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.     

Spectroscopic diagnostics of a laser erosion plasma formed from CuSbS<Subscript>2</Subscript> polycrystals

The spectra and dynamics of emission from regions of a laser plasma torch located at different distances from a polycrystalline CuSbS₂ target irradiated by a neodymium laser (W=(3–5)×10⁸ W/cm², ?=20 ns, f=12 Hz, 73x03BB;=1.06μm) were investigated. The emission data were used to estimate the average temperature (≤0.82 eV) and the electron density ((1.82?1.92)×10¹⁶ cm^?3) in the laser torch and the recombination times of ions (t _r(S²⁺)=15 ns, t _r(Cu⁺)=65?85 ns), as well as to analyze the efficiency of filling of excited atomic levels. A model describing the target destruction and the evolution of the processes accompanying spread of the laser plasma is proposed.     

Lithium diffusion in Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>-based electrodes: a joint analysis of electrochemical impedance,cyclic voltammetry,pulse chronoamperometry,and chronopotentiometry data

In order to establish the mechanism and to determine the parameters of lithium transport in electrodes based on lithium-vanadium phosphate (Li₃V₂(PO₄)₃), the kinetic model was designed and experimentally tested for joint analysis of electrochemical impedance (EIS), cyclic voltammetry (CV), pulse chronoamperometry (PITT), and chronopotentiometry (GITT) data. It comprises the stages of sequential lithium-ion transfer in the surface layer and the bulk of electrode material’s particles, including accumulation of lithium in the bulk. Transfer processes at both sites are of diffusion nature and differ significantly, both by temporal (characteristic time, τ) and kinetic (diffusion coefficient, D) constants. PITT data analysis provided the following D values for the predominantly lithiated and delithiated forms of the intercalation material: 10^?9 and 3 × 10^?10 cm² s^?1, respectively, for transfer in the bulk and 10^?12 cm² s^?1 for transfer in the thin surface layer of material’s particles. D values extracted from GITT data are in consistency with those obtained from PITT: 3.5–5.8 × 10^?10 and 0.9–5 × 10^?10 cm² s^?1 (for the current and currentless mode, respectively). The D values obtained from EIS data were 5.5 × 10^?10 cm² s^?1 for lithiated (at a potential of 3.5 V) and 2.3 × 10^?9 cm² s^?1 for delithiated (at a potential 4.1 V) forms. CV evaluation gave close results: 3 × 10^?11 cm² s^?1 for anodic and 3.4 × 10^?11 cm² s^?1 for cathodic processes, respectively. The use of complex experimental measurement procedure for combined application of the EIS, PITT, and GITT methods allowed to obtain thermodynamic E,c dependence of Li₃V₂(PO₄)₃ electrode, which is not affected by polarization and heterogeneity of lithium concentration in the intercalate.     

Spectroscopic diagnostics of the laser erosion plasma of an AgGaS<Subscript>2</Subscript> polycrystalline target

Results are presented from experimental studies of the radiation emitted from a plasma produced in vacuum after irradiating a polycrystalline target by 1.06-μm laser radiation with an intensity of (3–5)×10⁸ W/cm². Plasma radiation from regions located at distances of 1 and 7 mm from the target is analyzed. It is shown that the main contribution to the plasma radiation in the 220–600 nm spectral range is made by transitions from the excited states of single-charged Ag⁺ and S⁺ ions. The atomic component of plasma radiation is represented by intense spectral lines corresponding to transitions from the Rydberg states of Ag and Ga atoms, whereas no resonance lines of these atoms are observed.     

Pressure dependence of stimulated brillouin scattering from a hydrogen gas target

Backscattering of CO₂ laser radiation from underdense hydrogen plasma was found to increase from ～0.2 to ～12% as gas target filling pressure was increased from 20 to 130 torr. No marked change in growth constant was observed for increased radiation intensity from 1 × 10¹¹ to 3 × 10 ¹¹ W/cm².     

