Microwave energy channeling in plasma waveguides created by a high-power UV laser in the atmosphere

The grazing mode of microwave propagation in a hollow plasma waveguide formed by ionization of atmospheric air with a small easily ionized additive by strong UV pulses of the Garpun KrF laser (λ = 248 nm, the pulse duration and energy are ∼70 ns and ∼50 J) was experimentally demonstrated for the first time. The annular laser beam produced a hollow tube ∼10 cm in diameter with an electron density of ∼10¹² cm⁻³ in a plasma wall ∼1 cm thick, over whichmicrowave radiation with λ _mw ∼ 8 mm was transmitted to a distance of 60 m. Themicrowave signal transmitted by the waveguide was amplified by a factor of 6 in comparison with propagation in free space.     

Theoretical Study of Electro-Optic Effect and Elasto-Optic Effect in Chirped Fiber Grating with Uniaxial Crystal Materials Cladding

A chirped fiber grating with cladding made of uniaxial crystal material whose optical axis is parallel to the axis of grating, i.e., z-axis is proposed. Electro-optic effect and elasto-optic effect in this kind of fiber gratings are theoretically analyzed for the first time. The influences of the electric field and the strain applied to the fiber grating cladding along z-axis on Bragg wavelength λ _B and the reflectivity spectra of this kind of chirped grating are theoretically examined using coupled-mode theory and transfer matrix method. The curves of Bragg wavelength λ _B as a function of an external electric field or the strain are theoretically obtained respectively for three kinds of uniaxial crystal materials as the grating claddings. The calculated results indicate that when the axial electric field applied to the grating cladding varies from 1 × 10⁷ to 8 × 10⁷ V/m, λ _B has the decreases of 0.12 nm, and when the applied strain varies from 0 to 0.04, λ _B has the decreases of 0.45 nm.     

Investigation of Rb <Emphasis Type="Italic">D</Emphasis><Subscript>1</Subscript> atomic lines in strong magnetic fields by fluorescence from a half-wave-thick cell

It has been experimentally demonstrated that the use of the effect of significant narrowing of the fluorescence spectrum from a nanocell that contains a column of atomic Rb vapor with a thickness of L = 0.5λ (where λ = 794 nm is the wavelength of laser radiation, whose frequency is resonant with the atomic transition of the D ₁ line of Rb) and the application of narrowband diode lasers allow the spectral separation and investigation of changes in probabilities of optical atomic transitions between levels of the hyperfine structure of the D ₁ line of ⁸⁷Rb and ⁸⁵Rb atoms in external magnetic fields of 10–2500 Gs (for example, for one of transitions, the probability increases ∼17 times). Small column thicknesses (∼390 nm) allow the application of permanent magnets, which facilitates significantly the creation of strong magnetic fields. Experimental results are in a good agreement with the theoretical values. The advantages of this method over other existing methods are noted. The results obtained show that a magnetometer with a local spatial resolution of ∼390 nm can be created based on a nanocell with the column thickness L = 0.5λ. This result is important for mapping strongly inhomogeneous magnetic fields.     

Thermal conductivity of the diamond-paraffin wax composite

The thermal conductivity of diamond-paraffin wax composites prepared by infiltration of a hydrocarbon binder with the thermal conductivity λ _m = 0.2 W m⁻¹ K⁻¹ into a dense bed of diamond particles (λ _f ∼ 1500 W m⁻¹ K⁻¹) with sizes of 400 and 180 μm has been investigated. The calculations using universally accepted models considering isolated inclusions in a matrix have demonstrated that the best agreement with the measured values of the thermal conductivity of the composite λ = 10–12 W m⁻¹ K⁻¹ is achieved with the use of the differential effective medium model, the Maxwell mean field scheme gives a very underestimated calculated value of λ, and the effective medium theory leads to a very overestimated value. An agreement between the calculation and the experiment can be provided by constructing thermal conductivity functions. The calculation of the thermal conductivity at the percolation threshold has shown that the experimental thermal conductivity of the composites is higher than this critical value. It has been established that, for the composites with closely packed diamond particles (the volume fraction is ∼0.63 for a monodisperse binder), the use of the isolated particle model (Hasselman-Johnson and differential effective medium models) for calculating the thermal conductivity is not quite correct, because the model does not take into account the percolation component of the thermal conductivity. In particular, this holds true for the calculation of the heat conductance of diamond-matrix interfaces in diamond-metal composites with a high thermal conductivity.     

