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.

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.

Nanoripples with periods of 150 and 80 nm are formed on the surface of 6H-SiC crystals irradiated by the p-polarized 800 nm and the s-polarized 400 nm femtosecond lasers, respectively. When both of the two collinear laser beams focus simultaneously on the sample surface, nanoparticles are formed on the whole ablation area, and they array in parallel lines. We propose and confirm that the second harmonics in the sample surface excited by the incident lasers plays an important role in the formation of nanostructures.

The damage in fused silica and CaF2 crystals induced by wavelength tunable femtosecond lasers is studied. The threshold fluence is observed to increase rapidly with laser wavelength λ in the region of 250-800nm, while it is nearly a constant for 800<λ<2000nm. The ultrafast electronic excitation is also studied by a pump and probe method. The reflectivity increases rapidly in the latter half of pump pulse, which supports that impact ionization plays an important role in the generation of conduction band electrons (CBEs). We study the CBEs absorption via subconduction-band sub-CB transition, and develop a coupled avalanche model. Our results indicate that the CBEs absorption via sub-CB transition plays an important role in the damage in dielectrics irradiated by the visible and near ultraviolet femtosecond lasers. Our theory explains well the experiments.

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.

Single-shot laser damage threshold of MgO for 40-986 fs, 800 nm laser pulses is reported. The pump-probe measurements with femtosecond pulses were carried out to investigate the time-resolved electronic excitation processes. A theoretical model including conduction band electrons (CBE) production and laser energy deposition was applied to discuss the roles of multiphoton ionization (MPI) and avalanche ionization in femtosecond laser-induced dielectric breakdown. The results indicate that avalanche ionization plays the dominant role in the femtosecond laser-induced breakdown in MgO near the damage threshold.

We report the single-shot damage thresholds of MgF2/ZnS omnidirectional reflector for laser pulse durations from 50 fs to 900 fs. A coupled dynamic model is applied to study the damage mechanisms, in which we consider not only the electronic excitation of the material, but also the influence of this excitation-induced changes in the complex refractive index of material on the laser pulse itself. The results indicate that this feedback effect plays a very important role during the damage of material. Based on this model, we calculate the threshold fluences and the time-resolved excitation process of the multiplayer. The theoretical calculations agree well with our experimental results.

We present a simple route for ZnSe nanowire growth in the ablation crater on a ZnSe crystal surface. The crystal wafer, which was horizontally dipped in pure water, was irradiated by femtosecond laser pulses. No furnace, vacuum chamber or any metal catalyst were used in this experiment. The size of the nanowires is about 1-3μm long and 50-150nm in diameter. The growth rate is 1-3μm/s, which is much higher than that achieved with molecular-beam epitaxy and chemical vapor deposition methods. Our discovery reveals a rapid and simple way to grow nanowires on designed micro-patterns, which may have potential applications in microscopic optoelectronics.

We studied the single-shot damage in magnesium fluoride irradiated by 800 nm femtosecond (fs) laser. The dependence of damage thresholds on the laser pulse durations from 60 to 750 fs was measured. The pump-probe measurements were carried out to investigate the time-resolved electronic excitation processes. A coupled dynamic model was applied to study the microprocesses in the interaction between fs laser and magnesium fluoride. The results indicate that both multiphoton ionization and avalanche ionization play important roles in the femtosecond laser-induced damage in MgF2.

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.