Effect of wood properties on within-tree variation in ultrasonic wave velocity in softwood

The radial variations in the velocity of longitudinal waves propagating through Japanese cedar and Japanese cypress were experimentally investigated. In addition, the tracheid length (TL), microfibril angle (MFA), air-dried density (AD), and moisture content (MC) were measured in order to determine the effect of wood properties on velocity variations within the wood trunk. For both species, the longitudinal wave velocities measured in the longitudinal direction (V_L) exhibited minimum values near the pith. For Japanese cedar, V_L increased from 3600 m/s toward the bark and soon attained a constant value (=4500 m/s). On the other hand, for Japanese cypress, V_L kept increasing from 4000 m/s near the pith to 4800 m/s at the bark. These radial variations in V_L coincided with those in the tracheid length. V_L exhibited strong correlations with TL and MFA with a significant level of (p < 0.01). These findings suggest that the TL and MFA greatly affect the radial variation in the ultrasonic wave velocity in softwood.     

Design and VNA-measurement of coplanar waveguide (CPW) on benzocyclobutene (BCB) at THz frequencies

The low permittivity and the low loss tangent of the benzocyclobutene polymer (BCB) offers to coplanar waveguides (CPW) a low dispersive propagation properties at THz frequency. These transmission lines have been designed, modeled with a three dimensional (3D) solver of Maxwell equations based on finite element method (FEM) from 20 to 1000 GHz at various characteristic impedances (Z_c). Their dispersion and losses (radiation, conduction and dielectric) have been investigated separately versus the waveguide size, the nature of the substrate (dielectric or semiconductor) to optimize the THz signal propagation. Monomode CPW on BCB numerically designed for various Z_c were realized and measured with vector network analyzer (VNA). S-parameters of CPW are de-embedded by optimization of the accesses’ model. A good agreement is found between experimental and numerical results with low attenuation constants of 2.7 dB/mm and 3.5 dB/mm at 400 GHz and 500 GHz, respectively.     

Electromagnetically Induced Transparency and optical pumping processes formed in Cs sub-micron thin cell

The Electromagnetically Induced Transparency (EIT) effect in a Λ-system formed by Cs atoms (6S_1/2 ? 6P_3/2 ? 6S_1/2) confined in an extremely thin cell (ETC) (atomic column thickness L varies in the range of 800 nm –3 µm is studied both experimentally and theoretically. It is demonstrated that when the coupling laser frequency is in exact resonance with the corresponding atomic transition, the EIT resonance parameters weakly depend on L, which allows us to detect the effect at L = λ = 852 nm. EIT process reveals a striking peculiarity in case of the coupling laser detuned by Δ from the atomic transition, namely the width of the EIT resonance rapidly increases upon an increase in Δ (an opposite effect is observed in centimeter-scale cells). The strong broadening of the EIT resonance for large values of detunings Δ is caused by the influence of atom-wall collisions on dephasing rate of coherence. The influence of the coupling laser on the velocity selective optical pumping/saturation resonances formed in ETC has been also studied. The theoretical model well describes the observed results.     

Crosstalk in double-pumped fiber optic parametric amplifiers for wavelength division multiplexing systems

We study experimentally inter-channel crosstalk in double-pumped fiber optic parametric amplifiers constructed with conventional dispersion shifted fibers (DSFs) having different lengths (L_A = 13.8, L_B = 6.8, L_C = 4.3, and L_D = 0.8 km). For long fibers (L_A and L_B), eye diagram measurements in a 5-channel (100 GHz spacing) system show that in order to have negligible crosstalk, the output signal power per channel, P_s, should be limited to P_s < 0 dBm. By decreasing the fiber length (to L_C) it is possible to increase the output signal power and/or the number of signals while keeping the crosstalk on negligible levels. This trend was further confirmed by using a very short DSF (L_D = 0.8 km).Finally, we experimentally demonstrate that a general trend in 2P-FOPAs is that spurious FWM increases with the number of signal channels up to a given number of channels when a saturation regime is reached. This saturation of the generation of spurious tones occurs when the bandwidth occupied by the signals exceeds ∼4-5 nm.     

Beyond the pulse-area theorem: Role of the absorption and the dispersion in the propagation of weak ultrashort resonant pulses

We present here some fundamental but yet underlooked features of the propagation of weak ultrashort pulses (with Δ_dτ << 1, where Δ_d is the Doppler width and τ is the pulse duration) in resonant atomic media. We show that the pulse area behaviour and the pulse spectrum at resonance are governed by the usual optical depth (α₀L, where α₀ is the absorption coefficient at resonance and L the length of the medium), whereas the pertinent parameter that governs the severity of the dispersion effects and the distortion of the pulse is the dispersion parameter e_disp = (α₀L)Δ_dτ that we introduce. Paradoxical effect such as distortionless propagation (e.g. e_disp << 1) with vanishing pulse area (when α₀L >> 1) can then explained within this formalism.     