Neutron yield upon the irradiation of thin TiD<Subscript>2</Subscript> foils with a superintense laser pulse

The yield of neutrons from the thermonuclear-fusion reaction D(d, n)³He induced in a thin skin layer by the interaction of a high-intensity laser pulse of picosecond duration with thin TiD₂ foils is calculated. A multiple ionization of titanium atoms at the leading edge of the laser pulse is considered. The heating of free electrons proceeds via induced inverse bremsstrahlung in elastic electron scattering on multiply charged titanium ions. The electron temperature is calculated. It proves to be about 10 keV at the laser-pulse intensity of 5×10¹⁸ W/cm² at the peak. The neutron yield is estimated at 10⁴ per laser pulse. These results are in qualitative agreement with experimental data.     

Spectroscopic study of thulium-activated double sodium yttrium fluoride Na<Subscript>0.4</Subscript>Y<Subscript>0.6</Subscript>F<Subscript>2.2</Subscript>:Tm<Superscript>3+</Superscript> crystals: I. Intensity of spectra and luminescence kinetics

Na_0.4Y_0.6F_2.2:Tm³⁺ crystals with a thulium content from 1 to 100 at % have been grown by the Stockbarger-Bridgman method. The optical spectra of Na_0.4Y_0.6F_2.2:Tm³⁺ crystals were investigated in detail at room and low (10 K) temperatures, and the luminescence kinetics was analyzed using different excitation methods. The structure of the Stark splitting of thulium levels as “quasi-centers,” characterized by inhomogeneous broadening of the Stark components, is determined from analysis of the absorption spectrum at 10 K. The oscillator strengths of the transitions from the ground state to excited multiplets are determined from the absorption cross-section spectra at 300 K for ten transitions in the range 5000–38 500 cm^?1 and seven transitions in the range 5000–28 500 cm^?1. The transition intensity parameters Ω _t , obtained by the Judd-Ofelt method from the spectra due to the transitions to ten and seven excited levels, were found to be, respectively, (i) Ω₂ = 1.89 × 10^?20, Ω₄ = 2.16 × 10^?20, and Ω₆ = 1.40 × 10^?20 cm² and (ii) Ω₂ = 2.04 × 10^?20, Ω₄ = 2.01 × 10^?20, and Ω₆ = 1.44 × 10^?20 cm². These values of the intensity parameters were used to calculate the radiative transition probabilities and branching ratios and to estimate the multiphonon nonradiative transition probabilities for NYF:Tm. The luminescence decay kinetics from thulium radiative levels upon their selective excitation by nanosecond laser pulses has been studied and the lifetimes of thulium radiative levels in NYF crystals have been found.     

Variation of potential energy surface height and bound state depth induced by laser phase along the reaction path in atom-molecule reactions: Application to Li + CH<Subscript>4</Subscript> → LiH + CH<Subscript>3</Subscript>

Laser atom-molecule reaction interaction through polarizability and dipole moment contribution leads to potential energy surface barrier reshaping and bound states along the reaction path. The polarizability is maximum in the transition state. We will show here by using gauge representation (electric field gauge) for wave length λ = 20.6 μm, intensity I = 1 × 10¹² W/cm², I = 5 × 10¹² W/cm², I = 1 × 10¹³ W/cm², I = 3 × 10¹³ W/cm², that we can create laser induced potential energy surface barrier reshaping in the transition state region (–1–0.5 a. u.). We illustrate such effects for the LiH + CH₃ ? Li + CH₄ reaction with a barrier using ab-initio methods for calculating the reaction path, polarizability and dipole moment contribution of the atom-molecule reaction.     

