Fastscan z-scan system for determining optical non-linearities in semiconductors

Traditional z-scan techniques have been used to determine the non-linear optical coefficients of semiconductors. This is usually carried out by scanning a sample along the optical z-axis using a manual or computer-controlled linear translation stage. We report upon a novel z-scan method involving a fastscanning sampleholder: This gives real-time z-scan traces and greatly improves experimental efficiency and applicability. The technique is used here to study the two-photon absorption coefficient of ZnSe.     

Correlation and anticorrelation of fractal aggregates. A model for «spinodal-type» aggregation

Summary We present a simple phenomenological model, based on mass conservation arguments, in order to describe aggregation in dense solutions. It has been previously shown that fordiffusion-limited processes scattered intensity distributions exhibit a peak atq≠0 which grows in time and moves to smaller and smallerq vectors. According to the model, each aggregate is surrounded by a depletion region whose size depends on the aggregation kinetics. The form factor for such a cluster should satisfy local mass conservation and hence exhibit a pronounced depression atq=0. the initial stages of aggregation, when the clusters are far apart, are accounted on the basis of a form factor alone. The model shows good agreement with the data and also explains the experimental evidence that the peak does not appear inreaction-limited conditions. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

新型卟啉衍生物反饱和吸收研究

利用纳秒脉冲激光下的z扫描方法对两种新型卟啉衍生物在450 nm—490 nm范围内的反饱和吸收进行了研究.通过与四苯基卟啉的比较发现，在Soret和Q吸收带之间，两种新型卟啉衍生物都具有大的反饱和吸收.采用五能级模型对实验结果进行了分析，比较了在不同波长范围内各个样品的优势.实验结果表明，在450 nm—490 nm范围内，两种新型的卟啉衍生物有着比四苯基卟啉更好的光限制应用前景. 关键词： 反饱和吸收 z扫描')" href="#">z扫描 卟啉衍生物     

A new integral equation for treating high-intensity multiphoton processes

Summary In the present paper we derive a new integral equation for the non-perturbative evaluation of transition matrix elements for high-intensity multiphoton processes. Our method will be partly based on the Kramers-Henneberger transformation. In the course of the development of our theory we shall relate our approach to previous investigations along similar lines. As a specific example we shall apply our integral equation to the problem of light scattering and present a multiphoton generalization of the Kramers-Heisenberg dispersion formula. We shall then apply this general formula to the study of higher-harmonic generation from a short-range potential with a single bound state, and evaluate the ratesR _n of then-th-order harmonic for a set of specifically chosen parameters. This work has been supported by the East-West Program of the Austrian Academy of Sciences under Project Nr. 45.174/1-46a/93 of the ?sterreichische Bundesministerium für Wissenschaft und Forschung.     

A biomagnetic method for studying gastro-intestinal activity

Summary A novel method to investigate the gastro-intestinal activity by means of a superconducting instrumentation based on SQUID sensors developed for biomagnetic measurements is presented. The magnetic field generated by a small marker ingested by the subject is measured over the abdomen. The localization of the marker,i.e. a magnetic dipole, is carried out by means of a standard algorithm used in biomagnetic studies. In this way it is possible to noninvasively study the activity of the gastro-intestinal system. This new approach may lead to important clinical applications.     

z切LiTaO3单晶的冲击相变研究

利用二级轻气炮加载下的冲击Hugoniot线(冲击波速度D-粒子速度u关系)和粒子速度剖面测量，结合基于密度泛函理论的平面波赝势计算研究了z切LiTaO₃单晶的高压相变.实验发现，D-u关系在u=0.95km/s附近出现明显拐折；实测波剖面中25.9 GPa和32.6 GPa时观测到弹-塑性双波结构，而终态压力为42.7 GPa和53.0 GPa时则为三波结构.上述结果都清楚地表明z切LiTaO₃单晶冲击相变的发生，相变起始压力约为37.9 GPa.同时，理论计算的菱形相(R3c对称群)压缩线与低压实验数据符合较好，而正交相(Pbnm对称群)压缩线则与扣除热压贡献的高压实验数据相符，由此推断z-切LiTaO₃的高压相为正交结构.从实验和理论上澄清了z切LiTaO₃的相变起始压力和高压相晶体结构的认识，研究工作亦对类似单晶材料的冲击相变研究有参考价值.     

Cu/TiOx复合薄膜的电子态分析及其对亲水性的影响

在室温下,采用射频磁控溅射法制备了Cu/TiO_x纳米晶复合薄膜.利用X射线粉末衍射（XRD）、X射线光电子能谱（XPS）对其结构进行表征,并研究了Cu/TiO_x复合薄膜的UV-vis吸收谱和亲水性.结果表明,退火前后薄膜中钛元素皆以Ti³⁺形式存在.薄膜在可见区有吸收,吸收限为600 nm左右.Cu/TiO_x复合薄膜具有良好的亲水性.这主要是由于Cu的掺杂,使得薄膜的性能的亲水性变好. 关键词： x复合薄膜')" href="#">Cu/TiO_x复合薄膜 射频磁控溅射 XPS 亲水性     

On the role of secondary interactions in the production of bremsstrahlung spectra from a thick target

Bremsstrahlung emission, or radiation loss, is the dominant mechanism of energy dissipation of electrons at relativistic energies greater than a few MeV when it is subjected to acceleration in the field of the nucleus or of the electrons. In this study, the Monte Carlo calculations for bremsstrahlung spectra have been described for the case of a thick tungsten target with incident electron beams from 10 to 50 MeV, where secondary interactions induced by the electrons and photons in the target, such as energy loss, absorption, scattering, and (e ⁺, e ⁻)-pair production effects, were taken into account. The text was submitted by the authors in English.     

