The thresholds of optical breakdown in the volume of quartz glass were measured in relation to the number of pulses under irradiation by ultrashort laser pulses with different pulse repetition frequencies (1-400 kHz). Increasing this frequency from 10 to 400 kHz was found to substantially lower the breakdown threshold for 500-fs long pulses (at a wavelength of 1030 nm) and to lower to a smaller degree for 5-ps long pulses (515 nm). A strong frequency dependence of the breakdown threshold is observed under the same conditions as a manifold decrease of the breakdown threshold with increase in the number of pulses in a pulse train. The dependence of the optical breakdown on the number of pulses is attributable to the accumulation of point defects under multiple subthreshold irradiation, which affects the mechanism of collisional ionisation. In this case, the frequency dependence of the breakdown threshold of quartz glass is determined by the engagement of short-lived defects in the ionisation mechanism.

We report a detailed analysis of experimental parameters and fundamental mechanisms contributing to the strong nonlinear scattering of femtosecond and short picosecond laser pulses (50–6000 fs) at focusing in ambient air and protecting gases. The experimental conditions under consideration are typical for a variety of applications: micromachining of metals and dielectrics, high power laser exposure of targets. Such scattering, being the most noticeable manifestation of the complex phenomenon of conical emission in the focusing geometry, puts fundamental limitations to the incident power of precision laser targeting, restricting in this way potential productivity of micromachining in this pulse width range. The phenomena analyzed here are: ionization, the optical Kerr effect, the distortion of spectra via self-phase modulation, and the refractive ability of microplasma, namely the scattering. Characteristics of the last were investigated with respect to the incident wavelength and pulse width, for focusing of Gaussian and flat-top beams of different diameters. The obtained extensive data allowed us to trace the clue trends and relationships of this phenomenon and to find several ways to reduce or even eliminate the scattering in a broad range of the incident energies.

Nanosecond double-pulse laser drilling is reported in this paper. The double-pulse herein represents two closely conjoint pulses with 21ns pulse duration and about 52ns interpulse separation, which are acquired by temporal pulse shaping. Percussion drilling with such double-pulse is performed in stainless steel samples with different laser fluences, sample's thickness, repetition rates and ambient pressures. The experimental results show that the drilling rates of double-pulse drilling are more than one order of magnitude higher than that of conventional single-pulse drilling in air. Differences in the processing results between single-pulse and double-pulse with various processing parameters are investigated. In addition the ablation mechanisms of the double-pulse drilling are discussed.

Parameters of conical emission (CE), at focusing of femtosecond and short picosecond pulses in ambient gases, are characterized in a wide range of experimental conditions. Scattered and absorbed energy, the beam profiles and spectra are compared for different duration and wavelengths of laser radiation (harmonics of Ti:Sa and Yb:YAG laser). Shorter wavelengths were found to be most beneficial for elimination of CE. Nearly no scattering of short picosecond pulses was observed at 515 nm in the energy density range up to 300 J/cm2. The results are analyzed in terms of the contributing ultra-fast phenomena effecting refraction index of gases (the Kerr effect, different mechanisms of ionization). Advantages of the visible pulses are illustrated in deep drilling experiments.

The time dynamics of plasma-emission spectra is studied experimentally at different stages of the drilling of a steel plate by 100-fs and 5-ps laser pulses: from a shallow crater to a hole. The change in the time dependence of the plasma temperature caused by variations in the irradiated surface geometry is analysed. It is found that the time interval needed to reach a particular temperature (about 8000 K) drastically increases from 40–50 to 150–200 ns when a specific crater depth is achieved. The opposite tendency is observed as the crater depth grows further and a hole is produced. Strong self-absorption in a plasma plume inside a deep crater is experimentally confirmed which results in the appearance of line absorption against a continuous emission spectrum.

Parameters of conical emission are defined with respect to ambient gas, pulse-width and wavelength of focused radiation, which allows significant minimization of scattering in the case of ultra-short pulsed laser micromachining.

A novel approach to restrain the formation of the burr during nanosecond laser ablation is reported in this paper. An assistant laser pulse, separated from the primary processing laser pulse with the pulse duration of 21 ns by temporal pulse shaping method, is used to control the formation of the melt deposit. The effect of the assistant pulse on the morphologies of the melt pools is investigated with the aid of microscope. The results of machining grooves on steel samples with the shaped pulses show a reduction of the burr at the boundary of the ablation zone. The contribution indicates a potential method for obtaining an efficient ablation as well as good processing quality in short pulse laser microfabrication.

