Fabrications Of Periodic Nanostructures On Semiconductors Surface By Femtosecond Laser

Since femtosecond laser appeared, the interaction between ultrashort pulses and matters has attracted much attention. In this process, there is a lot of remarkable phenomenon, such as short periodic nanostructures induced by femtosecond pulses, of which the period is much less than laser wavelength. It has great applications on the fabrication of micro-optical elements of whose size breaks the diffraction limit, modification of material properties and 2D display. However, the formation mechanism is still a puzzling problem. Interference lithography (IL) is a ripe and effective method to fabricate 1D grating structure,2D and 3D photonic crystals. Furthermore, due to the talents of ultrashort time domain and ultrahigh power, interference lithography in use of femtosecond laser will be greatly developed. In this thesis, we focused on the studies of formation mechanism of femtosecond-laser-induced nanostructures on ZnO crystal and fabrications of abundant periodic nanopatterns on some semiconductors by the combination of femtosecond laser-induced surface nanostructures and multi-beam interference. The points are as follow:(1) The periods of femtosecond-pulses-induced nanostructures on ZnO crystal changed with the pulses energy and irradiation pulse number, and trended to the value ofλ/2n, where n is the refractive index. The thesis proposed a new theoretical model for the formation of femtosecond-laser-induced nanostructures. The sample should be divided into surface and subsurface layers in the processes of nanoripples formation. The surface layer directly interacted with laser pulses, where different ripple structures with distinct pulse energy and irradiation pulse number formed. In the subsurface layer, during the irradiation of femtosecond pulses, plenty of nanoplasmas were induced owing to nonlinear absorption and avalanche ionization. The dielectric constant was modified with laser parameters. The nanoplasmas strongly scattered the incident light, and gradually induced the standing wave field, which resulted in the formation of regular nanoripples. After the surface layer was peeled off by ablation, high-contrast and low-bifurcation nanoripples formed on the subsurface layer. We simulated the formation of surface layer structures and standing wave field with different pulse energies by finite element method, and compared the theoretical results with the experimental ones. They accorded well with each other.(2) Two-dimensional periodic structures were fabricated on many semiconductors by two-beam interference with applications of femtosecond pulses-induced nanostructures. Owing to the restriction effect of two-beam interferential intensity distribution on the length of nanoripples, regular nanoripples with better periodicity were obtained, which is hopeful in applications on fabrication of micro-gratings. Furthermore, when the laser parameters were adjusted to high pulse energy, a new structure, namely sub-micrometer pit array, was fabricated on many semiconductors by two-beam interference. By studying the evolution of sub-micrometer pits with the irradiation pulse number, we found that the sub-micrometer pits arose from the long-periodic ripples induced by laser pulses. Two-beam interference played an important role in the processes, of whose periodic intensity distribution limited the length of the ripples, and only the depth and width can be increased with the laser irradiation, and then the sub-micrometer pits formed. Furthermore, we studied the enhancement of absorption in a broadband spectrum resulting from the sub-micrometer pit array structures. The simulation results indicated that Mie scattering effects of ZnO surface structures played a role in the decrease of reflectivity, while the surface damage owing to laser ablation reacted on the decrease of transmissivity. Therefore, the absorption was enhanced, which is in favor of increasing the efficiency of solar energy.(3) Combining the fabrication of nanostructures induced by femtosecond laser pulses with multi-beam interference, we have obtained complex periodic nano/microstructures on many semiconductors surface by three-beam and four-beam interference. With different cross angles between beams as well as polarization combinations, various complex periodic nanostructures on many semiconductors were obtained. By changing the polarization combination, the interference patterns and nanopatterns were modulated. The laser polarization became an important factor in interference, which improved the flexibility and diversity of multi-beam interference. Then, we calculated the intensity and polarization distribution of multi-beam interference, and explained well the formation of complex periodic nanostructures. Furthermore, we studied the enhancement of photoluminescence of complex periodic nanostructures. Fluorescence microscopic photography indicated the great potential applications on 2D display.(4) The stimulated emission was found on part of ablated craters of ZnSe crystal after irradiation by femtosecond pulses. But the happen of this phenomenon is indefinite. It may be cause by the diffuse reflection of uneven surface. It is significant at the manufacture of mini laser. The formation mechanism of stimulated emission as well as the control of laser parameter to ablate the material for producing stimulated emission will be further studied.