Optical magnetometer with submicron spatial resolution based on Rb vapors

It is shown experimentally that use of fluorescence and transmission spectra obtained from nanocells with the thickness of column of rubidium atomic vapor L = λ/2 and L = λ, respectively (λ = 794 nm is the wavelength of laser radiation close to resonance with D ₁-line transition of Rb atoms), by means of a narrowband diode laser allows spectral separation and study of variations of probabilities of atomic transitions between ground and excited states of hfs of D ₁ lines of ⁸⁵Rb and ⁸⁷Rb atoms in the range of magnetic fields from 10 to 5000 G. Small thickness of atomic vapor column (∼390 nm and ∼794 nm) allows applying permanent magnets simplifying essentially creation of strong magnetic fields. Advantages of this technique are discussed as compared with the technique of saturated absorption. The obtained results show that a nanocell with submicrom thickness of vapor column may serve as a basis for designing a magnetometer with submicron local spatial resolution which is important in case of measuring strongly inhomogeneous magnetic fields. Experimental data are in good agreement with the theoretical results.     

Laser photoelectron projection microscopy of an organic conducting polymer

A conducting organic polymer is visualized on a laser photoelectron projection microscope, which is based on Letokhov’s concept and has a nanometer spatial resolution. Photoelectron images of polyaniline (which is the most promising representative of conducting polymers) with a magnification of ∼10⁵ have been obtained when a 100-nm quartz capillary coated with a film of this material was irradiated by femtosecond laser pulses. The projection photoelectron method using 400- and 800-nm laser radiation has made it possible to directly reveal the existence of the redox heterogeneity of the organic polymer, which is due to the contact of the sections of polyaniline with different oxidation degrees and strongly affects the electric conductivity of the sample.     

Tailor-made metal nanoparticles as SERS substrates

In this contribution we summarize recent experiments with the objective to generate optimized substrates for surface-enhanced Raman spectroscopy (SERS). For this purpose, the well-established laser-assisted growth technique has been applied, which relies on a precise control of the growth kinetics of supported metal nanoparticles. With this method reproducible and stable SERS substrates with tailor-made optical properties possing best field enhancements were produced for specific excitation wavelengths and detection ranges. Optimization of the SERS substrates has been achieved by stabilizing the localized surface plasmon polariton resonance (SPR) of gold nanoparticles in the vicinity of the laser wavelength of λ=647 nm and λ=785 nm used for SERS excitation. After nanoparticle preparation, SERS spectra of pyrene were obtained using naturally grown nanoparticles and nanoparticles prepared by laser-assisted growth. The most important result is that the optimized substrates prepared by laser-assisted growth exhibit a significantly higher signal-to-noise ratio as compared to naturally grown nanoparticles. They are even better than substrates whose SPR has been tuned to the excitation wavelength by an elevated temperature during preparation. Another important observation is that all SERS spectra exhibit excellent reproducibility and the substrates do not show degradation during the measurements. Finally, the SERS enhancement factors due to the optimized substrates have been estimated and are on the order of 10⁵ to 10⁶.     

Temperature dependence of electric resistance and magnetoresistance of pressed nanocomposites of multilayer nanotubes with the structure of nested cones

Bulk samples of carbon multilayer nanotubes with the structure of nested cones (fishbone structure) suitable for transport measurements, were prepared by compressing under high pressure (∼25 kbar) a nanotube precursor synthesized through thermal decomposition of polyethylene catalyzed by nickel. The structure of the initial nanotube material was studied using high-resolution transmission electron microscopy. In the low-temperature range (4.2–100 K) the electric resistance of the samples changes according to the law ln R ∝ (T ₀/T)^1/3, where T ₀∼7 K. The measured magnetoresistance is quadratic in the magnetic field and linear in the reciprocal temperature. The measurements have been interpreted in terms of two-dimensional variable-range hopping conductivity. It is suggested that the space between the inside and outside walls of nanotubes acts as a two-dimensional conducting medium. Estimates suggest a high value of the density of electron states at the Fermi level of about 5×10²¹ eV⁻¹ cm⁻³. Zh. éksp. Teor. Fiz. 113, 2221–2228 (June 1998)     

