Experimental study of optical gas-dynamic processes of polymeric material ablation by ultrashort laser pulses

Optical gas-dynamic processes occurring in polymeric targets ((CH₂O) _n , (C₂F₄) _n ) exposed to ultrashort laser pulses (τ _0.5 ∼ 45 − 70 fs; λ _I,II,III = 266, 400, 800 nm; and E/S ∼ 0.1 − 40 J/cm² at r ₀ ∼ 20 μm) were studied under normal conditions and in vacuum (p ∼ 10⁻² Pa). The dynamics of the mass flow from the target surface (m′ ∼ 10⁻⁵ − 10⁻⁴ g/J) was studied and the spectral-energy thresholds of laser ablation, the electron density distribution (n _e ∼ 10¹⁴ − 10¹⁸ cm⁻³), the mass-averaged velocity of the material flow from the target surface (∼ 10³ m/s), and the chemical composition and average temperature in the near-surface plasma formation (T ∼ 5000 K) were determined using interference microscopy, emission spectroscopy, and shadowgraphy.     

Decay of an electronic vortex of a collisionless shock wave as a possible mechanism of a “Coulomb explosion”

A new mechanism of a “Coulomb explosion,” where ions are accelerated by the electric field separating charges at the magnetic Debye radius r _B∼B/4πen _e, is proposed on the basis of a nonquasineutral model of electronic vortices in a magnetic field. It is shown by means of numerical calculations that in the process of acceleration of the ions a collisionless shock wave, whose front has an effective width of the order of δ∼r _B, determined by the breakdown of quasineutrality, is formed in a time of the order of ω _pi ⁻¹ , where ω_pi is the ion plasma frequency. The origin of such explosive dynamics is the formation of “holes” in the electron density at characteristic times of the order of ω _pe ⁻¹ (ω_pe is the electronic plasma frequency) as a result of the generation of electronic vorticity by the Weibel instability of an electromagnetic wave. Calculations for a laser pulse with intensity J∼6×10¹⁸ W/cm² show that the ions expand in the radial direction with velocities up to 3.5×10⁸ cm/s. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 10, 669–674 (25 November 1999)     

Small-scale instability of a pulse front traversing a resonant saturable absorber

Wave front of a light pulse is shown to be unstable as it propagates through a resonant saturable absorber, if its frequency is higher than the resonance frequency of the absorber. When ΔωT ₂∼1, a small-scale transverse instability with the dimension of (λl _abs)^1/2 grows rapidly. Its growth-rate is of the order of the small-signal-absorption length of the medium.     

Determination of nonlinear σ-ω-ρ model parameters in the relativistic mean-field theory by nuclear-matter properties

The parameters of the σ-ω-ρ model in the relativistic mean-field theory with nonlinear σ-meson self-interaction are determined by nuclear-matter properties, which are taken as those extracted by fits to data based on nonrelativistic nuclear models. The values of the relevant parameters are C _σ ²∼ 94, C _ω ²∼ 32, C _ρ ²∼ 26, b∼ - 0.09, c∼ 1, and the σ-meson mass m _σ∼ 370 MeV, while the value of the calculated nuclear- surface thickness is t∼ 1.4 fm. The field system is shown to be stable, since the σ-meson self-interaction energy is a lower bound in this whole parameter region with positive c. On the other hand, the effective nucleon mass M^* is larger than 0.73M, if the symmetry incompressibility K_s is assumed to be negative and the nuclear-matter incompressibility K₀ is kept less than 300 MeV. Received: 27 June 2001 / Accepted: 5 October 2001     

Fractal simulation of the resistivity and capacitance of arsenic selenide

The temperature dependences of the ac resistivity R and ac capacitance C of arsenic selenide were measured more than four decades ago [V. I. Kruglov and L. P. Strakhov, in Problems of Solid State Electronics, Vol. 2 (Leningrad Univ., Leningrad, 1968)]. According to these measurements, the frequency dependences are R ∝ ω^{−0.80±0.01} and ΔC ∝ ω^{−0.120±0.006} (ω is the circular frequency and ΔC is measured from the temperature-independent value C ₀). According to fractal-geometry methods, R ∝ ω^1−3/h and ΔC ∝ ω^−2+3/h , where h is the walk dimension of the electric current in arsenic selenide. Comparison of the experimental and theoretical results indicates that the walk dimensions calculated from the frequency dependences of resistivity and capacitance are h _R = 1.67 ± 0.02 and h _C = 1.60 ± 0.08, which are in agreement with each other within the measurement errors. The fractal dimension of the distribution of conducting sections is D = 1/h = 0.6. Since D < 1, the conducting sections are spatially separated and form a Cantor set.     

