O. G. Tsarkova

Ultrafast laser-induced plasma diagnostics with time-spatial-resolved shadow and interferometric techniques

The experimental studies of laser-induced plasma accompanying the laser ablation of material have been conducted with the developed shadow-interferometric technique. High intensity single picosecond pulses of YAP:Nd laser were applied to ablate tested samples and time-delayed probing pulses of second harmonic illuminating the interaction area were used to make snap-shots of the expanded plasma plume. Both shadow and interferometric images of hot plasma were captured simultaneously with a CCD camera providing approximately equals 1.5 micron spatial and approximately equals 10 ps temporal resolution of the investigated processes. By varying the intensity of ablating pulses and the time-delay of probing pulses it was possible to study a highly inhomogeneous small-scaled plasma density and refractive index distribution within the plume. The longer time-delays allowed study of laser-initiated shock wave expansion in the surrounding atmosphere. A special attention was paid to the plasma formation arising at a through-hole laser drilling process was observed. In particular, it was shown that the cluster explosion can efficiently block the laser radiation resulting in decreasing the ablation rate. A computer modeling of optical visualization of small-scale plasma objects has been conducted. The analysis si of the experimental and numerical results has revealed a number of characteristic features of plasma images that should be taken into account at the qualitative and quantitative evaluations of the plasma parameters.

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.

Picosecond 2D studies of CVD diamond film damage and graphitization

The interaction area of high-intensity tightly-focused 1078 nm pulses of 30 ps duration with CVD diamond film has been studied using second harmonic (540 nm, 20 ps) unfocused beam for backlighting. The 2D spatial distribution of energy density of probe radiation passed through the tested sample was recorded that allowed to measure the transient relative transmittance of the laser-affected zone as a function of temporal and spatial coordinates. The pump-probe interferometric measurements of laser induced refractive index in fused silica glass done with micron spatial and picosecond temporal resolution are also presented and discussed.

Optical and thermal properties of metals, ceramics, and CVD diamond films upon high-temperature laser heating

A complex optical method for simultaneous determination of optical and thermal properties of solids (true temperature, reflectance, transmittance, absorbance, extinction, specific heat capacity, thermal conductivity, and diffusivity) in a wide temperature range (800–2700°C) was proposed and experimentally demonstrated. The developed setup is characterized by a high processing rate (the time taken for measuring all parameters is from milliseconds to tens of seconds). It allows the study of rather small samples (about several milligrams), which is a significant advantage as applied to new or expensive materials. New and widely used materials (AlN, Al2O3, Si3N4, SiC, and Al2O3/SiC ceramics; C45, 16MnCrS5, 115CrV3, X5NiCr18 9, X12CrNi17 7, Sd, and Satellite 6 steels; E-Cu, CuSn, AlMgSi1, and AlCuMgPb copper and aluminum alloys; as well as chemical vapor deposition (CVD) diamond films) were studied at heating rates of 103−104K·s−1 in various atmospheres (air and argon at pressures of 1 and 10−3 atm). This study demonstrated high efficiency and informativity of the proposed method. The data obtained can be used to develop theoretical models describing the solid structure and properties at high temperatures in oxidizing, neutral, or regenerative atmospheres.

High-temperature measurements of reflectivity and heat capacity of metals and dielectrics at 1064 nm

