Establishment Of Femtosecond Laser Pump-Probe Thermoreflectance System And Study On Heat Transport In Metal Films

The microscopic mechanism of energy transport and conversion, as fundamental science, is very important in many fields, such as energy, information technology, material science and micromachining. The energy transport and conversion actually results from the interactions among microscopic carriers of energy. And the durations of these interactions are usually on femtosecond-picosecond time scales. Femtosecond transient thermoreflectance (FTTR) technique is capable of resolving and observing the interaction of carriers on sub-picosecond time scales. So the FTTR technique is inevitably used as an experimental means to observe and study the microscopic mechanism of energy transport and conversion. Furthermore, this technique is also the only means to observe the interaction of microscopic carriers.In this work, we have set up a femtosecond laser pump-probe thermoreflectance experimental system basing on the FTTR technique. This experimental system includes a femtosecond laser system, an optical system, a signal collection system and a control system. The femtosecond laser system, as the light source of the experimental system, consists of a femtosecond fiber laser, a frequency multiplier and a temperature controller. The optical system is the core of the whole experimental system, which includes a beam-splitting module, a pumping optical path, a probing optical path, a delay system, a modulating system and an incident optical path. In the optical system, the laser beam firstly splits into the pumping beam and the probing beam; then the pumping beam is modulated and the optical length of the probing beam is controlled by the delay system; finally, the pumping beam and the probing beam are focused on the same spot of the sample. The signal is collected and amplified by the signal collection system, which consists of a photodetector and a lock-in amplifier. The function of the control system is ensuring the auto-operation of the optical system and the signal collection system, which consists of the control soft ware in the IPC and the data lines between the IPC and other systems. We solved the key techniques in the process of zero calibration of the experimental system with auto-correlation method, designed the control software that ensures the auto-operation of the whole experimental system; and developed various implementing schemes for the key links of the optical system. The optical system with a collinear incident mode (also called collinear optical system) has independent intellectual property right.The use of ultra-short laser pulse heating may result in complicated temperature distribution in metals. If the heating time is on the order of the electron-phonon interaction time, the free electrons can exist at higher temperature than that of surrounding lattice, which causes the nonequilibrium heat transport in metals. By now, for this nonequilibrium heat transport there are two models: the parabolic two-step (PTS) model and the dual-phase-lag (DPL) model. According to the comparison of these two models, this dissertation finds that the DPL model is not consistent with the PTS model in evaluating the nonequilibrium heat transport in metals; the temperature described by the DPL model with a heating source term is different with either the lattice temperature or the actual electron temperature evaluated by the PTS model; and the DPL model has some difficulties in evaluating this nonequlibrium heat transport in metals heated by ultra-short laser pulses.At ultra-low temperature, heat may be dissipated by free electrons in a wavelike manner, as described by the CV-Wave model or the DPL model. An indirect numerical calculation method is given to simulate this wavy conduction behavior. With this method, the DPL model for short-pulse-laser heating problem is numerically simulated. And the one-dimensional numerical solutions in an rectangular system and the two-dimensional numerical solutions in an axially symmetric system are given to exhibit various thermal behaviors of conduction heat transfer, such as wave, wavelike, diffusion and over-diffusion.The femtosecond laser pump-probe thermoreflectance experimental system is used to study the energy transport in gold films with lower intensity laser pulse heating. In this experiment, the laser pulse duration is 140 fs and the maximum electron temperature change is less than 10K. This dissertation gives the comparison of the experimental results with the predicted results from the parabolic one-step (POS) model, the PTS model and the DPL model. The comparison shows that only the PTS model can well evaluate the measure data. We also found that the electron-lattice coupling factor gotten in the case of the lower intensity laser pulseheating equals to 4.5×10¹⁶ W/(m³K) , which is different from that(2.8×10¹⁶ W/(m³K)) gotten by Brorson and Qiu in the case of higher intensity laser pulse heating.