Influence of turbulence on train noise

The subject of this paper is the long distance propagation of train noise. The sound exposure level of train noise L_AE was measured. To describe the results of measurements, a semi-analytical model was used. It takes into account the wave-front divergence, air absorption, ground effect, and the turbulence destroying the coherent nature of the ground effect. The model contains three adjustable parameters that must be estimated at the site. To verify the model, we performed measurements of L_AE at the distance D = 450 m from the train track center. The difference between the calculated and measured mean values of L_AE equals 1.3 dB.     

A long-range hybrid THz plasmonic waveguide with low attenuation loss

Numerical solutions are obtained for the proposed novel hybrid terahertz plasmonic waveguide structure, namely the silicon metal silicon (SMS) waveguide. It is shown that the SMS waveguide can overcome the diffraction limit while still maintaining a sizeable propagation length. The geometric dependence of the mode characteristics of this structure is analyzed in detail, showing strong confinement and low loss with propagation lengths exceeding 14 mm at normalized mode areas of 1.72 × 10⁻². By using the FEM method (Comsol), the guiding properties of the hybrid terahertz surface plasmon polariton (HTSPP) waveguide are numerically analyzed at the THz frequency, and a combination of double-structured comparisons of the best features of the terahertz plasmonic waveguide is made. Depending on the height used and how the mode confinement is measured, various modal designs, such as double microwire structures, are developed. The structures indicate that we verified the possibility of low attenuation loss of hybrid THz plasmonics propagation. The effective mode area A_eff, energy distribution, and propagation length L_p versus height for waveguides with Si microwire and SiO₂ are shown. The numerical calculation results reveal a potential for use in applications such as optical force in trapping and transporting biomolecules, and in high-density integrated circuits.     

High brightness 735 nm tapered lasers – optimisation of the laser geometry

High brightness tapered laser diodes with different resonator geometries were fabricated and analysed. The devices consist of an index-guided straight section and a gain-guided tapered section. Lasers with a total length L = 2 and 4 mm and different length of the ridge waveguide L _RW (500 μm ≤ L _RW ≤ 1250 μm for L = 2 mm and 500 μm ≤ L _RW ≤ 2000 μm for L = 4 mm) were processed to study the influence of the straight section on the spatial mode filtering. The power–voltage–current-characteristics, the beam waist, the far field, and the beam propagation factor M ² were measured. From the experiments, it can be stated that the lasers with a small L _RW reach higher output powers compared to those with larger L _RW. Concerning the beam quality the length L _RW should exceed a minimal value to guarantee efficient spatial mode filtering. Devices optimised concerning maximum output power and excellent beam quality reach a beam propagation factor smaller than 2.1 at an output power P = 2 W.     

Proton dynamics in antiferroelectric CsH3(SeO3)2 single crystal investigated by 1H NMR measurements

¹H nuclear magnetic resonance (NMR) measurements have been performed to study the proton dynamics associated with the antiferroelectric transition of a hydrogen-bonded single crystal of CsH₃(SeO₃)₂. Herein, ¹H NMR spectrum, shift, linewidth, and spin-lattice relaxation rate 1/T₁ are measured in the temperature range of 80–296 K with the c-axis parallel to a magnetic field of ~4.85 T. The spectrum exhibits a composite structure with two narrow peaks at 296 K; at a low temperature, this structure evolves into a single broad shape with three humps. This complex shape and evolution are deconvoluted into five or six components based on the number of inequivalent and disordered hydrogen sites. By estimating the chemical shift and linewidth for each proton site, we identify all peaks. The spin-lattice relaxation recovery exhibits a double-exponential behavior with two relaxation times, short T_1S and extremely large T_1L. Both T_1S and T_1L follow Arrhenius behavior. From the respective 1/T₁(T), the activation energies for proton motion are measured to be small: 1.16 ± 0.1 and 0.83 ± 0.06 kJ/mol for T_1S and T_1L, respectively. While the static NMR data, chemical shift and linewidth, show no evidence for the transition, the dynamic data 1/T_1L highlights a clear increase across T_N = 145 K, which is possibly a signature of the transition.     

