Fundamental Study And Application Of Femtosecond Laser Solid Interactions

Femtosecond (fs, 1fs=10^(-15)s) laser pulses are used in a variety of fields from nano-machining to the quest for fusion energy. However, knowledge of quantitative details for femtosecond laser material interaction processes is still insufficient, and many unique applications remain unexplored. The first part of this thesis describes the quantitative study of the fundamental processes in early stages of femtosecond laser excited gold. The second part discusses the investigation of a unique application using femtosecond laser to tune the silicon microring resonators. In the fundamental investigation, a single state warm dense gold was generated by isochorically heating a free standing gold thin foil of 30nm thickness with a 45fs laser pulse at 400nm. The uniform heating of such a target was confirmed by simultaneous measurement of the reflectivity from its front and rear sides with two frequency chirped probe pulses, at energy density up to ~5MJ/kg. The frequency chirped pulse probe single shot technique was developed so that the evolutions of the reflectivities from front and rear surfaces of the heated foil were mapped in frequency domain. Optical pump-probe technique was then used to study the optical properties of gold in the first several picoseconds after laser excitation. The reflectivity (R) and transmissivity (T) of the excited target were measured simultaneously, and its AC conductivity was calculated from measured R and T. Experimental results were used to benchmark the first principle calculations based Density Functional Theory (DFT) and Quantum Molecular Dynamic (QMD) simulations. In the application study, the feasibility of using femtosecond laser pulses for tuning frequency in silicon microring resonators (SMR) was investigated. Due to fabrication imperfection, the designed resonant frequencies of SMRs are difficult to achieve accurately (e.g. 1nm variation in height can result in an SMR out of tune). It has been demonstrated that femtosecond laser pulses can tune the resonant frequency of SMRs permanently to either shorter or longer wavelengths, and controllable tuning is also possible.