Off-Bragg-angle light diffraction and structure of dynamic interband photorefractive gratings

Steady-state and time-resolved off-Bragg-angle diffraction experiments are used to determine the structure and the dynamics of photorefractive gratings induced by interband photoexcitation. In potassium niobate, we identify in such gratings basically a two-layer structure. Close to the surface, we find a space-charge electric field generated by a charge modulation stored directly in the bands. This grating component is typically 50 μm thick, the amplitude of the refractive index modulation is larger than 10^-4, and the response time is a few μs for resonant intensities of 100 mW cm^-2. This component is also robust under non-resonant illumination. Deeper in the crystal, a second holographic layer extends over a few hundreds of μm, its amplitude is smaller, and its slower response time is in the ms range. The mutual phase shift between the grating components is also determined. Received: 23 November 1998 / Revised version: 14 January 1999 / Published online: 12 April 1999     

Photoconduction in tunnel-coupled Ge/Si quantum dot arrays

The photoconduction in a tunnel-coupled Ge/Si quantum dot (QD) array has been studied. The photoconductance (PC) sign can be either positive or negative, depending on the initial filling of QDs with holes. The PC kinetics has a long-term character (10²?10⁴ s at T = 4.2 K) and is accompanied by persistent photoconduction (PPC), whereby the PC value is not restored on the initial level even after relaxation for several hours. These phenomena are observed upon illumination by light with photon energies both greater and smaller than the silicon bandgap. A threshold light wavelength corresponding to a long-term PC kinetics depends on the QD filling with holes. A model describing the observed PC kinetics is proposed, according to which the main contribution to the PC is related to the degree of QD filling with holes. By applying the proposed model to the analysis of PC kinetics at various excitation levels, it is possible to determine the dependence of the hopping conductance on the number of holes per QD. The rate of the charge carrier density relaxation exponentially depends on the carrier density.     

Low-density carbon material modified with pyrolytic carbon

Here we report a physicochemical investigation of low-density carbon materials modified with pyrolytic carbon (PC). Exfoliated graphite (EG) obtained by nitrate expandable graphite thermal destruction was pressed into low-density graphite materials (LDGMs) with densities of 0.045-0.50 g/cm³ and saturated with PC by impact CVI technique. LDGM infiltration with PC leads to sample weight and density growth. The amount of deposited PC strictly depends on synthesis conditions. The maximum surface and volume deposition of PC occurred for samples with density of 0.05 g/cm³. XRD, Raman spectroscopy and scanning electron microscopy revealed that the deposited PC is of smooth laminar (SL) type. Composite thermal conductivity is about 2-3.5 Wt/m K.     

沉积在液体衬底上连续铝薄膜的微观结构

研究了沉积在硅油衬底表面的连续铝薄膜的微结构及其表面形貌.与沉积在单晶硅表面的铝薄膜相比,两种铝薄膜均属颗粒结构,但硅油表面的铝薄膜具有颗粒尺寸较小、大小不均匀,表面起伏较大等特点,而且在该铝薄膜边缘的下表面,有一明显的波纹状楔型结构,其斜率仅为10^-4—10^-5.实验结果表明：这一现象是由于液体衬底的热膨胀行为引起的.此外对样品的晶态也进行了研究. 关键词： 铝薄膜 液体衬底 微观结构     

Intra-cavity spectroscopy with diode lasers

A multi-mode diode laser with an external cavity is studied experimentally and theoretically for its application to intra-cavity spectroscopy. One facet of a typical Ga_0.89Al_0.11As laser diode was antireflection-coated by deposition of HfO₂ such that 10^–3 residual reflectivity was left over. This diode was placed in an external optical cavity. The emission spectrum of this diode laser is highly sensitive to any frequency-dependent loss in the cavity, and the detectivity of such a loss grows with the pump rate. Even close to threshold, the absorption at 780 nm of Rb atoms with a density of 5×10¹⁰ cm^–3 has been detected. An adequate model for diode lasers based on rate equations and including frequency-dependent gain saturation is developed and applied to the calculations of output spectra. The sensitivity of these spectra to intra-cavity absorption is determined by the overall cavity loss — which is rather high — and the fraction of spontaneous emission in the total emission, in contrast with dye lasers where it is limited by nonlinear mode coupling. Various criteria for the sensitivity are suggested. The smallest detectable absorption with a perfectly antireflection-coated laser is 10^–6 cm^–1. Improvement of the characteristics of the laser diode would increase the sensitivity.     

