Short-pulsed Laser Induced Ablation Effects And Mechanism Of Silicon And Silicon-based EO Devices

This thesis is focused on short pulse laser interaction with silicon and silicon-based EO device.The major objectives of this research is to obtain an unified and complete understanding of the laser damage effect and mechanism to bring solid ground for laser ablation, machinery, laser protection and other engineering applications. In the introduction part, closely related technology progress in recent years about short and ultrashort/ultrafast lasers, silicon and its applications are comprehensively summarized briefly. Also, the progress in laser interaction with matter are also discussed.Then the research is carried out from simple single materials to complex film materials step by step, which means the interaction of short laser pulses with crystal silicon is studied in the first place, then the damage mechanism of EO devices under laser pulse irradiation. The contents of the thesis shall include:1)Firstly, DFT-based semi-classical laser interaction with silicon theory is deduced. The imaginery part of dielectric constant is used as a microscopical tool to study the physical processes since this parameter have macroscopical meaning as well.Then it is introduced with band electron excitation, Auger recombination effect and two photon excitation etc. to modify original Two-Temperature Model to adapt for the short pulse laser interaction with silicon. Then, the damage threshold is given and the thermal and non-thermal damage contributions are analyzed. Finally, two-pulse laser interaction with silicon is discussed. Also, several key parameters such as electron density and lattice temperature influence for heat accumulation effect are compared. Non thermal process is defined a semiconductor-to-metal transitions. With a laser of 800 nm wavelength and 100 fs, the laser damage fluence is about 0.25J/cm2. At this fluence level, thermal contribution dominates the process, which is identical to experiments. With a laser fluence larger than 0.53 J/cm2 non-thermal damage mechanism prevails. With two pulses shooting, the heat accumulation effect is predicted, which shows that a time span between two pulses smaller than 100 ns cannot be ignored and it can reduce the damage threshold significantly. It is identified an electron density after the first pulse does not significantly change the heating process. However, the lattice temperature after the first pulse(higher than 800K) can cause serious electron excitation.The theoretical study results can be confirmed with refelction experiment mutually.2) The optical spectrum analyzer is used to study the fluorescence of femtosecond laser ablation of silicon surface, and the damage threshold are analytically fitting. Femtosecond laser induced plasma spectrum method is applied to analyze damage threshold and multiple pulses damage mechanism. Single pulse femtosecond laser is shot on Si and Si O2/Si samples. The detection wavelength is Si(I) 390.55 nm line. The plasma radiations near damage threshold of both samples are acquired to build relations between laser fluences and spectrum intensities. The damage thresholds are found to be 0.86 J/cm2 and 1.11 J/cm2 for Si and Si O2/Si,respectively. With duo different wavelength laser pulses irradiation on silicon samples, the plasma radiation intensity is highly correlated with pulse spacing. Different from that of identical wavelength pulse process, the intensity peaks are found to be different. As 400 nm wavelength ahead of 800 wavelength pulse reaches the surface, the increasement of radiation intensity is sharp while the declining is also fast. For longer time spaces(±300 ps), the increasing ratio reaches 0.5.With two different wavelength laser pulses irradiation on silicon samples, the radiation increasement ration is much lower.3) Systematic experimental damage effects of single and multiple pulse picosecond laser interaction with interline charged coupled device is studied. Experimental methods are used including video output, SEM and electrical test, etc. The camera is installed on electric revolving stage to control laser and the test subject intersecting time and pulse numbers to manipulate multiple pulses impinging conditions. A secondary moment method is employed to obtain the laser diameters on CCD surface. Bearing the damage effects data a unified picture of EO device malfunction mechanism is induced. The experiment finds that the functional damage threshold of multiple-pulse damage is about 24~84.5m J/cm2, which is severely lower than that of single-pulse ablation for 1.5ns and 400 ps kilohertz-repetition-rate lasers, approximating 470~800m J/cm2. Two vast different CCD device malfunctioning mechanisms are presented. With multiple pulses reach the same pixel on CCD surface, the laser damage threshold is declined sharply, which is closely related with pulse numbers and laser fluencies. For instance, with 100 pulses the damage shreshold only amouts to 1/3 of the single-shot value. In this case, multiple pulse damage mechanism is found to be identical to sing-shot case. With multiple pulses reach different pixels, the situation is quite different which attributes only to multiple vertical line damage superposed effect without vertical transfer clock lines short circuit effect.In this thesis, short and ultrashort/ultrafast laser interaction with silicon and silicon-based EO devices are studied both from theoretical and experimental methods. In theorm aspects, advanced Density Functional Theory(DFT) based semi-classical approximation theory and improved Two Temperature Model(TTM) are adopted to study the instantaneous process between carriers and lattice, including temperature, carrier density and space-time evolution. The semi classical DFT theory and phenomenological methods used here are newly created. In the experiment part, advanced pumped detection method and instantaneous spectrum measurement are used to dynamically study the micro level processes. In multiple materials surface and multiple pulses damage mechanism research new results are given. Finally, multiple pulses damage Charged Coupled Devices(CCDs) are studied,and the damage mechanism under are analyzed from material and electronical levels. The research is probably useful for laser ablation, machinery, laser protection and other engineering applications.