V. V. Platonov

Laser synthesis of nanopowders with yttrium aluminum garnet stoichiometry

A nanopowder of yttria-alumina mixture with the yttrium aluminum garnet (YAG) stoichiometry has been synthesized for the first time by the laser evaporation method. A high-power CO2 laser with pulse duration above 200 μs, repetition frequency of 500 Hz, and pulse energy of about 1 J provided a high yield of powder at a rate of 24 g/h. The obtained nanopowder has been used to prepare YAG:Nd3+ ceramics with a cubic structure possessing an optical transmittance of about 77% at a wavelength of 1.06 μm. The successful synthesis of YAG nanopowder is based on the preliminary numerical simulation of the laser evaporation of a target using a three-dimensional model.

Dynamics and spectroscopy of the laser plume from solid targets

The dynamics and the spectral kinetic characteristics of the plume emerging in the vicinity of graphite targets, pressed pellets consisting of zirconium oxide powder stabilized with yttrium (YSZ) and yttrium-aluminum oxides with neodymium (YAO:Nd), and single-crystal YAG:Cr are studied. The targets are irradiated in air at room temperature using a repetitively pulsed CO2 laser with a wavelength of 10.6 μm, a peak power of up to 9 kW, a pulse energy of 1.69 J, and a pulse duration of 330 μs at a level of 0.1. The plume propagates normally to the target surface at an angle of 45° relative to the laser radiation. The spectral kinetic characteristics of the plume luminescence are discretely measured along the entire length. It is demonstrated that the plumes of all targets (except for the single-crystal YAG:Cr) represent the flows of a weakly nonequilibrium gas plasma with a temperature of 10 kK (graphite) and 3.1–4.7 kK (YSZ and YAO:Nd pressed pellets). The plume size is determined by the peak power of the laser pulse. The luminescence of the two-atom radicals (C2 in graphite; ZrO and YO in YSZ; and YO, AlO, and NdO in YAO:Nd) dominates in all of the plumes. A single radical (YO) and the spectral lines of atoms and atomic ions are observed in the YAG:Cr plume. A relatively high temperature of the graphite plume is maintained owing to the energy of the exothermic reaction involving the association of carbon atoms and the energy of the vibrationally excited molecules resulting from this reaction.

Effect of pulses from a high-power ytterbium fiber laser on a material with a nonuniform refractive index. I. Irradiation of yttrium oxide targets

The irradiation of Nd:Y2O3 targets with an absorption coefficient of 13–1.7 × 103 cm−1 using laser pulses with a duration of 0.1–3.5 ms and peak power of 200–700 W at a power density of (0.2–1.3) × 106 W/cm2 is studied. A relatively large spread of the delay times of laser plume, spike emission of the laser plume, cleavage of the front surface of the target, and greater ejection of substance from the crater in comparison with the effect of the CO2-laser radiation with almost the same power are demonstrated. A numerical model of the effect of radiation on a target with a nonuniform refractive index is proposed to interpret the destruction of dielectric material (cleavage of the front surface) and the large spread of the delay times of the plume.

Effect of pulses from a high-power ytterbium fiber laser on a material with a nonuniform refractive index. II. Production and parameters of Nd:Y2O3 nanopowders

The laser ablation of the Nd:Y2O3 target with substantially nonuniform refractive index leads to the formation of a needle-shaped surface with a needle height of 6–8 mm. An increase in the displacement velocity of the laser beam on the surface to 80 cm/s and an increase in the diameter of the laser spot at the central part of the beam waist to 430 μm lead to a more uniform relief of the target surface and an increase in the nanopowder yield and production rate to 22% and 23 g/h, respectively. In addition, an excess of the mole content of the low-melting Nd2O3 in the powder decreases from 174 to 11% in comparison with the target. At an air pressure in the evaporation chamber of 0.8 bar, the mean sizes of nanoparticles (13–14 nm) are virtually independent of the displacement velocity of the beam on the surface (7–81 cm/s) and the rate of air flow above the target (13–70 m/s) in spite of significantly different nanopowder production rates.

