A method, which utilizes the large difference in ionization potentials between successive ionization states of trace atoms, for injecting electrons into a laser-driven wakefield is presented. Here a mixture of helium and trace amounts of nitrogen gas was used. Electrons from the K shell of nitrogen were tunnel ionized near the peak of the laser pulse and were injected into and trapped by the wake created by electrons from majority helium atoms and the L shell of nitrogen. The spectrum of the accelerated electrons, the threshold intensity at which trapping occurs, the forward transmitted laser spectrum, and the beam divergence are all consistent with this injection process. The experimental measurements are supported by theory and 3D OSIRIS simulations.

http://absimage.aps.org/image/DPP09/MWS_DPP09-2009-000843.pdf

Injection and Trapping of Tunnel Ionized Electrons into Laser Produced Wakes

We present an overview of the different processes that can result from focusing an ultrafast laser light in the femtosecond–nanosecond time regime on a host of materials, e.g., metals, semiconductors, and insulators. We summarize the physical processes and surface and bulk applications and highlight how femtosecond lasers can be used to process various materials. Throughout this paper, we will show the advantages and disadvantages of using ultrafast lasers compared with lasers that operate in other regimes and demonstrate their potential for the ultrafast processing of materials and structures.

Ultrafast laser processing of materials: a review

Recent advances in the synthesis and modification of carbon-based 2D materials for application in energy conversion and storage

Laser-assisted synthesis, reduction and micro-patterning of graphene: Recent progress and applications

High average laser powers can have a serious adverse impact on the ablation quality in ultra-short pulsed laser material processing of metals. With respect to the scanning speed, a sharp transition between a smooth, reflective and an uneven, dark ablated surface is observed. Investigating the influence of the sample temperature, it is experimentally shown that this effect stems from heat accumulation. In a numerical heat flow simulation, the critical scanning speed indicating the change in ablation quality is determined in good agreement with the experimental data.

Heat accumulation in ultra-short pulsed scanning laser ablation of metals.

Effects related to the use of high repetition rate lasers in ablation of metals (aluminum, copper, stainless steel) and silicon
were investigated. The multi-pulse irradiation with the laser beam significantly lowered the ablation threshold and led to
a relative increase in the ablation rate at the higher repetition rate. The reason of alteration could be accumulation of
structural defects on the metal surface formed by irradiation with a laser of the sub-threshold fluence. The mean
volumetric ablation rate in laser milling experiments was a non-linear function of the pulse energy. Plasma shielding was
the main limiting factor in processing efficiency of metals with the high power picosecond lasers. Increasing the
repetition rate keeping the pulse energy below the plasma formation threshold is a way to increase the efficiency of
material removal with nanosecond lasers. Thermal management of the specimen could be a problem at high repetition
rates because of the laser energy wasted in the bulk. The reduction in the ablation threshold by irradiation with a series of
laser pulses might be useful in application of the high- repetition-rate lasers with the low pulse energy.

Accumulation effects in laser ablation of metals with high-repetition-rate lasers

Ultra-short laser pulses are frequently used for material removal (ablation) in science, technology and medicine. However, the laser energy is often used inefficiently, thus, leading to low ablation rates. For the efficient ablation of a rectangular shaped cavity, the numerous process parameters such as scanning speed, distance between scanned lines, and spot size on the sample, have to be optimized. Therefore, finding the optimal set of process parameters is always a time-demanding and challenging task. Clear theoretical understanding of the influence of the process parameters on the material removal rate can improve the efficiency of laser energy utilization and enhance the ablation rate. In this work, a new model of rectangular cavity ablation is introduced. The model takes into account the decrease in ablation threshold, as well as saturation of the ablation depth with increasing number of pulses per spot. Scanning electron microscopy and the stylus profilometry were employed to characterize the ablated depth and evaluate the material removal rate. The numerical modelling showed a good agreement with the experimental results. High speed mimicking of bio-inspired functional surfaces by laser irradiation has been demonstrated.

/pdf/advanced-laser-scanning-for-highly-efficient-ablation-and-435wre09bc.pdf

Advanced laser scanning for highly-efficient ablation and ultrafast surface structuring: experiment and model.

The extended focal depth of Bessel beams is a very attracting property for glass cutting applications. However, Bessel beam generation with a non-ideal conical lens induces beam pattern distortions. We present our novel results on bulk modifications of soda-lime glass using a non-ideal axicon-generated Bessel beam. Modelling of the Bessel beam pattern and experimental measurements indicated ellipticity of the central core diameter. That resulted in the formation of cracks in a transverse direction inside the bulk of glass. Furthermore, we demonstrate the possibility to control the transverse crack propagation direction, which is crucial in the case of glass cutting applications.

Non-ideal axicon-generated Bessel beam application for intra-volume glass modification

Ultrashort pulse laser, capable of varying pulse duration between 210 fs and 10 ps and producing a burst of pulses with an intra-burst pulse repetition rate of 64.5 MHz (time distance between pulses 15.5 ns), was used to investigate the ablation efficiency of the copper. The study on ablation efficiency was done for various numbers of pulses per burst between 1 and 40. The increase in the ablation efficiency by 20% for 3 pulses per burst compared to a non-burst regime was observed. The comparison was made between the beam-size optimised regimes. Therefore, the real advantage of the burst regime was demonstrated. To the best of our knowledge, we report the highest laser milling ablation efficiency of copper of 4.84 µm3/µJ by ultrashort pulses at ~1 µm optical wavelength.

/pdf/highly-efficient-laser-ablation-of-copper-by-bursts-of-vfxmim33zs.pdf

Paulius Gečys

Papers

Accumulation effects in laser ablation of metals with high-repetition-rate lasers

Advanced laser scanning for highly-efficient ablation and ultrafast surface structuring: experiment and model.

Non-ideal axicon-generated Bessel beam application for intra-volume glass modification

Highly-efficient laser ablation of copper by bursts of ultrashort tuneable (fs-ps) pulses.

Scribing of Thin-film Solar Cells with Picosecond Laser Pulses