Laser operation in bursts of pulses has recently attracted a lot of attention, as high average powers and pulse energies have become available. Bursts of pulses have become a mean to achieve different laser-matter interaction regimes with a single laser source. We have used an ultra-short pulse laser source in combination with an external array of birefringent crystals to generate near-THz bursts of single-picosecond pulses. Variability of the setup and high single pulse energy were exploited to generate bursts of up to 16 pulses at different delays between consecutive pulses. The experimental setup was applied for surface processing of different industrially relevant materials, including metals, ceramics, and polyimides. The goal of this study was to find out if the reduced ablation thresholds using near-THz bursts of pulses as reported in previous studies of other group could be utilized for increased ablation efficiency. Our findings show that near-THz bursts of pulses interact with materials in a similar fashion as a single, albeit longer, pulse would. We observed a clear trend on all the tested materials, showing a declining ablation efficiency with increasing total burst duration at a fixed pulse repetition frequency. Findings contradict the results of other groups concluding on an improved efficiency of laser to material energy transfer using bursts at near-THz intra-burst repetition rates. A simple model describing ablation efficiency decline with pulse prolongation was used to compare the experimental data with theoretical predictions. Detailed analysis of surface ablation effects on metallic materials also revealed a presence of melting processes typically associated with laser pulses above 10 ps in duration as opposed to original 1.5 ps pulses.

The article relates to SPS Operation entitled Building blocks, tools and systems for future factories-GOSTOP.

Laser ablation and modification using bursts of picosecond pulses and a tightly focused laser beam are used to manufacture structures in the bulk silicon. We demonstrate precise control of the surface crystallinity as well as the structure depth and topography of the processed areas, achieving homogeneous surface properties. The control is achieved with a combination of a well-defined pulse energy, systematic pulse positioning on the material, and the number of pulses in a burst. A custom designed fiber laser source is used to generate bursts of 1, 5, 10, and 20 pulses at a pulse repetition rate of 40 MHz and burst repetition rate of 83.3 kHz allowing for a fast and stable processing of silicon. We show a controlled transition through different laser-matter interaction regimes, from no observable changes of the silicon at low pulse energies, through amorphization below the ablation threshold energy, to the ablation with either complete, partial or nonexistent amorphization. Single micrometer-sized areas of desired shape and crystallinity were defined on the silicon surface with submicron precision, offering a promising tool for applications in the field of optics.

We have investigated the process of copper layer ablation with a tightly focused Q-switched 532 nm laser. Focusing 40 ns long laser pulses to a micrometer-sized spot results in high energy density and gives rise to ablation phenomena not seen during laser processing with larger beam diameters. Use of acousto-optic deflectors (AODs) enabled us to test different laser beam steering approaches in terms of choosing the position for each laser pulse independently of the previous pulses. Random addressing of desired positions across a microstructure proved to be the most efficient method compared to various scanning approaches. Assigning a random order to the spatial sequence of laser pulses resulted in the fastest microstructuring process and featured lowest residual heating of the substrate.

The intermediate pulse duration regime between typical ultra-short and nanosecond pulses has been investigated using MHz-range bursts of 70 ps pulses emitted from a custom-made fiber laser source. The goal of this study was to observe and understand the processes involved during laser ablation on the timescales from picoseconds to nanoseconds, relevant due to pulses in bursts. We developed material processing approaches that enable similar behaviour as single 70 ps pulse ablation to ultra-short pulses in terms of quality and burst-mode behaviour like nanosecond pulses in terms of efficiency. The variability of the fiber laser operation modes was studied and compared to both ultra-short and nanosecond pulses from standard laser sources.