Pengjun Liu

Continuous modulations of femtosecond laser-induced periodic surface structures and scanned line-widths on silicon by polarization changes

Large-area, uniform laser-induced periodic surface structures (LIPSS) are of wide potential industry applications. The continuity and processing precision of LIPSS are mainly determined by the scanning intervals of adjacent scanning lines. Therefore, continuous modulations of LIPSS and scanned line-widths within one laser scanning pass are of great significance. This study proposes that by varying the laser (800 nm, 50 fs, 1 kHz) polarization direction, LIPSS and the scanned line-widths on a silicon (111) surface can be continuously modulated with high precision. It shows that the scanned line-width reaches the maximum when the polarization direction is perpendicular to the scanning direction. As an application example, the experiments show large-area, uniform LIPSS can be fabricated by controlling the scanning intervals based on the one-pass scanned line-widths. The simulation shows that the initially formed LIPSS structures induce directional surface plasmon polaritons (SPP) scattering along the laser polarization direction, which strengthens the subsequently anisotropic LIPSS fabrication. The simulation results are in good agreement with the experiments, which both support the conclusions of continuous modulations of the LIPSS and scanned line-widths.

High-throughput rear-surface drilling of microchannels in glass based on electron dynamics control using femtosecond pulse trains

This study proposes a rear-surface ablation enhancement approach to fabricate high-aspect-ratio microchannels by temporally shaping femtosecond laser pulse trains. In the case study of K9 glass, enhancements of up to a 56 times higher material removal rate and a three times greater maximum drilling depth are obtained by the proposed method, as compared with conventional femtosecond laser drilling at the same processing parameters. The improvements are due to the changes of photon-electron interactions by shaping femtosecond pulse train, which can effectively adjust the photon absorption and localized transient material properties by changing electron dynamics such as free electron densities.

Self-organizing microstructures orientation control in femtosecond laser patterning on silicon surface

Self-organizing rippled microstructures are induced on silicon surface by linearly polarized femtosecond laser pulses. At a near threshold fluence, it is observed that ripple orientation is co-determined by the laser polarization direction and laser scanning parameters (scanning direction and scanning speed) in surface patterning process. Under fixed laser polarization, the ripple orientation can be controlled to rotate by about 40° through changing laser scanning parameters. In addition, it is also observed that the ripple morphology is sensitive to the laser scanning direction, and it is an optimal choice to obtain ordered ripple structures when the angle between laser scanning and laser polarization is less than 45°.

Direct writing anisotropy on crystalline silicon surface by linearly polarized femtosecond laser

An interesting anisotropy phenomenon in femtosecond laser processing of crystalline silicon is revealed by changing the angle between the writing direction and the laser polarization. The experimental results indicate the surface patterning is dependent on the laser polarization direction, showing that it is beneficial to forming continuous, ordered, and better-controlled ripples when the writing direction is parallel to the laser polarization. The anisotropy is attributed mainly to the elliptical shape of the induced ripples. The formation mechanisms of the elliptical ripples are also discussed. This observation promotes the fabrication of self-assembled subwavelength structures, which is important for electro-optic devices.

Controllable high-throughput high-quality femtosecond laser-enhanced chemical etching by temporal pulse shaping based on electron density control

We developed an efficient fabrication method of high-quality concave microarrays on fused silica substrates based on temporal shaping of femtosecond (fs) laser pulses. This method involves exposures of fs laser pulse trains followed by a wet etching process. Compared with conventional single pulses with the same processing parameters, the temporally shaped fs pulses can enhance the etch rate by a factor of 37 times with better controllability and higher quality. Moreover, we demonstrated the flexibility of the proposed method in tuning the profile of the concave microarray structures by changing the laser pulse delay, laser fluence, and pulse energy distribution ratio. Micro-Raman spectroscopy was conducted to elucidate the stronger modification induced by the fs laser pulse trains in comparison with the single pulses. Our calculations show that the controllability is due to the effective control of localized transient free electron densities by temporally shaping the fs pulses.

Etching rate enhancement by shaped femtosecond pulse train electron dynamics control for microchannels fabrication in fused silica glass

The dependence of the etching rate on the ultrafast pulse shaping is observed when microchannels are fabricated in fused silica glass using the method of femtosecond laser irradiation followed by chemical etching. In comparison with the conventional femtosecond pulses, the temporally shaped pulse trains can greatly enhance the etching rate under the same processing conditions. The enhancement is mainly attributed to the localized transient electron dynamics control by shaping the ultrafast pulse, resulting in higher photon absorption efficiency and uniform photomodification zone. Furthermore, processing parameters, including pulse delay and pulse energy distribution ratio, have also been investigated to optimize microchannels fabrication.

Ultrafast laser ablation size and recast adjustment in dielectrics based on electron dynamics control by pulse train shaping

The manipulation of the subpulse number, pulse delay, and pulse energy distribution of an ultrafast laser enables electron dynamics control by changing absorptions, excitations, ionizations, and recombinations of electrons, which can result in smaller, cleaner, and more controllable structures. This letter experimentally reveals that ablation sizes and recasts can be controlled by shaping femtosecond pulse trains to adjust transient localized electron dynamics, material properties, and corresponding phase change mechanisms.

Polarization-independent etching of fused silica based on electrons dynamics control by shaped femtosecond pulse trains for microchannel fabrication

We propose an approach to realize polarization-independent etching of fused silica by using temporally shaped femtosecond pulse trains to control the localized transient electrons dynamics. Instead of nanograting formation using traditional unshaped pulses, for the pulse delay of pulse trains larger than 1 ps, coherent field-vector-related coupling is not possible and field orientation is lost. The exponential growth of the periodic structures is interrupted. In this case, disordered and interconnected nanostructures are formed, which is probably the main reason of etching independence on the laser polarization. As an application example, square-wave-shaped and arc-shaped microchannels are fabricated by using pulse trains to demonstrate the advantage of the proposed method in fabricating high-aspect-ratio and three-dimensional microchannels.

Femtosecond laser microchannels fabrication based on electrons dynamics control using temporally or spatially shaped pulses

With ultrashort pulse durations and ultrahigh power densities, femtosecond laser presents unique advantages of high precision and high quality fabrication of microchannels in transparent materials. In our study, by shaping femtosecond laser pulse energy distribution in temporal or spatial domains, localized transient electrons dynamics and the subsequent processes, such as phase changes, can be controlled, leading to the dramatic increases in the capability of femtosecond laser microchannels fabrication. The temporally shaped femtosecond laser pulse trains can significantly enhance the material removal rate in both water-assisted femtosecond laser drilling and femtosecond laser irradiation followed by chemical etching. Besides, high-aspect-ratio and small-diameter microchannels are drilled by spatially shaped femtosecond laser pulses.