Kaihu Zhang

Femtosecond laser pulse-train induced breakdown in fused silica: the role of seed electrons

Femtosecond laser pulse-train induced breakdown in fused silica is investigated theoretically, with a focus on the role of ultrafast seed electrons during the pulse-train excitation. Material breakdown threshold is investigated by a model, which consists of both the excitation model and an improved optical model by including the optical absorption of self-trapped excitons (STEs). It is found that the evolution of a single train induced breakdown threshold is governed by the interplay of three competing sources of seed electrons initiating an electronic avalanche: residual conduction-band electrons left by the previous pulse, photoionization of atoms in dense media and photoionization of STEs by subsequent pulses. The third source provides a key to the understanding of some potential and existing problems involved, and leads to many pulse-separation independent phenomena (e.g. surface damage/ablation size) for pulse-train processing when it becomes dominant, and can contribute to the repeatable processing. For a single train of two or several femtosecond pulses, the third source can become dominant and sustained at large pulse-separations only when the first-pulse energy is over a critical value, ~65?75% of the single-pulse breakdown threshold. Our calculations are in agreement with the experimental data.

Nanopillar arrays with nanoparticles fabricated by a femtosecond laser pulse train for highly sensitive SERRS

This work presents a novel method for fabricating repeatable, uniform, large-area, highly sensitive, surface-enhanced resonance Raman scattering (SERRS) substrates combined with silicon nanopillar arrays and silver nanoparticles. The proposed method consists of two steps: (1) induce periodic ripples in deionized water using a linearly polarized femtosecond laser; and (2) generate dense 80-nm-diameter nanopillar arrays with silver nanoparticles in silver nitrate solution with a 90° rotated polarization, femtosecond double-pulse train. As the pulse delay increases from 0 to 1000 fs, the mean size of the silver nanoparticles reduces, and the average number of nanoparticles increases, which, in turn, increases the enhancement factor of SERRS signals up to 1.1×10(9). Furthermore, melamine (down to 125 ppb) was detected by the fabricated SERRS substrates.

Temporal femtosecond pulse shaping dependence of laser-induced periodic surface structures in fused silica

The dependence of periodic structures and ablated areas on temporal pulse shaping is studied upon irradiation of fused silica by femtosecond laser triple-pulse trains. Three types of periodic structures can be obtained by using pulse trains with designed pulse delays, in which the three-dimensional nanopillar arrays with ∼100–150 nm diameters and ∼200 nm heights are first fabricated in one step. These nanopillars arise from the break of the ridges of ripples in the upper portion, which is caused by the split of orthogonal ripples in the bottom part. The localized transient electron dynamics and corresponding material properties are considered for the morphological observations.

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.

Production rate enhancement of size-tunable silicon nanoparticles by temporally shaping femtosecond laser pulses in ethanol

This paper proposes an efficient approach for production-rate enhancement and size reduction of silicon nanoparticles produced by femtosecond (fs) double-pulse ablation of silicon in ethanol. Compared with a single pulse, the production rate is ~2.6 times higher and the mean size of the NPs is reduced by ~1/5 with a delay of 2 ps. The abnormal enhancement in the production rate is obtained at pulse delays Δt > 200 fs. The production-rate enhancement is mainly attributed to high photon absorption efficiency. It is caused by an increase in localized transient electron density, which results from the first sub-pulse ionization of ethanol molecules before the second sub-pulse arrives. The phase-change mechanism at a critical point might reduce nanoparticle size.

Generation and elimination of polarization-dependent ablation of cubic crystals by femtosecond laser radiation

We experimentally showed that the π/2-period oscillation of an ablation area with laser polarization direction can be observed in GaAs, ZnSe, MgO and LiF with cubic crystal by a femtosecond laser (800 nm, 100 fs) and that the modulation in the ablation area can be controlled by the laser fluence. While the polarization dependence is sustained in a wide range of laser fluences for a narrow band-gap crystal, it is strongly suppressed with a slight augmentation of laser fluence in a wide band-gap material. The polarization-dependent ablation is explained by the crystals orientation-dependent reduced-electron mass and the resultant contrasting nonlinear absorptions with slightly different reduced electron mass. The interplay between photoionization and avalanche ionization is discussed to interpret the influence of laser fluence on polarization-dependent ablation. Based on Keldyshs theory, polarization-dependent ablation occurs in a mixed regime between tunneling and multiphoton ionization.

Doping effects on ablation enhancement in femtosecond laser irradiation of silicon

We have conducted an experimental investigation on highly efficient femtosecond laser micromachining of silicon through N-type doping. We found that the material removal amount has a close relationship with the doping concentration rather than with the doping types. The amount of material removal was enhanced gradually as doping densities increased. When the doping density reached higher than 10(18) cm(-3), the ablation threshold was considerably reduced, up to 15%-20%. The results of the experiment indicate that the high density of initial free electrons by doping is the fundamental reason for efficiency improvement, and bandgap shrinkage also plays an important role. The electrons are excited more easily from the valance band to the conduction band and acquire higher initial kinetic energy, which then promotes the material ablation process.

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.

Control of surface ablation on fused silica with ultrafast laser double-pulse based on seed electrons dynamics control

The influence of pulse-separation (τs) between a pair of temporally separated femtosecond laser pulses (with near ablation-threshold energy) on surface ablation of SiO2 were experimentally studied. A τs range of τs≤20 ps was considered. It was shown that a τs-independent/-dependent crater ablation area can be flexibly controlled. Once the pulse energy of the pulse pair exceeds a threshold value, crater ablation area become quasi-τs-independent at τs> ~1 ps. This τs-independent phenomenon can even be observed when each pulse within the double-pulse pair has a sub-threshold energy, which leads to a further reduction in ablation size. The experimental findings have not only confirmed our previous calculation based on a modified model, but also greatly extended the results both quantitatively and qualitatively. A dominant amount of seed electron from photoionization of self-trapped excitons (STEs) is responsible for the appearance of τs-independent phenomena. For physical interest, it is inferred that destruction of STEs will tend to break the τs-independent ablation phenomena. Experiments performed on CdWO4, a material exhibiting similar electron dynamics to that in SiO2 but a faster decay in STE population, support this conjecture. A possible improvement for the relevant theoretical modeling is also suggested based on the experimental findings.