The information on optical and thermal properties of materials at high temperatures is the base for any theoretical analysis of laser matter interaction processes. To evaluate the absolute value of energy deposited in the interaction area and to estimate the laser damage threshold one need possessing the knowledge on reflection/absorption coefficients and heat composition, irradiation and environmental conditions, surface and bulk temperature etc. the task of its direct numerical calculation becomes very difficult and the obtained theoretical estimations are unreliable. Up to now the only way of get the reliable values of the parameters mentioned is the experimental procedure. Among the other experimental techniques developed for measuring reflection/absorption coefficients at high temperatures, the based on an integrating sphere method is the most suitable, reliable and accurate one. The installation for high sped, high temperature measurements of optical and thermal properties of metals and dielectric materials is described. The system includes a specially designed photometric sphere, a chopped CW:Nd:YAG laser with output power up to 250W, which heats samples and simultaneously serves as a probing beam, and a high speed optical pyrometer. The set-up is fully computerized. The real temperature of targets is determined by a simultaneous measuring of brightness temperature and reflectively at about the same wavelengths. Different metals, ceramics laser active media - Al2O3:Ti3+ single crystals, and diamond CVD films have been investigated in air, vacuum and argon atmosphere at heating rates of 103-104 K/sec. The obtained data on high temperature optical and thermal properties of materials can be used at surface laser damage processes modeling.

Ablation of ceramics by UV, visible and IR pulsed laser radiation

Multiparametric study of A1N ceramic ablation by high intensity (I less than or equal to 1013 W/cm2) nano and picosecond pulses of Nd:YAP laser have been performed. Ablation rates, surface morphology and element content, reflectivity and absorptivity of ceramic plates prior and after pulsed irradiation as well as high temperature material reflectivity have been measured. It is shown that high etch rate and good quality A1N ceramic microstructuring (cutting, hole drilling, pocket formation) can be obtained for the first, second and fourth laser harmonics if irradiation conditions are properly chosen. It was found that possibility to effectively process material, which initially weakly absorbs laser beam (first and second harmonic), it is determined by radiation and plasma induced surface modification. Comparison with other types of ceramics, such as Al2O3, Si3N4, SiC, is also made.

Estimation of the absorptivity of a constructive material at unknown losses in ablation plume during laser ablation

Computation of the processes of laser heating of carbon silicon carbide composite material (CSCCM) samples in air (to temperatures above 2000°C for 1 s) by IR laser radiation with a wavelength of 1.3 µm and intensity of 3 kWcm−2 in the presence of screening ablation plume have been carried out using the KARAT code. A comparison of the simulation results with the experimental dependences of spatial and temporal fields of sample temperatures made it possible to determine the absorptivity of thematerial, energy loss in the ablation plume, and, correspondingly, its influence on the heating and ablation of the material under study.

Thermal and optical properties of semitransparent ceramics : laser induced high temperature modifications

The laser combined express technique has been applied to high temperature measurements optical and thermal properties of different ceramics. It was found that the ambient atmosphere noticeably affects the physical properties of the heated ceramics that marks a noticeable structural and chemical modification of the material.

Lasers of near IR-range based on chromium-doped scandium garnet crystals

It is known, that the application of chromium ions for the effective sensitized luminescence of rare-earth ions in crystals with garnet structure has allowed to solve a number of problems in physics and technology of solid-state lasers. So, for example, the efficiency of pulsed neodymium lasers has been increased 2-3 times . and effective lasing on both erbium2 (=2.79 pin) and golmium (A,=2.088 pin) transitions with low probability was obtained. The possibility of sensitization has a principle character for erbium and golmium ions due to rather low pump efficiency into its absorption bands. It should be mentioned that the interest to 2-pam and 3-pin lasers especially increased whn5 the possibility of their medical application was demonstrated

Analysis of the sizes and number of microparticles in the ablation plume, which attenuates laser radiation during ablation of a constructive material

The effect of laser radiation with a wavelength of 1.3 µm, power of 25 kW, pulse width of 1 s, and irradiated spot area of 9 cm2 on carbon silicon carbide composite material (CSCCM) is analyzed. The formation of an ablation plume (which consists of vapor of irradiated material, burning products, drops and microparticles of various chemical composition and size) above the irradiated surface leads to a significant loss of laser energy. The fractions of the scattered and absorbed laser radiation in the plume are determined, the size and mass distributions of microparticles in the plume are estimated, their concentration is calculated, and the microparticle escape velocities from the irradiated CSCCM surface are evaluated.