Femtosecond Laser Induced Micro-/nano-structures On Silicon

Femtosecond laser induced surface micro/nano-structures change optical properity, electric properity, and hydrophilic/hydrophobic properities of material. Such modifications could dramatically extend scientific and industrial application of material. Therefore, this field becomes more attractive to scientists in recent years. In this thesis, I focus on the study of the interaction of femtosecond laser and silicon, which refers to the evolution of surface structures, and the formation mechanism. I also explore the application of femtosecond laser induced surface micro-/nano-structures.In the theory work, I have improved Two-Temperature Model (TTM) into a two dimensional TTM-Drude Model (2D TTM-Drude Model), which describes the space evolution and time evolution of carrier temperature, lattice temperature and carrier density of material excited by femtosecond laser pulse. The melting threthold, ablation threthold and transient reflectivity of silicon irradiated by femtosecond laser pulse are calculated by using2D TTM-Drue model. The nurmerical results fit well with experimental datas.The formation mechanism of femtosecond laser induced periodic surface structures (fs-LIPSS) are studied in details. We experimentally show that the generation and erasure of fs-LIPSS on rough silicon inducted by irradiation with a single laser pulse depend on the pulse fluence. We propose that this is due to competition between periodic surface structuring originating from the interference of incident light with surface plasmon polaritons and surface smoothing associated with surface melting. We show that SPPs stimulate periodic surface structuring and are hence crucial for the evolvement of fs-LIPSS, while surface melting produces smoothing and hence plays a counteracting role in preventing fs-LIPSS formation. Experimental results are supported by theoretical analysis of transient surface modifications based on combining the two-temperature model and the Drude model.We study the evolution of silicon surface structures irradiated by different number of femtosecond laser pulses. As the shot number increases, we devide the suface structures into four characteristic patterns:bubbles, LIPSS, droplets, and spikes or cones. By using two dementional Fourier transform methord, we analyse the relation between high space frequencies of the surface stuctures and laser polarization. We also observe the influences of gas pressure and gas species on the formation of silicon surface structures.A femtosecond laser machining system was established. By using this system, we can frabricate large area (3cm×3cm) black silicon samples. We also establish another femtosecond laser machining system which can fabricate sample in water condition. We successfully frabricated nano-squre arrays on silicon with deep-sub wavelength scale. The length of nano-squres is around90-95nm, the distance between nano-squres is about20-40nm.Large area (3×3cm2) substrates for surface-enhanced Raman scattering were fabricated by combining femtosecond laser microstructuring and soft lithography techniques. The resulting substrates exhibit strongly enhanced absorption (>40%)in the spectral region of350～1000nm and generate enhanced Raman signal with enhancement factor of the order of107. The main advantages of our substrates are low cost, large active area, and possibility for mass replication.