Nonequilibrium laser-produced plasma of volume-structured media and inertial-confined-fusion applications

We review the results of experimental and theoretical studies of the properties of a nonequilibrium plasma produced from volume-structured media, containing micro- and nano-size internal elements, under laser-pulse irradiation. We consider two types of materials, i.e., regularly and stochastically structured materials. The first type is either a set of flat layers or cylindrical and spherical shells of micrometer thickness, and the second type is either foams of light elements or light foams containing clusters of heavy elements with dimensions in the range of 10–100 nm. We study the properties of high-temperature laser-produced plasmas of such materials and applications directed to developing the design of inertial confinement fusion (ICF) targets and creating powerful sources of thermonuclear neutron and soft X-ray emission initiated by the laser pulse. The foam materials can be used as absorbers capable of providing homogeneity of laser-energy absorption by the target. A neutron yield up to 1014−1015 DT neutrons per shot can be achieved by heating regularly structured materials using a laser pulse in the regime of the consequent thermal explosions of solid elements containing isotopes of hydrogen. Laser-radiation conversion into soft X-ray emission with the efficiency controlled in a wide range may be realized in laser-produced plasmas of porous media doped with clusters of heavy elements. In particular, such a material can be used as an absorber–converter of laser radiation in inertial confinement fusion targets. Under direct irradiation of an ICF target by a laser pulse, such a converter can provide transformation of 20–30% of the absorbed laser energy into the energy of X-ray radiation transferred to thermonuclear capsules.     

Dynamics of radiation scattering by particles of condensed phase in quasicontinuous laser irradiation of metals

Radiation scattering by particles of condensed phase in an ablation plasma plume has been experimentally studied during quasicontinuous laser irradiation (λ = 1.06 μm, q = 0.1–9 MW/cm², τ ∼ 1.5 msec) of duraluminum D16T, aluminum A99, and bismuth. The particle size distribution and the nature of their dispersal during irradiation was studied in scattered light (λ = 0.69 μm) from individual particles that could be visually observed on photographs. It was found that under the pressure developed in the plume, large particles ejected from the irradiated zone can move backward and return to the target (D16T). The plume (Bi) becomes brighter due to ablation of particles in the path of the laser beam. The directional scattering coefficients for scattering from the local zone on the axis of the plume, measured during the laser pulse, were used to study the relationship between the dynamics of entry of condensed phase into the plume, shielding of the target by the particles, and brightening of the plume under the action of the incident laser radiation. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 2, pp. 210–219, March–April, 2006.     

Ponderomotive action of light in the problem of multiple scattering of light in a randomly inhomogeneous medium

The time correlation function of light reflected diffusely from a semi-infinite randomly inhomogeneous medium is calculated with allowance for the acceleration of the scatterers in the field of the laser beam incident on the medium. An analytical expression is found for the characteristic coherence time due to the ponderomotive action of light. It is shown that even with laser radiation power densities of the order of 1–10 W laser-acceleration effects substantially alter the character of the time autocorrelation function of the scattered light and must be taken into account in theoretical calculations. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 611–615 (10 May 1998)     

金等离子体M带谱强度的定量测量

在"神光II"多束高功率激光装置上利用列阵透镜匀滑钕玻璃波长0.53μm的强激光幅照平面金(Au)靶时产生X射线,本文给出了X射线绝对转换效率ξx。研究了多束倍频激光叠加驱动靶形成X射线背景光源辐射金M壳层1.8—3.1Kev带谱的特性,获得了不同激光功率密度及不同角度驱动靶面等几种条件下X射线能谱的定量测量结果和能谱分布。     

Self-grown fiber fabrication by two-photon photopolymerization

We demonstrate a new fiber growth mechanism in a photocurable resin by ultrafast laser illumination. A high-repetition rate (∼1 MHz) ultrafast laser beam at the wavelength of ∼523 nm was focused into an ultraviolet photocurable resin to trigger two-photon photopolymerization process. Time-resolved shadowgraphs and scattered light imaging revealed that the curing commenced in the neighborhood of the geometric focal point of the laser beam and that the fiber growth progressed mostly towards the laser source. The cured fiber was thinner and longer than the profile of the focused laser beam, facilitated by nonlinear propagation and absorption of the ultra-fast laser beam. The achieved aspect ratio of the fiber was higher than 180 with ∼10 μm mean diameter, and the average growth rate was up to ∼2 mm/s.     

