S. M. Klimentov

Laser ablation of metals and ceramics in picosecond–nanosecond pulsewidth in the presence of different ambient atmospheres

Abstract Ablation tests of AlN, Si3N4, SiC, Al2O3 ceramics, steel and aluminum have been carried out in vacuum, air and argon atmospheres using UV (270 nm), visible (539 nm) and IR (1078 nm) picosecond (100÷150 ps) and nanosecond (6÷9 ns) laser pulses. Ablation rate dependencies have been measured in the range of laser energy densities varied from (2÷5)×101 J/cm2 to (5÷10)×103 J/cm2. Peculiarities of laser ablation processes at different wavelengths, pulsewidths and ambient gases are discussed. In particular, the efficiencies of laser ablation in picosecond and nanosecond regions are compared. The scanning electron microscope (SEM) pictures of high quality microstructures, deep and narrow cuts and holes produced in ceramics with typical size of tens microns and aspect ratio as high as 20, are demonstrated.

Generation of long-living charged nanoparticles at ablation in air and their role in pulsed microdrilling

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.

Femtosecond Laser Ablation of Dielectrics: Experimental Studies of Fundamental Processes

We study experimentally the electronic excitation mechanisms involved in the breakdown and ablation of wide band gap dielectric materials. We use a femtosecond pump‐probe interferometry technique to measure the density of carriers which is excited by ultrashort intense laser pulses. In Al2O3, we observe a density dependent lifetime for electron hole pairs excited by the light pulse, indicating second order or bimolecular recombination kinetics. Then, we use a sequence of two excitation pulses: a first short pulse at 400 nm excites a large density of carriers, and a second one at 800 nm with different pulse duration, namely 50 fs and 10 ps, reaches an excited solid. The intensity of each pulse is kept just below the damage threshold, which is surpassed when both pulses are impinging the sample. We show that the total density of carriers is just the sum of the density excited by the two pulses sent independently. The interpretation of this result leads us to conclude that the second pulse deposit further en...

Effect of low-threshold air breakdown on material ablation by short laser pulses

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.

Optical Breakdown in Ambient Gas and Its Role in Material Processing by Short-Pulsed Lasers

Formation of gas breakdown plasma, staying apart from the surface of materials exposed to pulsed laser radiation, is shown to have a strong impact to productivity and accuracy of micromachining. Origin and effect of two types of such plasmas are described: one is induced by low threshold breakdown of ambient gas contaminated by charged ablated nanoparticles; which is characteristic of nano- and subnanosecond lased ablation; the second is caused by ionization of gas at the leading edge of focused pico- and femtosecond pulses. The ways to eliminate undesirable plasma effects are discussed and demonstrated.

Spectral dependences of conical emission in gases: Minimization of scattering for ultra-short pulsed laser ablation

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.

Ultrafast breakdown of dielectrics: new insight from double pump-probe experiments (Conference Presentation)

We investigate the mechanisms involved in the modification of dielectric materials by ultrashort laser pulses. We show that the use of a double pulse (fundamental and second harmonic of a Ti–Sa laser) excitation scheme allows getting new insight in the fundamental processes that occur during the interaction. We first measure the optical breakdown (OB) threshold map (intensity of first pulse versus intensity of second pulse) in various materials (Al2O3, MgO, α-SiO2). Using a simple model that includes multiphoton excitation followed by carrier heating in the conduction band, and assuming that OB occurs when a critical amount of energy is deposited in the material, we can satisfactorily reproduce this evolution of optical breakdown thresholds. The results demonstrate the dominant role of carrier heating in the energy transfer from the laser pulse to the solid. This important phenomenon is also highlighted by the kinetic energy distribution of photoelectrons observed in a photoemission experiment performed under similar conditions of double pulse excitation. Furthermore, we show, in the case of α-SiO2, that the formation of self-trapped exciton is in competition with the heating mechanism and thus play an important role especially when the pulse duration exceeds a few 100 fs. Finally, also in quartz or silica, we observe that the initial electronic excitation plays a key role in the formation of surface ripples and that their characteristics are determined by the first pulse, even at intensities well below OB threshold. The consequence of all these experimental results in the domain of UV or VUV induce damage will be discussed. In particular we demonstrate the possibility to dramatically increase the ablation efficiency by VUV light by using such double pulse scheme.

Semiconductor disk laser in the nanosecond lasing mode

Pulse generation (pulse duration 370 to 34 ns) is obtained for the semiconductor disk laser optically pumped by the Cr:LiCAF laser beam. Dynamics of its lasing spectrum during the pump pulse is investigated. Generation pulse variations under the mismatch of the pump spot and the region of the main disk lasing mode are studied.

Delocalization of focused intense ultra-short laser pulses in air and transparent solids

Ti:Al₂O₃ lasers emitting pulses with duration τ = 100-140 fs at basic wavelength λ = 800 nm were used in most of the experiments. Gaussian laser beams were focused either in atmospheric air (helium, argon) or inside the bulk of silicon or diamond samples with approximately equal spot radius r_{b ≈}10-15um. In the case of silicon target by means of OPA λ was shifted to 1200 nm. Measurements of scattered light were performed calorimetrically. Variation of medium refractive index (Δn) was controlled by pump-probe setup combined with sensitive interferometer.

Femtosecond laser ablation of dielectrics: Experimental studies of fundamental processes

In the field of laser ablation of wide band gap materials by ultrashort laser pulses, there has been a long debate regarding the excitation and energy deposition mechanism. This is due to the lack of direct experimental investigations, which have been mostly limited to the measurement of ablation threshold.