The ablation of steel in air by short laser pulses was shown to form a long-living cloud of electrically charged submicron particles. These particles, being resident in the atmosphere of deep laser-produced channels within tens of seconds and carrying an electric charge during portions of a second, are able to initiate low-threshold gas breakdown resulting in the significant screening of the following pulses with a duration of ∼100 ps. The clouds contained mostly positively charged particles shaped as ideal spheres. The statistics of their diameters nearly followed the Poisson law with the peak at 400 nm. The total volume of the charged particles was nearly equal to the volume of the ablatively removed material. A new approach was proposed to eliminate the screening, which implied the use of an external electric field. This enabled the enhancement of drilling rates by up to 50 times. The electric charge, mobility, and discharge rates of the particles were measured using a conductivity technique.

A significant improvement of drilling efficiency is observed in laser drilling thin steel plate with double-pulse laser technique. The laser used in the experiment is a 1047 nm Q-switched laser with pulse duration of 21 ns and interpulse separation of 52 ns. The maximum ablation rate of double-pulse is about 30 times higher than that in the conventional single-pulse drilling at room temperature. The enhancement ratio of ablation efficiency of double-pulse to single-pulse with the laser energy density, repetition rate, thickness of the samples and the ambient gas pressure is studied. These results can be important for practical applications in efficient laser drilling.

Ablative formation of channels in steel by picosecond and nanosecond pulses of Nd lasers was studied. It was found that significant screening of the incident energy (up to 80–90%) in this pulse duration range is caused by breakdown of air contaminated with ablated microparticles. The breakdown threshold, size of particles, and time of their settling down were estimated. It was shown that this kind of plasma screening results in a decrease in the ablation rate and significant channel widening. Practical approaches to eliminate the low-threshold breakdown induced by microparticles were proposed and implemented. These approaches are based on experimental results of the study of the dependences of laser ablation on the pressure and repetition rate. It was shown that a moderate decrease in the pressure below 300–400 mbar makes it possible to avoid screening. In high-repetition-rate ablation, it was found that values above several kilohertz correspond to quasi-vacuum conditions in the ablation spot.

Laser ablation of materials in air is able to introduce long living modification of the gas environment in vicinity of the exposed area. Such a modification implies formation and residence of small particles of the material and of ablation products. If the material is exposed to multipulse laser action, which is normally the case for micro-mashining, ignition of laser plasma at such particles is able to create significant screening of incident radiation, influencing this way formation rates and morphology of ablated structures. Effect of this low-threshold air breakdown is especially pronounced in long drilled channels, where convection is poor.

The laser microdrilling of steel samples by nanosecond single pulses and pulse trains was compared in wide range of repetition rates (up to 200 kHz). It was found that drilling by the short trains, or just by pairs of pulses can be more effective compared to conventional pico- and nanosecond (100ps÷100ns) laser processing in vacuum or drilling using ultra-high repetition rate lasers.

Laser-induced air breakdown near a steel target surface ablated by short (300 ps and 10 ns) laser pulses was studied with optical emission spectroscopy to determine conditions of the breakdown ignition and time-integrated spatial localization of the laser plasma. The air breakdown was found to occur in a wide range (5-1000 mbar) of ambient air pressures, if the energy density exceeds similar to 10 J/cm(2) for the IR radiation (lambda = 1078 nm) and similar to 40 J/cm(2) for the visible light (lambda = 539 nm). Both the crater bottom ablation and evaporation of residual metal droplets suspended in air can ignite the low-threshold air breakdown. The experiments on deep laser drilling of steel demonstrate a sharp decrease of the drilling rate and simultaneous increase of the crater entrance diameter after the crater depth reaches a critical value. The critical crater depth increases with the applied energy density, under decrease of the ambient air pressure and under shortening of the laser pulses. The critical depth correlates with longitudinal shift of the time-integrated maximum plasma emission away from the ablated surface. Noticeable etching of the crater walls by the laser-induced plasma is observed, if movement of the plasma plume away from the ablated surface is stopped by the ambient air before the plasma leaves the crater. Interaction of the plasma with the crater walls should block ejection of the melt phase which is proposed as a reason for the observed sharp decrease of the drilling rate in a deep crater.