Characterization of defect type and dislocation density in double oxide heteroepitaxial CeO2/YSZ/Si(001) films

In order to qualitatively and quantitatively analyze the structural defects including the defect types and their concentrations in oxide heteroepitaxial films, a new X-ray rocking-curve width-fitting method was used in the case of doubleCeO₂/YSZ/Si (YSZ=yttria-stabilized ZrO₂) films that were prepared by pulsed laser deposition. Two main defect types, angular rotation and oriented curvature, were found in both CeO₂ and YSZ. Dislocation densities of CeO₂ and YSZ, which were obtained from the angular rotations, are functions of the YSZ thickness. A distinct two-step correlation between dislocation densities of CeO₂ and YSZ was found that as the dislocation density of YSZ is higher than 2.4×10¹¹ cm^-2, the dislocation density of CeO₂ shows a high sensitivity with that of YSZ compared with the low relativity in lower dislocation density (<2.4×10¹¹ cm^-2). In addition, YSZ always has higher dislocation densities and oriented curvatures than CeO₂ in each specimen, which can be attributed to the smaller mosaic domain sizes in YSZ than in CeO₂ as observed by high-resolution transmission electron microscopy. Received: 12 August 2002 / Accepted: 14 August 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +81-3/5734-3369, E-mail: chun_hua_chen@hotmail.com     

多模虚共轭相干态与多模真空态组成的叠加态光场的等阶N次方H压缩

构造了由多模虚共轭相干态|{iZ_j^*}〉_q与多模真空态|{0_j}〉_q两者的线性叠加所组成的两态叠加多模叠加态光场|{Ψ_q⁽²⁾}〉_q.利用多模压缩态理论,首次对该态的等阶N次方H压缩特性进行了详细研究.结果发现:态|{Ψ_q⁽²⁾}〉_q是一种典型的多模非经典光场,它可呈现出周期性变化的、任意阶的等阶N次方H压缩效应.     

UHV-compatible magnetic material for atom optics

Magnetic videotape is of great interest for trapping and guiding cold atomic vapors, but was hitherto considered unsuitable for manipulating Bose–Einstein condensates (BEC) because of the presumed evolution of gas under vacuum. We have studied the outgassing in vacuum of the most promising tape, Ampex 398 Betacam SP. We find that after cleaning in ethanol and baking for 200 h at 100 °C the magnetic patterns are undisturbed and the outgassing is remarkably small: 4×10^-10 Torr l s^-1cm^-2, due mostly to hydrogen. This makes the tape exceedingly attractive for manipulation of BEC. Received: 12 February 2001 / Published online: 7 June 2001     

多模复共轭虚奇相干态光场的等阶高次压缩特性研究

本文构造了一种新型的多模复共轭虚奇相干态光场|Ψ_i,o^*(2)>_q,利用多模压缩态理论详细研究了态|Ψ_i,o^*(2)>_q的等阶N次方Y压缩与等阶N次方H压缩特性.结果发现:与多模奇相干态光场、多模复共轭奇相干态光场和多模虚奇相干态光场等所有的多纵模奇相干态光场的性质相同,多模复共轭虚奇相干态光场在任何条件下都不呈现等阶N次方Y压缩效应与等阶N次方H压缩效应,但在一定条件下却可恒处于等阶N-Y最小测不准态和等阶N-H最小测不准态.这表明所有的多纵模奇相干态光场之间,必定存在某些内在的必然性联系.     