Phénomènes de glissement à l'interface liquide–solide

The problem of liquid–solid slip is described here, in a simplified manner. Today, several experiments have shown that substantial slip appears when a non-wetting liquid flows along a surface which is smooth on an atomic scale. This phenomena is characterised by a length, called the slip length, or Navier length, generally denoted by L_s. A number of experiments indicate that this quantity may be as large as several hundreds of nanometers. Numerical simulations also show the existence of slip in non-wetting conditions, but the corresponding lengths found here are much smaller than those found experimentally. A theory, based on the existence of a gas film of nanometre thickness has been proposed, but has not yet been experimentally confirmed. Experiments on this are difficult, and sometimes controversial. To cite this article: P. Tabeling, C. R. Physique 5 (2004).     

Design optimization of tunneling field-effect transistor based on silicon nanowire PNPN structure and its radio frequency characteristics

Recently, a number of semiconductor devices have been widely researched in order to make breakthroughs from the short-channel effects (SCEs) and high standby power dissipation of the conventional metal-oxide-semiconductor field-effect transistors (MOSFETs). In this paper, a design optimization for the silicon nanowire tunneling field-effect transistor (SNW TFET) based on PNPN multi-junction structure and its radio frequency (RF) performances are presented by using technology computer-aided design (TCAD) simulations. The design optimization was carried out in terms of primary direct-current (DC) parameters such as on-current (I_on), off-current (I_off), current ratio (I_on/I_off), and subthreshold swing (SS). Based on the parameters from optimized DC characteristics, basic radio frequency (RF) performances such as cut-off frequency (f_T) and maximum oscillation frequency (f_max) were analyzed. The simulated device had a channel length of 60 nm and a SNW radius of 10 nm. The design variable was width of the n-doped layer. For an optimally designed PNPN SNW TFET, SS of 34 mV/dec and I_on of 35 μA/μm were obtained. For this device, f_T and f_max were 80 GHz and 800 GHz, respectively.     

Size effect on multiferroicity of GdMn2O5 nanorods

Four GdMn₂O₅ nanorod samples of various axial lengths (〈L_C〉) along the c axis were synthesized. The antiferromagnetic and ferroelectric ordering disappeared as 〈L_C〉 decreased to 66 and 55 nm. Various ferroic critical sizes were observed for the two types of domain sizes. Between T = 18 and 26 K, a charge ordering X-ray diffraction peak appeared at 〈L_C〉 = 79 nm. This peak was associated with structural distortion and axial length. The broken multiferroicity of the GdMn₂O₅ nanorods limits their practical application. For applications in memory devices, the estimated maximal capacity is approximately 650 Gbits/in².     

Wideband slow light achievement in MIM plasmonic waveguide by controlling Fano resonance

In this paper, we have investigated the characteristics of an asymmetric shaped Fano line in a metal–insulator–metal (MIM) plasmonic waveguide side coupled to two resonating stub structures. The spectral properties of Fano resonance are quite distinct due to the destructive interference between a two propagating plasmon modes. Two structural parameters are carefully adjusted: physical separation between both the resonating stubs and length of resonating stubs. By tailoring the separation between both the resonating structures, coupling between both the plasmon modes is controlled, and hence asymmetric nature of Fano line can be shaped accordingly. Resonance condition of Fano line can be tuned by scaling the length of stubs. A strong red shift in resonating wavelength with varying degree of asymmetry is observed, when length of resonating structures is increased. The sharp resonant peak, due to an asymmetric shaped Fano resonance is generally accompanied by large dispersion that results in reduction of group velocity of light near Fano resonance. By controlling the coupling between resonating stub, or by scaling the length of lower resonating stub, large value of group index (n_g = 75) and delay bandwidth product (DBP = 0.2533) is obtained. The structure can be modified to suit different applications in optical buffers, optical switches and nonlinear optics devices.     

Lewis number effects on laminar and turbulent expanding flames of NH3/H2/air mixtures at elevated pressures