On the variability of the transmission coefficient of the atmospheric optical window

Summary We use experimental data to show that the amplitudeA of the oscillations impressed upon upper atmospheric electrons by the electric field vectors of incident solar radiation increases with the solar energy wavelength λ over the visible spectrum. Calculations based upon experimental data show that the transmission coefficientT of the upper atmosphere for incident solar energy (say at intensityI ₀) also increases with λ over the visible spectrum, implying thatT increases withA over the latter range. Finally we show thatI ₀ ^1/2 λ² bears a positive linear relationship withT over the visible solar spectrum. The latter conclusion, which confirms theoretical results contained in a recent paper, is then used to show that the 100 000 year eccentricity cycle effectively causes a maximum change in the total solar energy reaching the lower atmosphere of approximately 4.5% which is enough to trigger an ice age.     

Spectral properties of backward stimulated scattering in liquid carbon disulfide

The spectral structure of backward stimulated scattering from a 10 cm-long CS₂-liquid cell is investigated by using Q-switched 10-ns and 532-nm laser pulses with different spectral linewidths. Under a narrow spectral line (∼0.1 cm⁻¹) pump condition, very strong sharp lines near the pump wavelength (λ ₀) position and the first-order stimulated Raman scattering (λ _s1) position can be observed. However, under a wide line (≈1 cm⁻¹) pump condition, only a strong and superbroadening spectral band can be observed mainly in the red-shift side of the pump wavelength. The different spectral features under these two conditions can be explained by a competition between stimulated Brillouin, Raman, and Rayleigh-Kerr scattering. Under both pump conditions, the broadening spectral distributions are not consistent with the predictions given by stimulated Rayleigh-wing scattering theories, but can be interpreted well utilizing the theoretical model of stimulated Rayleigh-Kerr scattering. Zh. éksp. Teor. Fiz. 112, 1563–1573 (November 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

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.     

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.     

Convective model of a microwave discharge in a gas at atmospheric pressure in the form of a spatially localized plasma

Experiments and a theoretical model consistent with them are presented which show that a stationary microwave discharge in a gas at atmospheric pressure under the action of free convection due to the action of the buoyant force on the heated air can be spatially localized, taking a spheroidal shape. Vortex motion inside the spheroid gives this localized plasma formation some of the properties of a material body which are manifested in a distinct material isolation from the surrounding space, in the formation of a narrow thermal boundary layer and flow separation, and in the formation of secondary vortices in the wake region. The characteristic radius of the stationary localized plasma is governed mainly by the wavelength of the microwave radiation a∼0.137λ. Energy balance is established to a significant degree by convective cooling of the microwave-heated structure. Zh. éksp. Teor. Fiz. 112, 877–893 (September 1997)     

Investigation of scaling laws as applied to the gas discharge in the case of a barrier-discharge-excited Kr/CCl<Subscript>4</Subscript> mixture

The electrical and luminescent characteristics of a barrier-discharge lamp filled with a Kr/CCl₄ (150: 1) mixture are experimentally studied versus the value of pd, which varies in the range (7.6–14) × 10³ Pa cm. When simulating the gas discharge using similarity parameters, the following relationships are fulfilled: for pd = const (p is the pressure, d is the interelectrode distance), the pulse duration and the mean current density are τ _j ∼ 1/p and 〈j〉 ∼ p; the surface charge density on the electrodes, σ ∼ const; the duration of the UV radiation pulse and the efficiency of UV radiation due to a KrCl* (222 nm) exciplex, τ_rad ∼ 1/p and η ∼ p ². The maximal radiation efficiency achieved in the experiments is about 13%. Deviations from the similarity laws for the gas discharge are related to the filamentary form of the observed discharge. Qualitative analysis indicates that similarity laws may be fulfilled for such a form of discharge as well but locally, within a single filament.     