Light-induced frequency shift of stimulated Raman scattering in Bessel light beams

The medium susceptibility related to stimulated Raman scattering (SRS) is known to be a complex value whose imaginary part determines the SRS gain coefficient whereas the real part constitutes a small fraction of the refractive index at the frequencies of the scattered emission that is proportional to the intensity of the exciting beam. Strong transverse nonuniformity of this beam causes focusing of high-frequency components of the scattered light and defocusing of low-frequency ones that inevitably affect their amplification efficiency. The effect for Gaussian pumping is responsible for a slight shift of the SRS gain spectrum towards higher frequencies. The present work analyzes theoretically SRS excited by a Bessel light beam and demonstrates that the axial component of the scattered emission experiences a frequency shift that is much greater than that for the case of Gaussian beam pumping. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 336–341, May–June, 2008.     

Planarity and stability of shock driven directly by the ninth laser beam from “Shenguang-II" laser facility

Using the ninth laser beam (converted to 2ω) of “Shenguang-II” laser facility and the beam smoothing technology of lens-array [Appl. Opt. 25, 377 (1986); Phys. Plasmas. 9, 3201 (1995)], a shock wave with 700 μm (the root-mean-square of shock breakout time (RMS) RMS ≈ 6.32 ps) flat top was created. An Al-Al four-step target was designed to do research on shock wave stability in an Al target. And the shock stability experiment with the Al-Al four-step target indicated that the shock wave steadily propagated in the Al target of thickness of about 20–45 μm under the power density of ~ 1.0×10¹⁴ W/cm².     

Laser/sol–gel synthesis: a novel method for depositing nanostructured TiN coatings in non-vacuum conditions

In this work results of experiments on the in situ production of titanium nitride by the reaction of titania sol–gel with a nitrogenous admixture under laser irradiation are reported. A diode laser beam at different powers and traverse speeds was applied to the mixture placed on EN43 mild steel and 316L stainless steel substrates. Composite coatings of titanium nitride and titanium oxide with a hardness of 17–21 GPa have been achieved by this new method. Surface morphology and microstructure of the deposited coatings and substrate surface layers were examined using optical microscopy, scanning electron microscopy, and field-emission gun scanning electron microscopy. Chemical composition was determined by energy-dispersive X-ray analysis. The phases were identified by X-ray diffraction. Results of microhardness and nanohardness at the top surface were evaluated. PACS 81.15.Fg; 81.20.Fw; 81.05.-t     

M<Superscript>2</Superscript> of MOPA CuBr Laser Radiation

The beam propagation factor, M² of the master-oscillator power-amplifier (MOPA) CuBr laser emission compliant with ISO 11146 is studied methodically. Statistical parameters of 2D intensity profile of the near and far fields of MOPA laser radiation are measured by a beam analyzing technique as functions of timing delay between MO and PA. For first time the influence of the gas buffer (causing the radiation profile to change from annular to top-hat and Gaussian-like) and light polarization on CuBr laser beam focusability (M²) was under investigation. The MOPA gain curve is found and the influence of gain on the input signal (from MO into PA) due to the absorption/amplification in PA on the field profiles is shown. For annular radiation M² range is from 13–14 (small delays) to 5–6 (large delays) and for filled-center radiation M² is 6–7 (small delays) and at the end of gain curve is as much as 4. With polarized light, M² drops to 3 at the end of gain curve. The brightness of laser emission with hydrogen goes up 3–5 times and the linearly-polarized beam is at least 40% brighter than that of partial or non-polarized beams.     

High-resolution spectroscopy with and frequency stabilization of a cw dye laser

The hyperfine splittings of the Na D₁ and D₂ lines were investigated using a single mode cw dye laser. The light of the laser was scattered by the atoms of an atomic beam and the fluorescent light was observed as the frequency of the laser was tuned across the D lines. The Doppler width of the atomic beam was reduced to about 2.5 MHz so that the absorption width of the atoms of the beam was essentially determined by the natural width of the 3²P_1/2 and 3²P_3/2 levels, which is about 10 MHz. Since the linewidth observed for the hyperfine transitions was 30 MHz, most of the hyperfine components of the D₁ and D₂ lines could be resolved. In another experiment the frequency of the dye laser was locked to a hyperfine transition of the D₁ line. The observed variation of the output frequency of the dye laser was less than ±1.5 MHz. In addition, the intensity of the dye laser was controlled to about 10⁻³, using an electro-optically variable transmission filter.     

