It is possible for core leaky mode to couple with cladding defect leaky mode when the cladding defect is close to fiber core. Dispersion properties and propagation loss of core mode will be affected in some extent once the coupling occurs. But a complete coupling between two leaky modes not always happens even the phase matching condition is satisfied. Leaky mode coupling in photonic crystal fiber with a hybrid cladding which includes low-index and high-index inclusions at the same time is numerically investigated based on a full vector finite element method. It is found that not only phase matching but also loss matching plays an important role in leaky mode coupling. The originally intersecting dispersion curves for the two leaky modes will split and become another two new curves due to the anti-crossing effect when both the real and imaginary parts of their mode effective refractive indices are equal. There is not splitting but some perturbation in dispersion curves for the two phase matching leaky modes when their losses are not equal. A theoretic explanation is also given to these phenomena.

The results obtained from the finite element model of laser-generated ultrasound are presented in terms of temperature and displacement. According to thermoelastic theory, considering the temperature dependence of the thermophysical parameters of the material, the transient temperature field can be precisely calculated by using the finite element method; then, laser-generated surface acoustic wave forms are calculated in Al plates of various thicknesses. The elastic waves excited by a pulsed laser in a thin plate are typical Lamb waves, and the numerical results demonstrate that the surface vibration is mainly determined by the lower frequency components of the symmetric mode $s_0$ and antisymmetric mode $a_0$ of the lowest order in very thin plate materials. It is also indicated that, when the sample thickness increases, both the higher frequency components of the lower Lamb wave modes and the higher order Lamb wave modes should be considered. In a relatively thicker plate, the numerical model can still capture the significant details of the transient features taking place in the plates. The surface skimming longitudinal and shear waves are relatively stronger in the near field, distorting the observed wave form of the Rayleigh wave. These effects must be considered when the quantitative near field is applied to determining elastic parameters or residual stresses.

Millisecond, nanosecond, and picosecond laser pulse induced damage thresholds on single-crystal are investigated in this study. The thresholds of laser-induced damage on silicon are calculated theoretically for three pulse widths based on the thermal damage model. An axisymmetric mathematical model is established for the transient temperature field of the silicon. Experiments are performed to test the damage thresholds of silicon at various pulse widths. The results indicate that the damage thresholds obviously increase with the increasing of laser pulse width. Additionally, the experimental results agree well with theoretical calculations and numerical simulation results.

Several metal plates with different thickness including copper, iron, aluminum, and stainless steel have been drilled in the surroundings of air and water, respectively, by a Q-switched pulsed Nd:yttrium–aluminum–garnet laser. It is observed that for the same metal plate less energy is needed to drill a hole in water than that in air, and the surface morphology of hole drilled in water is improved greatly than that in air by comparison of the scanning electron micrographs. The underlying mechanisms behind the efficiency and quality enhancement in water are further investigated by means of optical beam deflection technique. The experimental results show that due to the water confinement the peak amplitude and duration of the laser-ablation-generated impact underwater is much larger than that in air. During the underwater laser drilling, besides laser ablation effect, both the first and second liquid-jet-induced impulses by cavitation bubble collapse in the vicinity of a solid boundary are also observed and their amplitudes are, respectively, about 12.4 and 5.2 times that of the laser ablation impact in air. Cavitation bubbles are the special dynamic phenomenon occurring in liquids. Therefore, it is concluded that in-air-drilling laser ablation-produced impact is a dominant mechanism; while during laser underwater drilling, it is the result of a combination of ablation-produced impact effect and liquid-jet-induced impact, especially the latter. Thus, the efficiency and quality of laser processing in the surrounding water can be greatly increased and improved compared with that in air.

Pulsed laser ablation in liquid (PLAL) is gradually becoming an attractive approach for nanomaterial fabrication because it is a chemically simple and clean method with high product purity. We studied the laser interaction with bulk target and particle products in detail, including mechanism process, target morphology and nanoparticle products. We captured three oscillations of one bubble after laser ablates the bulk target and calculated the variation of pressure and temperature inside the bubble. The results show that the first bubble oscillation has greatest impact on the nanomaterial synthesis, and the most powerful stages for the material synthesis during all the bubble oscillations are the beginning of each expansions and the end of each shrinks. Nanomaterial releases from the bubble at the end of each oscillations. In addition, based on the analysis of ablation cavity on the target, it is found that the cavity depth increases with the number of laser pulses, and then the depth tends to be under saturation status, which means it is difficult to obtain great improvement of the nanomaterial productivity just by prolonging the laser irradiation time. More importantly, the strong interaction between laser and particle products is presented clearly by time-resolved shadowgraphy, which can contributed to the modification of nanoparticle products.