Thulium doped monoclinic KLu(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> single crystals: growth and spectroscopy

This paper presents the crystal growth and optical characterization of thulium-doped KLu(WO₄)₂ (KLuW). Thulium-doped KLuW macrodefect-free monoclinic single crystals (a^*×b×c≈10×7×15 mm³) were grown by the top seeded solution growth slow cooling method with dopant concentrations of 0.5%, 1%, 3% and 5% atomic in solution. The evolution of unit cell parameters in relation with thulium doping was studied by X-ray powder patterns. Thulium energy levels in the KLuW host were determined by 6 K polarized optical absorption. The Judd–Ofelt parameters determined were Ω₂=9.01×10^-20 cm², Ω₄=1.36×10^-20 cm² and Ω₆=1.43×10^-20 cm². The maximum emission cross section for the 1.9 μm emission, calculated by Füchtbauer–Ladenburg method, is 1.75×10^-20 cm², at 1845 nm with E//N_m. The intensity decay time from the emitting levels ¹ G ₄ and ³ H ₄ levels in relation to the concentration were studied. For the lowest thulium concentration, the measured decay times from ¹ G ₄ and ³ H ₄ emitting levels are 140 μs and 230 μs, respectively. PACS 42.55.Rz; 78.20.-e; 78.55.-m     

Linear and nonlinear optical spectroscopy of gadolinium iron borate GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The optical spectra and the second-harmonic generation (SHG) are studied in a noncentrosymmetric GdFe₃(BO₃)₄ magnet. In the region of weak absorption (α～20–400 cm^?1) below ～3 eV, three absorption bands are distinguished, which can be unambiguously assigned to forbidden electronic transitions from the ground ⁶A₁ state of the Fe³⁺ ion to its excited states ⁴T₁(～1.4 eV), ⁴T₂(～2 eV), and ⁴A₁, ⁴E(～2.8 eV). Intense absorption begins in the region above 3 eV (α～2–4×10⁵ cm^?1), where two bands at ～4.0 and 4.8 eV are observed, which are caused by allowed electric dipole charge-transfer transitions. The spectral features of SHG in the 1.2–3.0-eV region are explained by a change in the SHG efficiency caused by a change in the phase mismatch. It is shown that in the weak absorption region, phase matching can be achieved for SHG.     

Nonlinear optical characterization of LaEr(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> thin films using the <Emphasis Type="BoldItalic">Z</Emphasis> -scan technique

The nonlinear optical properties of thin films of LaEr(MoO₄)₃ were studied using a ∼30 ps Nd:YAG laser at 532 nm with a repetition rate of 250 Hz. Closed aperture Z-scan measurement revealed a negative nonlinearity in the LaEr(MoO₄)₃. The nonlinear refractive index γ=1.38×10^-10 cm²/W and nonlinear absorption coefficient β=16.8×10^-6 cm/W were calculated from the Z-scan data. The fluorescent upconversion spectra were recorded with 980 nm excitation. An optical switching mechanism based on nonlinear absorption is also presented experimentally. PACS 81.15.Fg; 77.84.Bw; 33.50.Dq; 42.70.Mp     

Energetic ion generation by Coulomb explosion in cluster gas and a low-density plastic foam with an intense femtosecond laser

The energy distributions of protons emitted from the Coulomb explosion of hydrogen clusters by an intense femtosecond laser have been experimentally obtained. Ten thousand hydrogen clusters were exploded, emitting 8.1-keV protons under laser irradiation of intensity 6 × 10¹⁶W/cm². The energy distributions are interpreted well by a spherical uniform cluster analytical model. The maximum energy of the emitted protons can be characterized by cluster size and laser intensity. The laser intensity scale for the maximum proton energy, given by a spherical cluster Coulomb explosion model, is in fairly good agreement with the experimental results obtained at a laser intensity of 10¹⁶–10¹⁷ W/cm² and also when extrapolated with the results of three-dimensional particle simulations at 10²⁰–10²¹ W/cm². Energetic proton generation in low-density plastic (C₅H₁₀) foam by intense femtosecond laser pulse irradiation has been studied experimentally and numerically. Plastic foam was successfully produced by a sol-gel method, achieving an average density of 10 mg/cm³. The foam target was irradiated by 100-fs pulses of a laser with intensity 1 × 10¹⁸ W/cm². A plateau structure extending up to 200 keV was observed in the energy distribution of protons generated from the foam target, with the plateau shape explained well by Coulomb explosion of lamella in the foam. The laser-foam interaction and ion generation were studied qualitatively by two-dimensional particle-in-cell simulations, which indicated that energetic protons are mainly generated by the Coulomb explosion. From the results, the efficiency of energetic ion generation in a low-density foam target by Coulomb explosion is expected to be higher than in a gas-cluster target.     

