| Popis: |
Wide band gap dielectrics remain transparent for low intensity near-infrared light. However, when light intensity increases, highly nonlinear absorption takes place. The absorption occurs due to the transfer electrons from the valence to the conduction band in the dielectric. We investigate the physics of this ionization process and the material changes induced in the sample by multiple femtosecond laser pulses. We show how transmission relates to the ionization prbabi1ity in high density material. By controlling various experimental parameters we obtain qualitatative and quantitatative information about ionization and different mechanisms involved. Analyzing transmission for a variety of pulse durations, we obtain the field dependent collisional ionization rates for fused silica. Using elliptically polarized light, we probe the sub-laser-cycle dynamics of the ionization. The variation in the ionization rate within a laser cycle is translated, through the differential absorption between the major and the minor axes of the polarization ellipse, into the polarization changes of the transmitted light. In crystalline samples we observe a dependence of the ionization on the alignment of the field with the lattice. The modulation in transmission with the alignment angle of the crystal is analyzed using Fourier transformation and it provides the information on the lattice symmetry. We find that the directionally dependent electron effective mass plays an exponentially dominant role in defining the ionization probabilities. Chemical changes inside the focal volume accumulate over many laser shots. We show that these material changes lead to a localized increase in the ionization, providing a feedback mechanism that leads to the formation of nano-structures in the interaction region. With longer exposure time, defocusing micro-lenses are formed in the bulk. These micro-lenses change the divergence of the laser beam as well as the pulse energy required for the ionization, therefore, leading to enhanced transmission. We observe this effect in a variety of transparent dielectrics. In crystalline quartz we find a novel phenomenon associated with repeated ionization. The permanent changes in the sample create favourable phase matching conditions for strong second harmonic generation. |