Spectra of the selective heat radiation of Yb2O3 under the laser thermal excitation

The dependence of the spectra of the near-IR and visible selective heat radiation (SHR) of the Yb₂O₃ polycrystals that are synthesized using the laser thermal method on the excitation intensity of the CW electric discharge CO₂ laser at the wavelength λ = 10.6 μm is experimentally studied. The SHR spectra are determined by the multiphonon excitation of the vibronic states of the ² F _5/2 term in the Yb³⁺ ions and the radiative transitions to the vibronic states of the ground term ² F _7/2.The laser heating of the polycrystals to the melting point causes the anomalous growth of the SHR spectral peaks in the wavelength range λ ≥ 1040 nm due to an increase in the intensity related to an increase in the probabilities of the radiative vibronic transitions owing to the thermal increase in the phonon density. The effective conversion of the thermal energy of the Yb₂O₃ polycrystals into the SHR indicate the significant role of the radiative cooling of surface.     

Preliminary report on the large halo event TADJIKISTAN

Summary A large halo event, TADJIKISTAN, observed recently by the Pamir Collaboration, consists of a halo (diameter ∼2 cm) and of a large number (∼2000) of high-energy showers, distributed over an area ofR∼20 cm. The total observed energy in the halo is estimated to be 4.8·10⁴ TeV. The characteristics of the event are compared with those of ANDROMEDA, another large event observed by the Chacaltaya Collaboration. The observed attenuation mean free path of hadrons, Λ/λ_p = 1.41 _−0.32 ^+0.21 is compatible with that of pions, indicating that the hadrons in the event are not exotic on average. Paper presented at the Special Session on very high-energy cosmic-ray interactions (superfamilies) of the XXIV International Cosmic-Ray Conference, Rome, August 28–September 8, 1995.     

Appearance of a band of delocalized D − states in uncompensated silicon in an electric field

The impurity photoconductivity spectra of uncompensated silicon at liquid-helium temperatures under conditions of strongly suppressed background radiation (background) are studied in different electric fields E. It is established that the delocalization arising in the D ⁻ band as E increases is not associated with any changes of the fluctuation potential and is due to the direct action of the field E. A delocalization band of finite width appears abruptly at a critical value E _c (∼100 V/cm) of E. The critical field E _c increases with the density of charged centers in the sample. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 56–59 (10 January 1997)     

Nonlinear optical absorption and refraction in Ru, Pd, and Au nanoparticle suspensions

We present the results of studies of the nonlinear optical properties of Pd, Ru, and Au nanoparticles. We studied the nonlinear refraction and nonlinear absorption of suspensions of these nanoparticles at 1064-nm wavelength. A relatively strong nonlinear absorption of the Pd nanoparticles was observed in the case of 1064-nm, 50-ps pulses (β=2×10⁻⁹ m W⁻¹). The Ru and Pd nanoparticles showed weak negative nonlinear refraction (γ∼−(6–8)×10⁻¹⁶ m² W⁻¹) in this spectral range. In the case of the Au nanoparticles, a saturated absorption at 532 nm dominated over other nonlinear optical processes.     

Changes in the optical properties of conducting polydiacetylene THD brought on by doping

Changes in the optical properties of conducting polydiacetylene THD (poly-1,1,6,6-tetraphenylhexadiindiamine) brought on by doping are investigated for the first time. Spectral dependences of the extinction coefficients were studied in the range 400–25 000 cm⁻¹ both for the undoped polymer (σ<10⁻⁹ S/cm) and at various doping levels (up to σ∼5×10⁻³ S/cm). The results obtained attest to the appearance of high carrier concentrations in polydiacetylene THD with conductivities σ⩾10⁻⁴ S/cm. The relatively low observed macroscopic conductivity is explained by the complex hierarchy of structural formations that are intrinsic to polymers. The results obtained are compared with the corresponding data for conducting polyacetylene. Fiz. Tverd. Tela (St. Petersburg) 40, 1162–1166 (June 1998)