Direct patterning of gold oxide thin films by focused ion-beam irradiation

For direct writing of electrically conducting connections and areas into insulating gold oxide thin films a scanning Ar⁺ laser beam and a 30 keV Ga⁺ focused ion beam (FIB) have been used. The gold oxide films are prepared by magnetron sputtering under argon/oxygen plasma. The patterning of larger areas (dimension 10–100 μm) has been carried out with the laser beam by local heating of the selected area above the decomposition temperature of AuO_x (130–150 °C). For smaller dimensions (100 nm to 10 μm) the FIB irradiation could be used. With both complementary methods a reduction of the sheet resistance by 6–7 orders of magnitude has been achieved in the irradiated regions (e.g. with FIB irradiation from 1.5×10⁷ Ω/□ to approximately 6 Ω/□). The energy-dispersive X-ray analysis (EDX) show a considerably reduced oxygen content in the irradiated areas, and scanning electron microscopy (SEM), as well as atomic force microscopy (AFM) investigations, indicate that the FIB patterning in the low-dose region (10¹⁴ Ga⁺/cm²) is combined with a volume reduction, which is caused by oxygen escape rather than by sputtering. Received: 30 May 2000 / Accepted: 31 May 2000 / Published online: 13 July 2000     

Localized insulator-conductor transformation of graphene oxide thin films via focused laser beam irradiation

By scanning a focused laser beam over graphene oxide (GO) film deposited on SiO₂/Si substrates, conductive strips as small as 1 μm can be patterned directly either as a channel in the insulating matrix, or as a stand-alone micro belt. The conductivity was increased by at least two orders of magnitude with the mobility estimated in the range of 1–10 cm²/V s. Raman mapping and X-ray photoelectron spectroscopy studies demonstrated the reduction of GO in the laser-irradiated area. The conductance of the patterned channel was independent of the change in oxide-electrode contact resistance of the graphene, and increased linearly with increasing channel width. Increasing irradiation power by repeated scanning initially increased the conductivity of the irradiated area and saturated at a conductivity of ∼36 S/cm. Partial oxidative burning combined with photothermal reduction was identified as the underlying mechanism for the enhancement of the conductivity after laser irradiation on the GO film. Oxidative burning can be controlled by varying the film thickness and laser power.     

Dynamics of femtosecond-laser-ablated liquid-aluminum nanoparticles probed by means of spatiotemporally resolved X-ray absorption spectroscopy

We investigated spatiotemporal evolution of expanding ablation plume of aluminum created by a 100-fs, 10¹⁴–10¹⁵-W/cm² laser pulse. For diagnosing dynamic behavior of ablation plume, we employed the spatiotemporally resolved X-ray absorption spectroscopy (XAS) system that consists of a femtosecond-laser-plasma soft X-ray source and a Kirkpatrick–Baez (K–B) microscope. We successfully assigned the ejected particles by analyzing structure of absorption spectra near the L _II,III absorption edge of Al, and we clarified the spatial distribution of Al⁺ ions, Al atoms, and liquid droplets of Al in the plume. We found that the ejected particles strongly depend the irradiated laser intensity. The spatial distribution of atomic density and the expansion velocity of each type of particle were estimated from the spatiotemporal evolution of ablation particles. We also investigated a temperature of the aluminum fine particles in liquid phase during the plume expansion by analyzing the slope of the L _II,III absorption edge in case of 10¹⁴-W/cm² laser irradiation where the nanoparticles are most efficiently produced. The result suggests that the ejected particles travel in a vacuum as a liquid phase with a temperature of about 2500 to 4200 K in the early stage of plume expansion.     

