We report on the stable and continuous operation of a kilohertz laser-plasma accelerator. Electron bunches with 2.6 pC charge and 2.5 MeV peak energy were generated via injection and trapping in a downward plasma density ramp. This density transition was produced in a specially designed asymmetrically shocked gas jet. The reproducibility of the electron source was also assessed over a period of a week and found to be satisfactory with similar values of the beam charge and energy. Particle in cell simulations confirm the role of the shock and the density transition in the electron injection mechanism. These results show that the reproducibility and stability of the laser-plasma accelerator are greatly enhanced on the long-term scale when using a robust scheme for density gradient injection.

The evidence of multi-photon absorption enhancement by the dual-wavelength double-pulse laser irradiation in transparent sapphire was demonstrated experimentally and explained theoretically for the first time. Two collinearly combined laser beams with the wavelengths of 1064 nm and 355 nm, inter-pulse delay of 0.1 ns, and pulse duration of 10 ps were used to induce intra-volume modifications in sapphire. The theoretical prediction of using a particular orientation angle of 15 degrees of the half-wave plate for the most efficient absorption of laser irradiation is in good agreement with the experimental data. The new innovative effect of multi-photon absorption enhancement by dual-wavelength double-pulse irradiation allowed utilisation of the laser energy up to four times more efficiently for initiation of internal modifications in sapphire. The new absorption enhancement effect has been used for efficient intra-volume dicing and singulation of transparent sapphire wafers. The dicing speed of 150 mm/s was achieved for the 430 μm thick sapphire wafer by using the laser power of 6.8 W at the repetition rate of 100 kHz. This method opens new opportunities for the manufacturers of the GaN-based light-emitting diodes by fast and precise separation of sapphire substrates.

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Understanding of material behaviour at nanoscale under intense laser excitation is becoming critical for future application of nanotechnologies. Nanograting formation by linearly polarised ultra-short laser pulses has been studied systematically in fused silica for various pulse energies at 3D laser printing/writing conditions, typically used for the industrial fabrication of optical elements. The period of the nanogratings revealed a dependence on the orientation of the scanning direction. A tilt of the nanograting wave vector at a fixed laser polarisation was also observed. The mechanism responsible for this peculiar dependency of several features of the nanogratings on the writing direction is qualitatively explained by considering the heat transport flux in the presence of a linearly polarised electric field, rather than by temporal and spatial chirp of the laser beam. The confirmed vectorial nature of the light-matter interaction opens new control of material processing with nanoscale precision.

Laser–plasma acceleration at kilohertz repetition rates has recently been shown to work in two different regimes with pulse lengths of either 30 fs or 3.5 fs. We now report on a systematic study in which a large range of pulse durations and plasma densities were investigated through continuous tuning of the laser spectral bandwidth. Indeed, two laser–plasma accelerator (LPA) processes can be distinguished, where beams of the highest quality, with a charge of 5.4 pC and a spectrum peaked at 2–2.5 MeV, are obtained with short pulses propagating at moderate plasma densities. Through particle-in-cell (PIC) simulations, the two different acceleration processes are thoroughly explained. Finally, we proceed to show the results of a 5-h continuous and stable run of our LPA accelerator accumulating more than 18×106 consecutive shots, with a charge of 2.6 pC and a peaked 2.5 MeV spectrum. A parametric study of the influence of the laser driver energy through PIC simulations underlines that this unprecedented stability was obtained thanks to micro-scale density gradient injection. Together, these results represent an important step toward stable laser–plasma accelerated electron beams at kilohertz repetition rates.

Bursts of femtosecond laser pulses were used to record internal modifications inside fused silica for selective chemical etching. Two-pulse bursts with a variable energy ratio between those pulses at a fixed inter-pulse duration of 14.5 ns were applied for the first time. The selective chemical etching rate of the laser-modified material with the burst of two pulses was compared to the single-pulse regime when etching in HF and KOH etchants. The advantage of the burst-mode processing was demonstrated when etching was performed in the KOH solution. More regular nanogratings were formed, and the etching initiation was more stable when burst pulses were applied for fused silica modification. The vertical planar structures were obtained using the two-pulse bursts with an energy ratio of 1:2, increasing the etching rate by more than 35% compared to the single-pulse processing. The highest ever reported selectivity of 1:2000 was demonstrated by introducing the two-pulse burst mode.

Shocks in supersonic flows offer both high density and sharp density gradients that are used, for instance, for gradient injection in laser-plasma accelerators. We report on a parametric study of oblique shocks created by inserting a straight axisymmetric section at the end of a supersonic “de Laval” nozzle. The effect of different parameters, such as the throat diameter and straight section length on the shock position and density, is studied through computational fluid dynamics (CFD) simulations. Experimental characterizations of a shocked nozzle are compared to CFD simulations and found to be in good agreement. We then introduce a newly designed asymmetric shocked gas jet, where the straight section is only present on one lateral side of the nozzle, thus providing a gas profile well adapted for density transition injection. In this case, full-3D fluid simulations and experimental measurements are compared and show excellent agreement.

In this paper, we report results of our research on the formation of micro-channels with the rectangular cross section inside bulk fused silica. The selective etching of channels was performed by the technique called the femtosecond laser induced chemical etching. Hydrofluoric acid was used as an agent for selective removal of laser-modified regions in fused silica samples. The method of the channel cross section control based on multiple scanning is proposed. The effect of scanning speed, laser pulse energy, and polarization on the etching selectivity was investigated and discussed.