Complex Micro/Nanostructures On Semiconductor Surfaces Induced By The Interference Of Three Femtosecond Laser Beams And The Investigations On Their Photoluminescence Properties

Recently, people pay much attention to various kinds of two-dimensional (2D) and three-dimensional (3D) structures induced by ultra-short laser pulses for their applications in high density optical storage, light-emitting diode (LED) and photonic crystals. This paper focuses on the fabrications of2D periodic structures induced by the interference of three femtosecond laser beams and the optical properties. The main results are as follows:(1) By adjusting the laser polarization combinations, we fabricated different types of2D periodic structures on semiconductor surfaces by the interference of three femtosecond laser beams. By changing the cross angles between any two laser beams,2D periodic structures achieve different periods. The theoretical results of the patterns of light intensity, polarization ellipticity, and polarization orientation indicate that the long-period micro patterns are determined by the interferential intensity distributions and the short-periodic nanopatterns are determined by the interferential polarization patterns. Interference polarization pattern makes the fabrication of periodic structures much more diversiform and flexible, which have potential applications in the high density optical storage and material modification.(2) Excited by800nm femtosecond laser pulses, the2D nanostructures of ZnO crystals revealed an ultraviolet (UV) luminescence enhancement through a multi-photon absorption (MPA) process. The luminescence of2D nanostructures is enhanced by a factor of5-7, which is comparable to that of nanorods fabricated by a chemical vapor deposition method. We studied the mechanisms of the UV luminescence enhancement. Our results indicate that the enhancement is caused by an increase in optical absorption and by the formation of surface defect states. Meanwhile, the red shift and broadening of the PL spectra depend on the excitation laser wavelengths and fluences. Excited by an800nm femtosecond laser pulses, photoluminescence (PL) micrographs with blue-light emission arrays accord well with the periodic structures, for example, emission nanodot, emission nanoring, and uniform blue-light emission plane. Due to the enhancements of optical absorption and the UV luminescence, these structures have potential applications in the optical storage and display in2D and3D geometries. (3) We make some investigations on PL luminescence of2D periodic structures in ZnSe crystal surface as a function of the incident angle, laser polarization and wavelength. Compared with the case of ZnSe plane surfaces, the2D periodic structures revealed a near band-gap luminescence enhancement excited by an800nm femtosecond laser pulses. For the incident angle of0°, the laser polarizations has no influences on the near band-gap luminescence, and the polarization component in the horizontal direction is stronger compared with the vertical direction. For the incident angle of80°, the laser polarization has strong influences on the near band-gap luminescence of the2D periodic structures. The emission spectra of ZnSe sample excited by1206nm laser pulses have two emission peaks, which are attributed to a near band-gap emission and a second harmonic generation (SHG). Compared with the case of ZnSe plane surfaces, the2D periodic structures reveal a very strong near band-gap luminescence enhancement excited by a1206nm femtosecond laser. Meanwhile, the luminescence has a stronger polarization component in the horizontal direction. Excited by667nm and520nm laser beams, the results of polarizations dependences are similar to those observed by the excitation of800nm laser beam. For different photon energy excitations, the slops of the excitation powers dependence of PL intensities indicate that the exciton transitions are apt to a higher energy level of4.7-5.0eV rather than a lower energy level of2.7-3.1eV.(4) At last, we research the formation of periodic nanoripples on GaAs surface induced by femtosecond laser pulses in four different circumstances of air, purified water, vacuum with30K and vacuum with295K. In addition, we make an investigation on the influence of laser powers on the formation of periodic nanoripples in same circumstance. This work provides experimental results for the future investigation on the formation mechanisms of nanoripples induced by femtosecond laser pulses.