Lyman alpha (1216 Å) generation in krypton

We measured the nonlinear susceptibilities of Kr up to fifth order via nonresonant third-harmonic generation (THG) of a frequency-doubled dye laser at 3648 Å. In agreement with theory the optimum intensity conversion efficiency saturates at 10^-5 in pure Kr. The phase matching of the third harmonic and fundamental wave breaks up owing to intensity-dependent changes of refractive indices. Phase matching in high-density Kr/Ar mixtures raised the 3648 Å to 1216 Å third harmonic conversion obtained in pure Kr by more than two orders of magnitude, as predicted by theory but hitherto never experimentally confirmed. With laser intensities of 5 × 10⁹ W/cm² at 3648 Å we measured conversion efficiencies of 1.4 × 10^-4 limited by gas breakdown. Taking into account the Lyman alpha absorption of Kr₂ and KrAr molecules in the high-density regime, a conversion efficiency of 10^-3 is expected from extrapolating the experimental results to Kr/Ar mixtures of up to 3 × 10³ torr. It is limited by self-focusing, assuming the minimum threshold intensity of gas breakdown is 10¹¹ W/cm², as predicted by theory.     

Conduction through localized states in a single crystal of the TlGa<Subscript>0.5</Subscript>Fe<Subscript>0.5</Subscript>Se<Subscript>2</Subscript> alloy

Layered single crystals of the TlGa_0.5Fe_0.5Se₂ alloy in a dc electric field at temperatures ranging from 128 to 178 K are found to possess variable-range-hopping conduction along natural crystal layers through states localized in the vicinity of the Fermi level. The parameters characterizing the electrical conduction in the TlGa_0.5Fe_0.5Se₂ crystals are estimated as follows: the density of states near the Fermi level N_F = 2.8 × 10¹⁷ eV^?1 cm^?3, the spread in energy of these states ΔE = 0.13 eV, the average hopping length R_av = 233 Å, and the concentration of deep-lying traps N _t = 3.6 × 10¹⁶ cm^?3.     

Spectroscopic study of double sodium-yttrium fluoride crystals doped with erbium Na<Subscript>0.4</Subscript>Y<Subscript>0.6</Subscript>F<Subscript>2.2</Subscript>:Er<Superscript>3+</Superscript>: I. Intensities of luminescence spectra and kinetics of luminescence