This study clarifies the effects of Lewis number (Le) on laminar and turbulent expanding flames of NH₃/H₂/air mixtures. The laminar burning velocity (S_L) and turbulent burning velocity (S_T) were measured using a medium-scale, fan-stirred combustion chamber with ammonia/hydrogen molar ratio (NH₃/H₂) of 50/50 and 80/20 under the maximum pressures of 5 atm. The lean laminar flame with NH₃/H₂ = 50/50 is significantly accelerated by the diffusional–thermal instability, which dominated the trend of S_T,c=0.1 with the equivalence ratio (ϕ). The lean normalized turbulent burning velocity (S_T/S_L) increases with the decrease of hydrogen content due to the weakening effects of S_L. However, the S_T/S_L reaches peak with hydrogen volumetric content less than 20% due to effects made by diffusional–thermal instability than S_L did. The turbulent flame of NH₃/H₂/air mixtures is characterized by self-similar acceleration propagation, and propagation with Le < 1 is faster. A modified correlation considering the effects of Le was proposed, as (d<r>/dt)/σS_L = 0.118(Re_T,flameLe⁻²)^0.57, which was able to predict not only the self-similar propagation of NH₃/H₂/air but also the previous syngas/air flames. The Kobayashi correlations modified by three kinds of Le power exponents were used to clarify the effects of Le by comparing their fitting parameters and predictive powers on experimental data and literature data. Similar pre-factors, power exponents and the goodness of fit (R²) were obtained with Le ranging from 0.58 to 1.62, which suggested that the determination of Le power exponent had no significant effect on the prediction accuracy of the S_T/S_L trend with data of Le near unity. This might be attributed to the fact that the variation ranges of the dimensionless number that characterizes the experimental conditions is much larger than that of the Le.     

Slant path intensity distribution of focusing J0-correlated Schell-model vortex beams in atmospheric turbulence

The intensity distribution of the J₀-correlated Schell-model (JSM) vortex beams focused by a lens and propagation in weak-to-strong turbulent atmosphere are investigated. It is shown that the beam spreading increases with the increase in topological charge n, the source coherent length α−1, turbulent outer scale L₀ and propagation distance z. The center hollow depth of intensity distribution of the J₀-correlated Schell-model (JSM) vortex beams decrease with the increase of topological charge n, turbulent outer scale L₀ and propagation distance z or the decrease of the source coherent length α−1.     

Effect of grain boundary on resistive magnetodielectric property of polycrystalline <Emphasis Type="Italic">γ</Emphasis>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>

We have investigated the role of the grain boundary on the resistive magnetodielectric property of polycrystalline γ-Fe₂O₃ through impedance spectroscopy measurements. Depending on the sample preparation temperature, the dielectric constant of γ-Fe₂O₃ is significantly different especially at low frequencies (<10⁴ Hz) and high temperatures (>200 K). The value of the magnetodielectric effect at a specific frequency and the resonance frequency for the maximized magnetodielectric effect are different, although polycrystalline γ-Fe₂O₃ samples show a quite similar magnetoresistance. Through the experimentally obtained resistance ratio between the grain and the grain boundary, we can reproduce the magnetodielectric curves based on the Maxwell–Wagner model and the measured magnetoresistance.     

A model of enhanced STM current due to semiconductor optical absorption

We present a simple model for the change in tunneling current between a semiconductor surface and a metal tip under spectroscopic illumination in a scanning tunneling microscope. This model predicts a sharp increase in the tunneling current due to the increase in the conduction band carrier density when the photon energy exceeds the optical band gap. The tunneling current for a large diffusion length has a more pronounced onset than for a small length. Our model should provide, when combined with experiments, a method of determining localized effective stoichiometry, and therefore provides a localized alternative to the use of optical absorption measurements. Our theoretical tunneling current versus photon energy curves are in good qualitative agreement with the existing experimentally measured curves for Si, GaAs, and InP obtained by Qian and Wessels. In addition, we have examined the effects of temperature, surface recombination velocity, and degeneracy on our theoretical results for the Hg_1−xCd_xTe, Hg_1−xZn_xTe, and Hg_1−xZn_xSe ternary narrow gap semiconductor systems.     

Theoretical study of surface plasmons coupling in transition metallic alloy 2D binary grating

The excitation of a surface plasmon polariton (SPP) wave on a metal–air interface by a 2D diffraction grating is numerically investigated. The grating consists of homogeneous alloys of two metals of a formula A_xB_1−x, or three metals of a formula A_xB_yC_z, where A, B and C could be silver (Ag), copper (Cu), gold (Au) or aluminum (Al).It is observed that all the alloys of two metals present a very small change of surface plasmon resonance (SPR) irrespective of composition x. Moreover, the addition of 25% of Al to two metals alloy is insufficient to change the SPR curves. The influence of the different grating parameters is discussed in details using rigorous coupled-wave analysis (RCWA) method. Furthermore, the SPR is highly dependent on grating periods (d_x and d_y) and the height of the grating h. The results reveal that d_x= d_y= 700 nm, h=40 nm and duty cycle w=0.5 are the optimal parameters for exciting SPP.