“Quasi-Planck” spectra of capillary turbulence on the surface of liquid hydrogen

We report the first observation of “quasi-Planck” spectra of capillary turbulence on the surface of liquid hydrogen in the dissipation domain. Capillary waves have been driven by low-frequency random force. We have observed that the frequency spectrum of surface elevation changes its dependence from power-like 〈|η_ω²|〉 ∼ ω^−2,8 at middle-frequency domain to “quasi-Planck” distribution ∼e ^ω/ω _d at higher frequencies. The frequency ω _d is proportional to the boundary frequency between inertial interval and dissipation domain and it is scaled up with the increase of driving force.     

Sub-THz radiation room temperature sensitivity of long-channel silicon field effect transistors

Room temperature operating n-MOSFETs (n-type metal-oxide silicon field effect transistors) used for registration of sub-THz (sub-terahertz) radiation in the frequency range ν = 53−145 GHz are considered. n-MOSFETs were manufactured by 1-μm Si CMOS technology applied to epitaxial Si-layers (d ≈15 μm) deposited on thick Si substrates (d = 640 μm). It was shown that for transistors with the channel width to length ratio W/L = 20/3 μm without any special antennas used for radiation input, the noise equivalent power (NEP) for radiation frequency ν ≈76 GHz can reach NEP ∼6×10⁻¹⁰ W/Hz^1/2. With estimated frequency dependent antenna effective area S_est for contact wires considered as antennas, the estimated possible noise equivalent power NEP_pos for n-MOSFET structures themselves can be from ∼15 to ∼10³ times better in the specral range of ν ∼55–78 GHz reaching NEP_pos ≈10⁻¹² W/Hz^1/2.     

Level statistics inside the core of a superconductive vortex

A microscopic theory of the Efetov supermatrix sigma-model type is constructed for the low-lying electron states in a mixed superconductive-normal system with disorder. This technique is used for the study of the localized states in the core of a vortex in a moderately clean superconductor with τ ⁻¹≫ω ₀∼Δ²/E _F . At low energies ε≪ω _Th∼ (ω ₀/τ)^1/2, the energy level statistics is described by the “zero-dimensional” limit of this supermatrix theory, and the result for the density of states is equivalent to that obtained within Altland-Zirnbauer random matrix model. Nonzero modes of the sigma model increase the mean interlevel distance by the relative amount [2 ln (1/ω ₀ τ)]⁻¹. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 78–83 (10 July 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Two-photon fluorescence spectrum in an atom + dielectric microsphere system

A strong resonant interaction of a two-level atom with a dielectric microsphere is studied on the basis of quantum electrodynamics. The initial condition considered is one in which the atom is initially excited and the resonant mode of the microsphere has been excited by a single photon. The spectrum of two emitted photons depends strongly on the method used to excite the microsphere, i.e., on the spatial distribution of the photon energy. The most characteristic feature of the two-photon fluorescence spectrum is a strong energy correlation of the emitted photons. This correlation is expressed in the fact that the energies of the emitted photons are related by the equation of an ellipse (ω+ω ₂−2ω _vA )²+3(ω ₁−ω ₂)²= 4Ω _Rabi ² . The relation between the results obtained and the predictions of the theory of dressed states is discussed. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 3, 192–197 (10 August 1999)     

Characteristics of a multi-keV monochromatic point x-ray source based on vacuum diode with laser-produced plasma as cathode

Temporal, spatial and spectral characteristics of a multi-keV monochromatic point x-ray source based on vacuum diode with laser-produced plasma as cathode are presented. Electrons from a laser-produced aluminium plasma were accelerated towards a conical point tip titanium anode to generate K-shell x-ray radiation. Approximately 10₁₀ photons/pulse were generated in x-ray pulses of ∼18 to ∼28 ns duration from a source of ∼300 μm diameter, athυ = 4.51 keV (K _α emission of titanium), with a brightness of ∼10²⁰ photons/cm²/s/sr. This was sufficient to record single-shot x-ray radiographs of physical objects on a DEF-5 x-ray film kept at a distance of up to ∼10 cm.     