Pulsed laser drilling in steel was explored in a wide range of repetition rates (up to 200 kHz) at variable pressure of the ambient air. A critical repetition rate was established, exceeding of which drastically increased productivity of ablative drilling. The phenomenon was attributed to formation of a long-living domain (> 100 μs) with hot rarefied atmosphere in the vicinity of the ablated spot or inside the drilled channel. This rarefication was shown to reduce the plasma screening effect, which allows creation of quasi-vacuum conditions for laser ablative technology avoiding vacuum-pumped facilities.

Influence of pulse duration on microprocessing of Al is studied. Results show noticeable differences in terms of quality, burr height and remolten or recast matter into micromachined grooves at high fluence regime for 120fs and 4,5 ps pulse duration. At 120 fs experimental results of penetration depth are found to be 2 or 3 times higher than the theoretical optical penetration depth and is lowered to this value with increasing pulse duration. At high fluence regime up to 2 J/cm2, ablation thresholds are found to be in the range 10 times higher than for the case of 1 J/cm2. Penetration depths are higher by a factor 10 to 20 than the theoretical optical penetration depth. The ablation rate is nearly constant until 1 ps and then falls down to 2 times lower values and decreases regularly until 4,5 ps. This time is supposed to correspond to a critical pulse width between ultrashort and short regime.

Some basic features of ultrashort laser drilling of deep holes in Fe are investigated using molecular dynamics simulation model. The process is simulated for laser pulse duration of 0.1 ps at aspect ratio of the hole (hole depth/hole width) higher than 1. The interaction of the ablated material with the hole's wall affects its shape. The number of re-deposited, reflected, and secondary ejected particles from the walls is estimated as a function of the laser fluence. Significant secondary ejection of material from the walls is observed at fluences above 5 J/cm2.

Processes of graphitisation of laser-irradiated polycrystalline diamond surface exposed to multipulse irradiation are studied experimentally. The thickness of the laser-modified layer as a function of the laser-pulse duration ranging from 100 fs to 1.5 μs and the effect of the radiation wavelength on this thickness are studied. It is shown that the diamond graphitisation during multipulse laser ablation is a thermally stimulated process. The dependences of the diamond-ablation rates on the radiation energy density under the action of laser pulses of various durations are presented.

A theoretical model is developed and a molecular dynamics simulation technique is applied for the description of ultrashort laser ablation of metals. The ablation of Al, Ni, and Fe using 0.1, 0.5 and 5 ps laser pulses at wavelengths of 248 and 800 nm is studied. The process is investigated at fluences up to 0.5 J/cm2. The analysis based on the temporal evolution of the ablation, the temperature, and the pressure distributions into the material reveals that a thermo-mechanical mechanism (spallation) takes place near the threshold. However, phase explosion is found to be the dominant mechanism of material removal at fluences higher than several hundreds of mJ/cm2. The influence of the laser parameters (wavelength and pulse duration) is obtained and discussed. The ablation depth as a function of the laser fluence and the ablation threshold value are evaluated and compared with the experimental data available. Good agreement between the theory and experiments is observed.

A passively Q-switched laser with a nonlinear mirror on the basis of stimulated Brillouin scattering (SBS), generates bursts of pulses with a few 10 ns pulse duration and a separation between 20–90 μs. Percussion drilling and trepanning are performed in different materials with 1 mm thickness. The optimum parameter set of these pulse trains with regard to the burr height and ablation rate is investigated. Differences in the processing results between single pulse and multi pulse structures are discussed. In addition the laser allowed for transiently mode locked operation. Results for mode locked and merely Q-switched operation were compared.

Micromachining of steel, Cu and Al is studied. Ablation depths per pulse are deduced for laser pulse durations between 100fs and 5ps for fluences in the range of 150mJcm−2 to 20Jcm−2. The evolution of ablation rates allows to evidence a low and a high fluence regime. Ablation thresholds and penetration depths are deduced as functions of pulse duration. While in the low fluence regime the penetration depth is close to the theoretical optical penetration depth, at higher fluences the effective heat penetration depth is 10–20 times bigger with also higher ablation thresholds. Even in the femtosecond range thermal ablation processes occur and reduce quality, accuracy and efficiency of micromachining. Additionally, the latter are influenced by strong beam distortions due to nonlinear interaction between the radiation and the atmospheric gas. In the case of steel and Cu, the pulse duration seems not to affect microprocessing, but it is demonstrated to play a role for Al for pulses between 1 and 5ps.