Na_0.4Y_0.6F_2.2:Er³⁺ (NYF:Er) crystals with an erbium concentration as high as 100 at. % (Na_0.4Er_0.6F_2.2) were grown by the Bridgman-Stockbarger method. The optical spectra were investigated at low (6 K) and room temperatures. It is shown that the absorption spectrum of NYF:Er crystals contains wide bands (790–801 and 965–980 nm) corresponding to the emission range of laser diodes. The peak absorption cross section σ_a for the band peaked at λ=970.4 nm is 0.15×10^?20 cm². On the basis of the analysis of the absorption and luminescence spectra at low (6 and 12 K) temperatures, the structure of the Stark splitting of erbium levels was determined as a structure of quasi-centers for which Stark components are inhomogeneously broadened. The oscillator strengths of the transitions from the ground state of erbium to excited multiplets were calculated from the absorption spectra measured at T=300 K, and the intensity parameters Ω_t were determined by the Judd-Ofelt method: Ω₂=1.65×10^?20 cm², Ω₄=0.56× 10^?20 cm², and Ω₆=1.01×10^?20 cm². These values of the intensity parameters were used to calculate the probabilities of radiative transitions and the branching ratios. The rates of multiphonon nonradiative transitions in NYF: Er were estimated. The luminescence decay kinetics for radiative levels of erbium ions upon their selective excitation by nanosecond laser pulses was studied. The intracenter lifetimes of radiative levels of erbium ions were determined from the luminescence kinetics upon selective ion excitation by low-intensity light in a sample with a low erbium concentration (0.5%). It is demonstrated that, with an increase in temperature from 6 to 300 K, luminescence from the ⁴ G _11/2, ² G(H)_9/2, and ⁴ F _9/2 levels is quenched as a result of multiphonon nonradiative transitions. Luminescence from the ⁴ I _9/2 level is quenched only insignificantly with increasing temperature, and no quenching of luminescence from the ⁴ I _11/2 and ⁴ I _13/2 levels is observed. The spectra of steady-state luminescence of NYF:Er(0.5–15%) crystals were investigated upon broadband excitation by UV and UV-visible lamp light and selective time-resolved laser excitation. It is shown that low-lying levels of erbium ions separated by an energy gap smaller than 2500 cm^?1 are populated via cascade mechanisms. On the basis of the results obtained, it is concluded that NYF:Er ³⁺ crystals are promising candidates for active media of tunable diode-pumped lasers.     

A study of the nonlinear refractive index in amorphous Ge–As–S layers and nanocomposites with gold nanoparticles

Nonlinear refractive indices of simple and composite chalcogenide glasses with gold nanoparticles are measured by the Z-scan method using a femtosecond Ti:sapphire laser with a central wavelength of 800 nm and a pulse duration of 40 fs. It is shown that introduction of nanoparticles into thin layers of amorphous GeS₂, As₃₀Ge₂₀S₅₀, and As₃Ge₃₅S₆₂ leads to a decrease in the total nonlinear refractive index due to additional absorption of femtosecond laser radiation without efficient excitation of localized plasmons outside the resonance region. For example, the nonlinear refractive index decreases with addition of gold nanoparticles from 16.1 × 10^–12 to 13.0 × 10^–12 cm²/W for GeS₂, from 3.9 × 10^–12 to 3.2 × 10^–12 cm²/W for As₃₀Ge₂₀S₅₀, and from 5.8 × 10^–12 to 4.7 × 10^–12 cm²/W for As₃Ge₃₅S₆₂.     

Spectroscopic study of erbium-activated crystals of double calcium yttrium fluoride Ca<Subscript>0.89</Subscript>Y<Subscript>0.11</Subscript>F<Subscript>2.11</Subscript>:Er<Superscript>3+</Superscript>: I. Intensity of spectra and luminescence kinetics

Ca_0.89Y_0.11F_2.11:Er³⁺ (CYF:Er) crystals with an erbium content of 1–15 at % have been grown. The optical spectra and luminescence kinetics of CYF:Er crystals have been investigated at low (～5 K) and room temperatures. Based on an analysis of the absorption spectra at low temperature, the structure of Stark splitting of erbium levels in CYF:Er crystals is determined. Room-temperature absorption spectra are used to calculate the spectra of absorption cross sections and oscillator strengths of transitions from the erbium ground state to excited multiplets. It is shown that the absorption spectrum of CYF:Er crystals contains broad bands in the ranges of 790–815 and 965–980 nm, which correspond to the range of emission of laser diodes. For the band peaking near 967 nm, the peak absorption cross section is σ _abs ^max = 2.7 × 10^?21 cm². The intensity parameters are determined by the Judd-Ofelt method to be Ω₂ = 1.39 × 10^?20, Ω₄ = 1.34 × 10^?20, and Ω₆ = 2.24 × 10^?20 cm². The radiative transition probabilities, radiative lifetimes, and branching ratios are calculated with these values. The luminescence decay kinetics from excited erbium levels upon selective excitation is investigated and the experimental lifetimes of the 4F _9/2, ⁴ S _3/2, and ⁴ G _11/2 radiative erbium levels are determined. The dependences of multiphonon relaxation rates on the energy gap in CYF:Er crystals are obtained. The rates of nonradiative multiphonon relaxation from radiative erbium levels are determined.     