Very high resolution CO laser spectrometer and first sub-Doppler line-shape studies near 60 THz (5 μm)

2 laser standards. Using this technique, we can tune the CO laser frequency with absolute frequency control within the gain profile of each laser transition. The frequency uncertainty is smaller than 15 kHz, corresponding to Δν/ν=2.5×10^-10. Moreover, we obtain a reduction of the CO laser linewidth by a factor of 2 down to 65 kHz, corresponding to a spectral resolution of δν/ν=1×10^-9. With this outstanding accuracy and resolution we studied the shape of saturation dips in rovibrational lines of CO and carbonyl sulfide (OCS) at low pressure (<5 Pa). The self-pressure-broadening rate of CO was found to be γ_c=+83(7) kHz/Pa in this pressure region. This value is about four times higher than values resulting from previous measurements at much higher pressures. To our knowledge the measurements described here are the first line-shape studies with sub-Doppler resolution in the 5 μm spectral region. Received: 4 November 1996     

Electron capture and scaling anomaly in polar molecules

We present a new analysis of the electron capture mechanism in polar molecules, based on von Neumann's theory of self-adjoint extensions. Our analysis suggests that it is theoretically possible for polar molecules to form bound states with electrons, even with dipole moments smaller than the critical value D₀=1.63×¹⁰⁻¹⁸ esu cm. We argue that the quantum mechanical scaling anomaly is responsible for the formation of these bound states.     

脉冲γ射线对光纤的辐射效应

介绍了光纤的损耗机制和γ射线对光纤的辐射效应,设计了针对脉冲γ射线作用于光纤而产生辐射感生损耗的实验测量系统。利用平均光子能量为0.3 MeV、脉冲宽度25 ns、剂量率2.03×107Gy.s-1,和平均光子能量为1.0 MeV、脉冲宽度25 ns、剂量率5.32×109Gy.s-1的2种脉冲γ射线分别作用于多模和单模光纤,分别采用波长为405,660,850,1 310和1 550 nm的激光光纤传输系统对辐射感生损耗进行了测量。获得了光纤辐射感生损耗和辐射剂量的关系,并对实验结果进行分析。从实验结果可以看出:在近红外到可见光范围内,脉冲γ射线对光纤作用产生的辐射感生损耗随探测波长减小而增大;在0.1~3.5 Gy剂量范围内,多模光纤辐射感生损耗和辐射剂量呈线性关系。分析辐射对光纤的作用机制和实验结果后得出:光纤基质原子的电子能级对传输光子的共振吸收而造成吸收损耗增加;光纤折射率分布的改变从而导致波导损耗增加。     

Effect of cathode structure on neutron yield performance of a miniature plasma focus device

In this Letter we report the effect of two different cathode structures - tubular and squirrel cage, on neutron output from a miniature plasma focus device. The squirrel cage cathode is typical of most DPF sources, with an outer, tubular envelope that serves as a vacuum housing, but does not carry current. The tubular cathode carries the return current and also serves as the vacuum envelope, thereby minimizing the size of the DPF head. The maximum average neutron yield of (1.82±0.52)×¹⁰⁵ n/shot for the tubular cathode at 4 mbar was enhanced to (1.15±0.2)×¹⁰⁶ n/shot with squirrel cage cathode at 6 mbar operation. These results are explained on the basis of a current sheath loading/mass choking effect. The penalty for using a non-transparent cathode negates the advantage of the smaller size of the DPF head.     

A Study on the Rheological and Magnetic Properties of Polymer-Bonded Magnets Using Polycarbonate as the Bonding Material

The effect of three metal oxides on the magnetic properties of polymer bonded magnets (PBMs) was studied. The three PBMs, using polycarbonate (PC) as binder and 5 wt% of Fe₃O₄, Fe₂O₃, or CuO nanoparticles, were prepared by melt extrusion in a twin screw extruder followed by compression molding. Transmission electron microscopic (TEM) images showed a better dispersion for the PC/Fe₃O₄ nanocomposite compared with that of the other nanocomposites. The dynamic intersection frequency (ω_c), which is related to the crossing of the G′ and G^″ curves, showed that there was more homogeneity in the PC/Fe₃O₄ and PC/Fe₂O₃ nanocomposites. The curves of saturation magnetization for the three nanocomposites showed that there was a relationship between the magnetic properties and the homogeneity of the nanoparticles studied by rheometry. Because the magnetic strength of PC/Fe₃O₄ was greater than that of the other nanocomposites, it was concluded that not only the intrinsic magnetic property of the filler was an important factor to increase the magnetic property, but also the homogeneity of the filler within the matrix had an important role.     

