamics of plasma expansion.