Study On Transient Reflectivity Phenomenon On The Surfaces Of Metal Films Induced By Femtosecond Laser

With the development of laser technology, femtosecond laser is widely used in energy, materials, information technology and micro machining field. The microscopic mechanism of energy conversion and transfer process in materials by femtosecond laser heating is an important research content. The energy conversion and transfer process embody the interactions between the carriers at a micro level, the time scale of the interactions is very short, generally in picosecond to femtosecond level. The femtosecond transient reflection technique (FTTR) is often called the femtosecond pump probe technique, it can distinguish and observe the interactions between the carriers, so it is the main experimental method to observe and research the micro energy transfer processing and the regularities. This paper mainly studied on the transient reflectivity changes on some metal films’surface, and discussed the heat transport process of electron-electron and electron-phonon in the metal films by femtosecond laser heating. It was summarized as follows:1. We introduced three kinds of models when the ultrafast pulses laser heat the metal films:the two temperature model, the three temperature model and the nonthermal model. They aimed at the nonequilibrium heat transport process of different carriers in metal films.2. The preparations of the pump probe experiment was introduced in details. It contained two parts:the building of the pump probe experiment system and the preparation of samples. The working principle of the main instruments was discussed in details, and also the samples preparing using magnetron sputtering method was introduced, which included the principle of plating, the coating equipment and the annealing equipment etc..3. We performed pump probe experiments by using single pump beam to heat Ag films, Cr films, Co films and NiFe alloy films. We expounded the specific experimental process, described the trend of the transient reflectivity curve, and analyzed the movement of electrons and phonons in the thin films. We discussed the variation regularities of transient reflectivity curves for the electron-phonon nonequilibrium relaxation occuring, and analysis the electron thermalization time, the heating time et al.. We found and discussed the multiple transient reflectivity mutation phenomenon on the Cr films and NiFe alloy films of glass substrates, which were not discussed in detail in other pump probe experiments when the pump beam was single.4. In order to deeply analysis the multiple transient reflectivity mutation phenomenon, we set up the experimental system of double pump beams. The Pt films and the Ag films were also studied by pump probe technology. By analysis the transient reflectivity signals on Pt films, we found that the transient reflectivity curves of platinum film of20nm on glass substrates had a different phenomenon:after the Pt films heated by the double laser pulses, the AR/R curves sharply declined to a low value, the lasting time was approximately130ps when the ΔR/R curves remained at the low reflectivity level, and then the ΔR/R curves rose to the thermal equilibrium state. The phenomenon was not experienced in single pump beam experiments. Due to the electron temperature of films was proportional to the transient reflectivity of the films, so the electron temperature of the Pt film maintain at low temperature state in130ps. In addition, when the silver films were heated by the double pump beams, the transient reflectivity changed sharply up to4times in the delay time of less than15ps, so the electron temperature of the Ag thin films rapidly increased or declined many times in less than15ps. In a word, the emergence of these phenomena was important to research the interactions of femtosecond laser and metals.