An experimental investigation of femtosecond laser through-hole drilling of stainless-steel 304 with and without transverse magnetic assistance was conducted. The characteristics of the through-hole geometry and sidewall as well as the chemical composition of the through-hole sidewall surface were analyzed. In addition, a theoretical analysis of magnetic-field-assisted femtosecond laser through-hole drilling is proposed. The results showed that transverse magnetic assistance could improve both the femtosecond laser through-hole drilling quality (through-hole geometry and sidewall characteristics) and efficiency. The primary reason is that transverse magnetic assistance changes the distribution of plasma and reduces the plasma density, which weakens the shielding effect of the plasma. However, compared with nanosecond laser drilling, the effect of the magnetic field on the femtosecond laser through-hole drilling was not obvious. A noticeable thermal effect appeared near the through-hole entrance at a pulse repetition rate of 500 kHz, and a heat affected zone and oxidation zone were produced, which is disadvantageous to laser drilling. This research has good prospects for industrial applications.

This paper presents a compound laser surface modification strategy to enhance the tribological performance of biomedical titanium alloys involving femtosecond laser nitriding and femtosecond laser texturing. First, high-repetition-rate femtosecond pulses (MHz) were used to melt the surface under a nitrogen atmosphere, forming a wear-resistant TiN coating. Subsequently, the TiN layer was ablated in air with low-repetition-rate femtosecond pulses (kHz) to create squared textures. The effects of the combined nitriding and texturing treatment on bio-tribological performance was investigated. Results show that compared with the untreated samples, the single femtosecond laser nitriding process increased the surface hardness from 336 HV to 1455 HV and significantly enhanced the wear resistance of titanium, with the wear loss decreasing from 9.07 mg to 3.41 mg. However, the friction coefficient increased from 0.388 to 0.655, which was attributed to the increased hardness, roughness within the wear scars, and the formation of hard debris. After combined treatment, the friction coefficient decreased to 0.408 under the optimal texture density of 65%. The mechanisms for the improvement in friction behavior are the reduction in contact area and the trapping of hard debris.

A strongly coupled finite element model of the optical breakdown during femtosecond laser pulse interaction, with different morphology of aluminum nanoparticles in water, was developed. This model provided new insight into the optical breakdown dependence on the nanoparticles’ morphology and assembly. This model was used to theoretically investigate a 300 fs laser pulse interaction with uncoupled and plasmon coupled aluminum coated silica shell nanoparticles. This study revealed how the nanoparticles’ one-dimensional assembly affected the optical breakdown threshold of its surrounding mediums. The optical breakdown threshold had much stronger dependence on the optical near-field enhancement than on the nanostructure’s extinction cross-section. The maximum electric field that is outside of the aluminum nanoparticles, with 2 nm silica shell and 2 nm gap, was more than 4 times greater to the one inside of the aluminum nanoparticles. For dimer and trimer configuration, the calculated lattice cross-section temperatures at each breakdown threshold were below their melting point. It is suggested that water could be ionized by aluminum/silica (core/shell) nanostructure during femtosecond laser exposures without nanoparticles consumption. This model could increase understanding of the aluminum nanoparticle-mediated optical breakdown in water.

By taking into account the dynamic thermophysical and optical properties, the ultrafast thermoelastic response of thin copper film irradiated by femtosecond lasers has been researched. The temperature and stress fields of copper film irradiated by femtosecond lasers are analyzed in this work. The simulation results reveal that the degree of thermomechanical response is much underestimated, especially with higher laser fluence and smaller pulse duration. It is necessary to employ dynamic properties in ultrafast thermoelastic simulation for accuracy.