Study On Thermal Transportation Mechanism Of Nano-Scale Materials And Interfaces By Femtosecond Laser Pump And Probe Method

A two-color femto-second pump-probe thermoreflectance system (TDTR) is established for the study of thermal transport behaviour within nano-structures or on interfaces. In this system, the second-harmonic generator (SHG) is used to double the frequency of the probe pulses, which produces a light train with a central wavelength of 400 nm. Such a near infrared (NIR)-blue two-color system has significant advantages over a single-color one since it is easier to isolate the scattered pump light from the detector by using dielectric mirrors and color filters which could have a transmission of 10-9 at the wavelength around 800 nm, much more efficient than a polarization arrangement. This also allows us to use a simple coaxial geometry where pump and probe beams are focused by the same objective lens, simplifying the overlap of pump and probe pulses and producing less deformation of the laser profile.The experimental system is used to study the energy transport in gold films with lower intensity laser pulse heating. In this experiment, the maximum electron temperature change is less than 10K, which makes our measurement more accurate than others', since this temperature change could be as large as several hundred K in their experiments. By the comparison between the measurement data and PTS model, the electron-lattice coupling coefficient is given.Different kinds of nano-thin films, like SiO2, polymer, and molecular layer deposition (MLD), are systematically measured. The results show that there is no size-effect in the thermal conductivity of SiO2 thin films with a thickness from tens to hundreds nanometers, in the mean time, there is in MLD thin films.Based on the TDTR method, a thermal mapping technique is applied to get the distribution of thermal conductivity on the surface of materials with micro-scale structures.The frequency domain pump-probe thermoreflectance system (FDTR) has also been established, and successfully used to measure the thermal conductivity of SiO2 thin films with different thicknesses and their interface thermal conductance with Si substrate. FDTR measurement results agree well with the TDTR measurements, but promises to be a much easier implementation than TDTR measurements.