Low-frequency fluctuations in a pure toroidal magnetized plasma

A magnetized, low-β plasma in pure toroidal configuration is formed and extensively studied with ion mass as control parameter. Xenon, krypton and argon plasmas are formed at a fixed toroidal magnetic field of 0.024 T, with a peak density of ∼10¹¹ cm⁻³, ∼4 × 10¹⁰ cm ⁻³ and ∼2 × 10¹⁰ cm ⁻³ respectively. The experimental investigation of time-averaged plasma parameter reveals that their profiles remain insensitive to ion mass and suggests that saturated slab equilibrium is obtained. Low-frequency (LF) coherent fluctuations (ω < ω _ci) are observed and identified as flute modes. Here ω _ci represents ion cyclotron frequency. Our results indicate that these modes get reduced with ion mass. The frequency of the fluctuating mode decreases with increase in the ion mass. Further, an attempt has been made to discuss the theory of flute modes to understand the relevance of some of our experimental observations.     

Lattice mechanical properties of noble and transition metals

A model pseudopotential depending on an effective core radius but otherwise parameter free is used to study the interatomic interactions, phonon dispersion curves (inq and r-space analysis), phonon density of states, mode Grüneisen parameters, dynamical elastic constants (C ₁₁,C ₁₂ andC ₄₄), bulk modulus (B), shear modulus (C′), deviation from Cauchy relation (C ₁₂–C ₄₄), Poisson’s ratio (σ), Young’s modulus (Y), behavior of phonon frequencies in the elastic limit independent of the direction (Y ₁), limiting value in the [110] direction (Y ₂), degree of elastic anisotropy (A), maximum frequencyω _max, mean frequency 〈ω〉, 〈ω ²〉^1/2=(〈ω〉/〈ω ⁻¹〉)^1/2, fundamental frequency 〈ω ²〉, and propagation velocities of the elastic constants in Cu, Ag, Au, Ni, Pd, and Pt. The contribution of s-like electrons is calculated in the second-order perturbation theory for the model potential while that of d-like electrons is taken into account by introducing repulsive short-range Born-Mayer like term. Very recently proposed screening function due to Sarkar et al. has been used to obtain the screened form factor. The theoretical results are compared with experimental findings wherever possible. A good agreement between theoretical investigations and experimental findings has proved the ability of our model potential for predicting a large number of physical properties of transition metals.     

Study of opto-mechanical characteristics of interaction of ultrashort laser pulses with polymer materials

The opto-mechanical characteristics, such as the specific mechanical recoil momentum, the specific impulse, and the energy efficiency, of the laser ablation of flat polymer targets ((C₂F₄) _n , (CH₂O) _n ) have been determined experimentally for the first time for the case of excitation with femtosecond pulses (τ ∼ 45–70 fs) of UV-IR (λ ∼ 266, 400, 800 nm) laser radiation (I ₀ up to 10¹⁵ W/cm²) under normal atmospheric and vacuum (p ∼ 10⁻⁴ mbar) conditions. The efficiency of mechanical recoil momentum generation is analyzed for various regimes of the laser irradiation.     

Wavelength modulation and cavity enhanced absorption spectroscopy using ～1.9?μm radiation produced by difference frequency generation with a MgO doped PPLN crystal

We have demonstrated the production of ∼1.9 μm near-infrared radiation by using difference frequency generation within a 5% MgO doped PPLN crystal by coupling ∼735 nm radiation from a tunable external cavity diode laser with relatively high powered 532 nm radiation from both Nd:YVO₃ and Nd:YAG lasers. The radiation produced is of low power, ∼15 μW, and was used in conjunction with the sensitivity enhancing techniques of wavelength modulation spectroscopy (WMS) and cavity enhanced absorption spectroscopy (CEAS). Experiments were carried out on rotationally resolved transitions in the combination bands of NH₃ and CO₂ in the 1.9 μm region. An α _min value of 3.6×10⁻⁶ cm⁻¹ Hz^−1/2 was achieved for WMS measurements on CO₂. A comparable α _min value of 2.2×10⁻⁶ cm⁻¹ Hz^−1/2 was achieved for NH₃ using CEAS. The low NIR power indicates that despite the level of MgO doping quoted for the crystal, under prolonged exposure photorefractive damage has occurred.     

