Study On Surface Domain Inversion And Ablation Of Ferroelectric Crystal By Femtosecond Laser

With the development of femtosecond laser, the application of laser and the research area of interaction between laser and materials have been widened. In this dissertation, a novel method for domain inversion of ferroelectric crystal, domain inversion induced by femtosecond laser, has been proposed with simulation study and experimental investigation. Besides, the surface ablation of lithium niobate and Mg-doped lithium niobate by femtosecond laser has also been studied, which can help to get a deeper understanding of the anti-damage characteristic of ferroelectric crystal and also has important reference on the microfabrication in such materials by femtosecond laser.In our simulation model of domain inversion, which describes the process of electric field of laser acting on the crucial ions in domain inversion, that the domain inversion can actually be induced by laser in domain inversion window has been found, which alternates with un-inversion window as the energy of laser growing. Study of the parameters of the model shows that the decrease of damping constant, coupling spring constant, temperature, duration of the pulse and frequency of the laser field, will more or less reduce the inversion threshold, while a smaller laser waist radius will bring to a smaller domain inversion area. Carrier Envelope Phase (CEP) also plays an important part in the inversion process. When the value of CEP is about 0.5π, more laser energy will be devoted onto the domain inversion which can help to induce the possibility of domain inversion Simulation of the model for lithium niobate clarifies that the domain inversion by infrared femtosecond laser has a threshold in the range of 1~100μJ.Ring-like domain inversion window of lithium niobate induced by 800nm femtosecond laser, which has a polarization parallel to z axis of the specimen, has been observed. However the inversion becomes harder when the laser polarization turns an angle of 90 degree, which means the domain inversion is caused by electric field as is simulated in the model. Results show that the inversion threshold is between 1~4J/cm2 corresponding to laser energy of 30~60μJ, which is agreeable with our theoretical predictions. Besides, the experiment has been improved by using lens with focal length of 150mm and adjusting the position of focal point 10mm away from the specimen. Such a domain inversion induced totally by laser creates a path to achieve domain inversion structure in 3-dimension.Finally, the surface ablation of lithium niobate and Mg-doped lithium niobate under irradiation by single and multiple femtosecond laser pulses has been studied, and their ablation thresholds have been found out by using the logarithm relation of the ablation area and the laser energy fluence. Results show that when lithium niobate is Mg (5mol%) doped, its single pulse ablation thresholds increases 50% and its multi pulse ablation thresholds reduces more slowly as the pulses number increases. Another research for the comparison of Mg (5mol%): LiNbO3 and Mg (6mol%): LiNbO3 shows that the former one has a better anti-damaged characteristic.