Uniform ZnSe nanowires are observed on the ablation crater on ZnSe crystal surface irradiated by femtosecond lasers in air, while other parts of the sample surface are not polluted. The nanowire growth rate is about 5μm/s, it is higher than that fabricated by chemical vapor deposition method by a factor of 104. The nanowire length and diameter can be controlled by varying laser pulse energy and pulse number. The formation mechanism is studied and found to be self-catalyzed vapor-liquid-solid process.

Two collinear femtosecond laser pulses, one at wavelength of 800nm and the other at 400nm (double frequency), simultaneously irradiated the surface of ZnSe crystal, which resulted in regular nanograting with period of 180 nm on the whole ablation area. We attribute the formation of the nanograting to be due to the interference between the surface scattered wave of 800 nm lasers and the 400nm light. The period of the nanograting is aboutλ/2n, where n is refractive index of the sample, andλ, the laser wavelength. This mechanism is supported by observation of rotation of the nanograting with the polarization of 400nm light, and by the dependence of λ of the nanoripples on the surface of semiconductors and dielectrics.

Two-dimensional periodic nanostructures on ZnO crystal surface were fabricated by two-beam interference of 790 nm femtosecond laser. The long period is, as usually reported, determined by the interference pattern of two laser beams. Surprisingly, there is another short periodic nanostructures with periods of 220-270nm embedding in the long periodic structures. We studied the periods, orientation, and the evolution of the short periodic nanostructures, and found them analogous to the self-organized nanostructures induced by single fs laser beam.

A pump and probe system is developed, where the probe pulse durationγis less than 60 fs while the pump pulse is stretched up to 150-670 fs. The time-resolved excitation processes and damage mechanisms in the omnidirectional reflectors SiO2/TiO2 and ZnS/MgF2 are studied. It is found that as the pump pulse energy is higher than the threshold value, the reflectivity of the probe pulse decreases rapidly during the former half, rather than around the peak of the pump pulse. A coupled dynamic model based on the avalanche ionization (AI) theory is used to study the excitation processes in the sample and its inverse influences on the pump pulse. The results indicate that as pulse duration is longer than 150fs, photoionization (PI) and AI both play important roles in the generation of conduction band electrons (CBEs); the CBE density generated via AI is higher than that via PI by a factor of 102-104. The theory explains well the experimental results about the ultrafast excitation processes and the threshold fluences.

The one- and the two-photon absorption rates of conduction-band electrons (CBEs) in dielectric materials are calculated by second-and third-order perturbation theory, respectively. Here fused silica irradiated under 526 nm ultra-short pulse laser is used as an example. Compared with the case that only the one-photon absorption of CBE assisted by a phonon is considered. the rate of total energy gain of CBE from laser field will be enhanced by a factor of 3 when both of the one-and the two-photon absorption are included. The avalanche rate includes not only the term proportional to laser intensity, but also the terms proportional to the square of laser intensity and to the product of laser intensity and hole density. The damage threshold is calculated on the basis of the avalanche model, and find it is lowered by about 15% compared with the case for only the one-photon absorption of CBE assisted by a phonon is considered. The theoretical value agrees well with experimental results.