Interaction of the radiation of the high-power ytterbium-fiber laser with inhomogeneous dielectric targets

The action of the radiation of the ytterbium-fiber laser (λ = 1.07 μm) on the Nd3+Y2O3 target with nonuniform transparency in the course of the nanopowder production is studied. It is demonstrated that the laser irradiation leads to an extremely rough surface with the stalagmite roughness due to a relatively large melting depth. The resulting powder consists of two fractions. The first fraction (99% of the total mass of the powder) consists of nanoparticles with a mean size of 29 nm (BET data). The second fraction consists of micro- and submicroparticles that represent circular drops condensed from the melt and shapeless debris of the target. The peaks on the diameter distribution of the drops at 2, 8, and 80 μm are determined by different effects. The laser heating of the inhomogeneous target with the nonlinear refractive index is numerically analyzed. It is demonstrated that the melting of the target is initiated at a mean laser power of 700 W, a power density of 5.6 × 105 W/cm2, and an irradiation time of 150 μs.

Nonlinear dynamics of a plasma torch generated by a laser pulse of large width

Experimental data on the nonlinear dynamics of a plasma torch generated by a laser pulse of large width acting upon a graphite target are presented and discussed. The mushroom shape of the luminous region and the duration of emission observed in experiment are explained by the development of the Richtmyer-Meshkov instability at the carbon plasma-air interface and by the formation of nanoparticles in the plasma expanding into the buffer gas.

Laser plume evolution in the process of nanopowder preparation using an ytterbium fibre laser

We study the laser plume dynamics by means of high-speed shooting in the intrinsic light, as well as using the shadow method with a laser monitor. It is found that the laser plume arising under the impact of a radiation pulse from an ytterbium fibre laser with a power of on a target with an Nd concentration of 1 mol % is first a plasma consisting of the target material vapour, and then becomes a mixture of vapour and droplets. The first droplets in this plasma appear in after the formation of the laser plume, and in the major part of the substance is removed in the form of liquid droplets. We have also found that the depth of the laser-produced crater linearly depends on the laser pulse duration, thus confirming the absence of essential shielding of laser radiation by melt droplets. The higher the target transparency, the longer the delay time of the formation of the laser plume and the greater its spread. Sometimes, instead of the laser plume formation, one can observe a light flash inside a semitransparent target. The explanation of these results is presented.

Fabrication and Properties of Neodymium-Activated Yttrium Oxide Optical Ceramics

About new technology of production of transparent ceramics including laser synthesis of nanopowders, their magnetic pulsed compaction and vacuum sintering is reported. The results of investigations of the synthesized samples of transparent ceramics made from neodymium-activated yttrium oxide are presented. It has been shown that in a 1.1 mm thickness sample with optical loss coefficient α1.07μm = 0.03 cm−1 laser generation at λg ∼ 1.08 μm with a slope efficiency of 15% at laser diode pumping at a wavelength of 807 nm has been obtained.

Spectroscopy of a laser plume arising under radiation of a ytterbium fiber laser

The spectra of luminescence of plumes that occur near targets of Nd: Y2O3, YSZ, and Al2O3 when they are irradiated by pulses of a ytterbium fiber laser with a wavelength of 1.07 μm, duration of 1450 μs, and intensity of 0.4 MW/cm2 are studied. Craters with a diameter of 400 μm and a depth of 600 μm appeared under such exposure in the targets. It is shown that the bands of the cation’s radicals of the targets, the intensities of which are distributed according to a law close to Planck’s law, predominate in the spectra of the plumes. On this basis, the temperature of the plumes was determined. It was about 2200–2280 K at the surface of the target, which is below the boiling temperature of the target due to cooling of the vapor during the passage of the deep laser crater.

On the mechanism of deep craters formation under the action of high power ytterbium-fiber laser

Stability of a liquid crater wall formed under the action of an ytterbium-fiber laser in the course of the Nd3+:Y2O3 nanopowder production is studied theoretically. It has been shown that hydrodynamic instability can develop on the melt-vapor interface as a result of the tangential discontinuity of the velocity between the vapor stream and molten crater wall. The characteristic spatial and temporal scales are estimated in the framework of the proposed qualitative model, they are found to be 20-90 μm and 20-50 μs, respectively, that is in good agreement with experimental data. Thus, the droplet formation time (during which the amplitude of the boundary perturbation reaches the wavelength order) is much smaller than a pulse duration of the ytterbium-fiber laser (1360 μs). This means that a significant amount of material can be removed from the crater due to formation of microscale droplets during the irradiation. This mechanism can explain the much greater crater depth for the fiber laser than for CO2 laser (a pulse duration for which is 370 μs).