Polarization Sensitive Ultrafast Laser Material Processing

Abstract

In this thesis, I will concentrate on ultrafast laser interactions with various materials such as fused silica, crystalline silicon, amorphous silicon and nonlinear crystal.

The first polarization sensitive ultrafast laser material interaction to be illustrated was second harmonic generation in lithium niobate by tightly focused cylindrical vector beams. The generated second harmonic patterns were experimentally demonstrated and theoretically explained. Existence of the longitudinal component of the fundamental light field was proven. The same beams were used for modifying fused silica glass. Distribution of the electric field in the focal region was visualized by the presence of self-assembled nanogratings. Also in this experiment, crystalline and amorphous silicon were modified by the focused cylindrical vector beams. The generated modifications matched well with the theoretical simulations.

Polarization dependent structure was not observed under single pulse irradiation above the silicon surface. The generated isotropic crater structures with their smooth surface can be implemented as a wavefront sensor. Unexpectedly, an entirely different modification was observed after the double pulse laser irradiation. The size and orientation of the structure can be independently manipulated by the energy of the first pulse and polarization of the second pulse.

Theoretical analysis was conducted and the formation mechanism of the polarization dependent structures was explained. This structure on silicon surface can be used for the polarization-multiplexed optical memory.

One type of polarization sensitive ultrafast laser modification in fused silica is nanogratings. This modification exhibits form birefringence and therefore can be implemented for multi-dimensional optical data storage. Optimized data recording parameters were determined by sets of experiments. Stress-induced birefringence was observed and explained by material expansion at different conditions.

Finally, the multilevel encoding of polarization and intensity states of light with self-assembled nanostructures was illustrated. A new writing setup was designed and involved a spatial light modulator, a half-wave plate matrix and a 4F optical system. The data recording rate was increased by 2 orders of magnitude compared to conventional laser direct writing setup using polarization optics. The recording and readout of digital information was experimental demonstrated. We successfully recorded across three layers a digital copy of a 310KB file. The benefits of 5D optical data storage, such as long lifetime and high capacity were illustrated. In addition, the theoretical limitations of the current writing system and readout system were discussed and several upgraded systems were proposed.

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