Surface nanostructuring of silicon

Irradiation with polarized laser light of 248-nm wavelength induces the formation of periodic undulations ∼10-nm-highon flat silicon substrates. The wavelength of these periodic structures is a function of the light wavelength and the angle of incidence of the laser beam. Linear arrays of silicon nanoparticles with fairly uniform size that extended up to a millimeter were formed if the irradiation was performed using polarized light. When non-polarized laser light with the same fluence was used to illuminate an initially flat surface, non-aligned nanoparticle strings were obtained. However, if part of the irradiated area was microstructured, nanoparticle linear arrays resulted in the vicinity of the microstructured region. An analysis on the evolution of these nanostructures is presented. Nanocolumns could be grown on top of every cone of a microstructured surface upon cumulative laser irradiation with non-polarized light, reaching a height of ∼3 μm and a diameter of 100–200 nm. The mechanisms of nanocolumn origin and growth are analyzed. Received: 16 December 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-865/974-4115, E-mail: apedraza@utk.edu     

Energy of a shock wave generated in different metals under irradiation by a high-power laser pulse

The energies of a shock wave generated in different metals under irradiation by a high-power laser beam were determined experimentally. The experiments were performed with the use of targets prepared from a number of metals, such as aluminum, copper, silver and lead (which belong to different periods of the periodic table) under irradiation by pulses of the first and third harmonics of the PALS iodine laser at a radiation intensity of approximately 10¹⁴ W/cm². It was found that, for heavy metals, like for light solid materials, the fraction of laser radiation energy converted into the energy of a shock wave under irradiation by a laser pulse of the third harmonic considerably (by a factor of 2–3) exceeds the fraction of laser radiation energy converted under irradiation by a laser pulse of the first harmonic. The influence of radiation processes on the efficiency of conversion of the laser energy into the energy of the shock wave was analyzed.     

Femtosecond laser plasma in CaF<Subscript>2</Subscript> crystal microchannel and effective generation of characteristic X-ray radiation

The dependence of the characteristic X-ray radiation yield from CaF₂ crystal on the formed microchannel depth under highly intensive (I ∼ 3 × 10¹⁵ W/cm²) laser pulses with different contrast was obtained. The maximum of the characteristic X-ray radiation yield at these experimental conditions corresponded to the microchannel depth of 30–50 μm. The efficiency of the laser radiation conversion to the characteristic X-ray radiation increased from 6 × 10⁻⁸ for the surface up to 10⁻⁷ in the microchannel. The dependence of the characteristic X-ray radiation yield on the viewing angle showed that the source of X-ray radiation was located near the surface inside the microchannel.     

Enhancement of hard X-ray yield under the interaction of Cr: Forsterite femtosecond laser radiation with Xe clusters

Experiments on the interaction of femtosecond laser radiation (1240 nm, 140 fs,10¹⁶W/cm²) with a Xe cluster beam in a Xe-Ne binary mixture are carried out. The formation of Xe clusters in the presence of light carrier gas (Ne) is found to narrow the Xe cluster beam and enhance the x-ray yield (at about 4 keV). X-ray generation efficiency is about 10⁻⁸. Original Russian Text ? A.P. Golubev, V.M. Gordienko, M.S. Dzhidzhoev, I.A. Makarov, D.N. Trubnikov, 2009, published in Vestnik Moskovskogo Universiteta. Fizika, 2009, No. 2, pp. 112–114.     

Enhanced spatiotemporal laser-beam smoothing in gas-jet plasmas

Spatiotemporal smoothing of large-scale laser intensity fluctuations is observed for a laser beam focused into underdense helium plasmas. This smoothing is found to be severely enhanced when focusing the laser beam into a helium gas jet. In contrast to other experiments with preformed plasmas, the average and the peak laser intensities are well below the threshold for ponderomotive self-focusing. The coherence characteristics of the transmitted light are measured for various electron densities, and the smoothing effect is explained by multiple scattering of laser light on self-induced density perturbations.     

Self-action of Bessel light beams in cubic gyrotropic photorefractive crystals

We have studied conversion and interaction of a Bessel laser beam of zero order in photorefractive crystals. We have established that in a cubic gyrotropic crystal, two Bessel light beams propagate with different wave vectors and cone angles, but with identical conicity parameters. As the power in the Bessel light beam increases, we have experimentally observed a new optical effect: self-action of a Bessel light beam. We have studied the effect of external influences and the parameters of the beam itself on the process of self-refraction in photorefractive crystals. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 4, pp. 488–493, July–August, 2006.