Spectroscopic properties of Nd<Superscript>3+</Superscript>:CaNb<Subscript>2</Subscript>O<Subscript>6</Subscript> crystal

The absorption spectra, fluorescence spectrum and fluorescence decay curve of Nd³⁺ ions in CaNb₂O₆ crystal were measured at room temperature. The peak absorption cross section was calculated to be 6.202×10⁻²⁰ cm² with a broad FWHM of 7 nm at 808 nm for E//a light polarization. The spectroscopic parameters of Nd³⁺ ions in CaNb₂O₆ crystal have been investigated based on Judd-Ofelt theory. The parameters of the line strengths Ω _t are Ω ₂=5.321×10⁻²⁰ cm²,Ω ₄=1.734×10⁻²⁰ cm²,Ω ₆=2.889×10⁻²⁰ cm². The radiative lifetime, the fluorescence lifetime and the quantum efficiency are 167 μs, 152 μs and 91%, respectively. The fluorescence branch ratios are calculated to be β ₁=36.03%,β ₂=52.29%,β ₃=11.15%,β ₄=0.533%. The emission cross section at 1062 nm is 9.87×10⁻²⁰ cm².     

Charge carrier mobility in crystals of the Pb<Subscript>0.67</Subscript>Cd<Subscript>0.33</Subscript>F<Subscript>2</Subscript> superionic conductor

The frequency (ν = 10^?1–10⁷ Hz) dependences σ(ν) of the conductivity of single crystals of the Pb_0.67Cd_0.33F₂ superionic conductor with the fluorite-type structure (CaF₂) in the temperature range of 132–395 K have been studied. The dependences σ(ν) have been discussed in the framework of the hopping relaxation of ionic carriers, which are mobile anions F^?. From experimental curves σ(ν), the direct-current (dc) conductivity σ_dc and the average charge carrier hopping frequency ν_h have been determined. This has made it possible to calculate the charge carrier mobility μ_mob and charge carrier concentration n _mob in these crystals. At room temperature (293 K), the electrical parameters are σ_dc = 1.6 × 10^?4 S/cm, ν_h = 2.7 × 10⁷ Hz, μ_mob = 2.0 × 10^?7 cm²/(s V), and n _mob = 5.1 × 10²¹ cm^?3.     

Mechanisms of low-temperature high-frequency conductivity in systems with a dense array of Ge<Subscript>0.7</Subscript>Si<Subscript>0.3</Subscript> quantum dots in silicon

High-frequency (HF) conductivity in systems with a dense (with a density of n = 3 × 10¹¹ cm^?2) array of self-organized Ge_0.7Si_0.3 quantum dots in silicon with different boron concentrations n_B is determined by acoustic methods. The measurements of the absorption coefficient and the velocity of surface acoustic waves (SAWs) with frequencies of 30–300 MHz that interact with holes localized in quantum dots are carried out in magnetic fields of up to 18 T in the temperature interval from 1 to 20 K. Using one of the samples (n_B = 8.2 × 10¹¹ cm^?2), it is shown that, at temperatures T ≤ 4 K, the HF conductivity is realized by the hopping of holes between the states localized in different quantum dots and can be explained within a two-site model in the case of

Open image in new window

, where ω is the SAW frequency and τ₀ is the relaxation time of the populations of the sites (quantum dots). For T > 7 K, the HF conductivity has an activation character associated with the diffusion over the states at the mobility threshold. In the interval 4 K < T < 7 K, the HF conductivity is determined by a combination of the hopping and activation mechanisms. The contributions of these mechanisms are distinguished; it is found that the temperature dependence of the hopping HF conductivity approaches saturation at T* ≈ 4.5 K, which points to a τ₀ ≤ 1. A value of τ₀(T*) ≈ 5 × 10^?9 s is determined from the condition ωτ₀(T*) ≈ 1.