Fluctuation-induced pseudogap in high-temperature superconductors

We study the effect of fluctuations on the ac conductivity of a layered superconductor for c-axis electromagnetic wave polarization. The fluctuation contributions of different physical nature and sign (paraconductivity, Maki-Thompson anomalous contribution, one-electron density-of-states renormalization) are found to be suppressed by the external field at different characteristic frequencies (ω _AL∼T-T _c , ω _MT∼max{T-T _c ,τ _ϕ ⁻¹ }, ω _DOS∼min{T, τ ⁻¹}). As a result, the appearance of a nonmonotonic frequency dependence (pseudogap) in the infrared optical conductivity of high-temperature superconductor is predicted. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 6, 397–401 (25 September 1996) Department of Theoretical Physics Moscow Institute of Steel and Alloys. Published in English in the original Russian journal. Edited by Steve Torstveit.     

Kl3γ+ decays revisited: Branching ratios and T-ODD momenta correlations

We calculate the branching ratios of the K ⁺ → π ⁰ l ⁺ ν _lγ (l = e, μ) decays, and the T-odd triple momenta correlations ξ = q · [p _l × p _π ]/M _K ³, due to the electromagnetic final-state interaction, in these processes. The contributions on the order of ω ⁻¹ and ω ⁰ to the corresponding amplitudes are treated exactly. For the branching ratios, the corrections on the order of ω are estimated and demonstrated to be small. We compare the results with those of other authors. In some cases our results differ considerably from the previous ones.     

Surface vibrations and the nature of the adsorption state: C2H2 on Pt(111)

Molecular vibrations of C₂H₂ and C₂D₂ adsorbed on Pt(111) at 140 K and ∼300K have been measured by high resolution electron energy loss spectroscopy. The comparison of C₂H₂ and C₂D₂ spectra allows an unambiguous assignment of the observed losses to the excitation of C−H bending, C−H stretching, and C−C stretching modes of nondissociatively adsorbed acetylene. From the relative intensities of losses the hybridisation state is determined to be nearsp ². The C−C stretching frequency indicates a C−C bond order of ∼1.8.     

Direct photons in nuclear collisions at fair energies

Using the extrapolation of existing data, estimations of prompt-photon production at FAIR energies have been made. At y = y _c.m. the rapidity density of prompt photons with p _t > 1.5 GeV/c per central Au + Au event at 25 A GeV is estimated as ∼10⁻⁴. With the planned beam intensity 10⁹ per second and 1% interaction probability, for 10% of most central events one can expect the prompt-photon rate ∼10² photons per second. Direct photons from the hadron scenario of ion collisions generated by the Hadron-String-Dynamics (HSD) transport approach with implemented meson scatterings πρ → πγ, ππ → ργ have been analyzed. Photons from short-living resonances (e.g., ω → π ⁰ γ) decaying during the dense phase of the collision should be considered as direct photons. They contribute significantly in the directphoton spectrum at p _t = 0.5–1 GeV/c. At the FAIR energy 25 A GeV in Au + Au central collisions the HSD generator predicts, as a lower estimate, γ _direct/ ≃ 0.5% in the region p _t = 0.5–1 GeV/c. At p _t = 1.5–2 GeV/c γ _prompt/ ≃ 2%. Thermal direct photons have been evaluated with the Bjorken Hydro-Dynamics (BHD) model. The BHD spectra differ strongly from the HSD predictions. The direct-photon spectrumis very sensitive to the initial temperature parameter T ₀ of the model. The 10-MeV increase in the T ₀ value leads to ∼2 times higher photon yield. The text was submitted by the authors in English.     

Effect of ionizing irradiation on the mechanism of current passage in TlInSe<Subscript>2</Subscript> single crystals

The temperature dependence of the electrical conductivity and current-voltage characteristics of γ-irradiated TlInSe₂ single crystals with an electrical resistivity of ∼10⁸ Ω cm have been investigated. It has been established that the anomalies of the conductivity observed in weak electric fields and at low dozes of irradiation are related to the decomposition of neutral complexes containing an interstitial cation atom. In strong electric fields, a thermal-field ionization of traps occurs. The main mechanism of radiation defect formation is the formation of complexes [V _In⁻In _i ⁺], [V _Se⁻Se _i ⁻], and others with the structural defects characteristic of unirradiated crystals. The activation energy, trap concentrations, and the potential well shape near the traps have been determined.