Three-body decay of a rubidium Bose–Einstein condensate

We have measured the three-body decay of a Bose–Einstein condensate of rubidium (⁸⁷Rb) atoms prepared in the doubly polarized ground state F=m _F =2. Our data are taken for a peak atomic density in the condensate varying between 2×10¹⁴ cm^-3 at initial time and 7×10¹³ cm^-3, 16 s later. Taking into account the influence of the uncondensed atoms on the decay of the condensate, we deduce a rate constant for condensed atoms L=1.8 (±0.5) ×10^-29 cm⁶ s^-1. For these densities we did not find a significant contribution of two-body processes such as spin dipole relaxation. Received: 24 November 1998 / Revised version: 26 June 1999 / Published online: 8 September 1999     

Top-hat beam Tm<Superscript>3+</Superscript>-doped fiber laser using an intracavity abrupt taper

The top-hat beam clad-pumped Tm³⁺-doped fiber laser was realized simply using an intracavity multi-mode abrupt taper. The ratio of the flat-top diameter to the spot diameter reaches 53%, with a small intensity variation less than 6%, and the top-hat beam’s half-divergence angle is only 5.3°. The fiber laser has a maximal output power of 5 W with slope efficiency of 39.7%, pumped by the 792 nm diode laser (LD). The abrupt taper is directly made on the multi-mode double-clad Tm³⁺-doped fiber near the fiber laser output end with the 0.45 ratio of taper waist diameter to fiber clad diameter, and this fiber end 4% Fresnel reflection is used to be the output coupler. The fiber laser’s high reflective coupler is an intracore multi-mode FBG, which is directly written into the multi-mode Tm³⁺-doped fiber core using femtosecond laser and phase mask, at the other fiber end. The abrupt taper has no obviously influence on the fiber laser output power, and the output laser spectrum.     

Mass scales for grand unified theories: Are there testable alternatives to SU(5)?

We consider the effects of the existence of new interactions consistent with the Weinberg angle, quark masses, and grand unification of all interactions except gravity. With one very specific exception, we find that the mass scales for such new interactions are greater than 10⁷ GeV. We conclude that if proton decay is not found at the SU(5) rate, then experiments at extremely high energies or sensitive to microweak or smaller effects will be necessary to decide among the alternatives. The exceptional case is viable only if new weak interactions exist with a mass scale below 10⁵ GeV.     

多模虚共轭相干态与多模真空态的叠加态光场的等阶N次方Y压缩

利用多模压缩态理论,研究了多模虚共轭相干态|{iZ_j^*}〉q与多模真空态|{O_j}〉q的叠加态|{Ψ_p⁽²⁾}〉q的广义非线性等阶N次方Y压缩特性.结果发现态|{Ψ_p⁽²⁾}〉q是一种典型的多模非经典光场,它在一定的条件下,可呈现出周期性变化的、任意奇数阶和任意偶数阶的等阶N次方Y压缩效应,而在另外的条件下,则可呈现出等阶N次方Y相似压缩现象.     

Raman scattering in layered compound 2H-WS2

Raman scattering of layer-type compound 2H-WS₂ has been studied at room temperature. The first-order Raman peaks are observed at 27.4, 357 and 423 cm^-1. The low-frequency peak at 27.4 cm^-1 is a rigid-layer mode, from which the interlayer shear force constant is estimated. The central force model is applied to the high-frequency phonons. The interlayer shear force constant is much smaller than the intralayer force constants. We also find several peaks due to the second-order processes.     

一种强度不等的新型两态叠加多模叠加态光场的等阶N次方Y压缩效应

本文构造了由多模虚共轭相干态{iZ_j^(a)*}>_q与多模虚共轭相干态的相反态|{-iZ_j^(b)*}>_q这两者的线性叠加所组成的一种强度不等的新型两态叠加多模叠加态光场 |Ψ^(ab)>_q.利用多模压缩态理论,对态|Ψ^(ab)>_q的等阶N次方Y压缩特性进行了详细研究.结果发现:该态在一定条件下可呈现出周期性变化的任意奇数阶和任意偶数阶的等阶N次方Y压缩效应,而在特殊条件下,则呈现出等阶N次方Y压缩简并现象.