Bacteriochlorin <Emphasis Type="Italic">e</Emphasis><Subscript>6</Subscript> as a sensitizer for photodynamic therapy

A simple scheme of modification of the native pigment bacteriochlorophyll a, extracted from the biomass of phototropic bacteria, for the purpose of obtaining a water-soluble derivative — bacteriochlorin e ₆ — which is similar in molecular structure to the effective sensitizer chlorin e ₆ and, in addition, has an intense longwave absorption band in the near-infrared region of the spectrum where biological tissues weakly absorb light — has been developed. The ease of production of bacteriochlorin e ₆ in combination with its physicochemical and spectral properties allow it to be considered as a promising sensitizer for photodynamic therapy.Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 71, No. 6, pp. 745–748, November–December, 2004.This revised version was published online in April 2005 with a corrected cover date.     

The effective absorption cross-section of thermal neutrons in a medium containing strongly or weakly absorbing centres

The structure of a heterogeneous system influences diffusion of thermal neutrons. The thermal-neutron absorption in grained media is considered in the paper. A simple theory is presented for a two-component medium treated as grains embedded in the matrix or as a system built of two types of grains (of strongly differing absorption cross-sections). A grain parameter is defined as the ratio of the effective macroscopic absorption cross-section of the heterogeneous medium to the absorption cross-section of the corresponding homogeneous medium (consisting of the same components in the same proportions). The grain parameter depends on the ratio of the absorption cross-sections and contributions of the components and on the size of grains. The theoretical approach has been verified in experiments on prepared dedicated models which have kept required geometrical and physical conditions (silver grains distributed regularly in Plexiglas). The effective absorption cross-sections have been measured and compared with the results of calculations. A very good agreement has been observed. In certain cases the differences between the absorption in the heterogeneous and homogeneous media are very significant. A validity of an extension of the theoretical model on natural, two-component, heterogeneous mixtures has been tested experimentally. Aqueous solutions of boric acid have been used as the strongly absorbing component. Fine- and coarse-grained pure silicon has been used as the second component with well-defined thermal-neutron parameters. Small and large grains of diabase have been used as the second natural component. The theoretical predictions have been confirmed in these experiments.     

Photoproduction of η -mesons off nuclei for Eγ ? 2.2 GeV

The photoproduction of η -mesons off ¹²C , ⁴⁰Ca , ⁹³Nb , and ^nat Pb nuclei has been measured with a tagged photon beam with energies between 0.6 and 2.2GeV. The experiment was performed at the Bonn ELSA accelerator with the combined setup of the Crystal Barrel and TAPS calorimeters. It aimed at the in-medium properties of the S ₁₁(1535) nucleon resonance and the study of the absorption properties of nuclear matter for η -mesons. Careful consideration was given to contributions from ηπ final states and secondary production mechanisms of η -mesons, e.g. from inelastic πN reactions of intermediate pions. The analysis of the mass number scaling shows that the nuclear absorption cross-section for η -mesons is constant over a wide range of the η momentum. The comparison of the excitation functions to data off the deuteron and to calculations in the framework of a BUU model show no unexplained in-medium modifications of the S ₁₁(1535) .     

Dispersive optical constants of a-Se100−xSbx films

Thin films of amorphous Se_100−xSb_x (x=5,10 and 20 at%) system are deposited on a silicon substrate at room temperature (300 K) by thermal evaporation technique. The optical constant such as refractive index (n) has been determined by a method based on the envelope curves of the optical transmission spectrum at normal incidence by a Swanpoel method. The oscillator energy (E_o), dispersion energy (E_d) and other parameters have been determined by the Wemple–DiDomenico method. The absorption coefficient (α) has been determined from the reflectivity and transmitivity spectrum in the range 300–2500 nm. The optical-absorption data indicate that the absorption mechanism is a non-direct transition. We found that the optical band gap, E_g^opt, decreases from 1.66±0.01 to 1.35±0.01 eV with increase Sb content.     

Influence of the helium autoionization structures on the single/double ionization signal

Calculations of intense field (around 10¹⁶ W/cm²) single- and double-ionization processes in helium at XUV wavelengths are presented. The laser wavelength is chosen near the | 2s2p ¹ P autoionization structure and the dynamics are explored. Single and double ionization yields, as well as the photoelectron energy spectrum for photon energies around the autoionization structure are calculated. In the case of a pulse of few femtoseconds duration, no significant enhancement of the double ionization yield has been found in tuning the photon frequency around the peak of the resonance. It is also shown that in the case of a long pulse (and hence narrow compared with the relevant autoionization width), the branching ratio of double to single ionization yield can be relatively enhanced by tuning to the absorption minimum of the resonance. Received 19 February 2002 / Received in final form 2 May 2002 Published online 19 July 2002     

Two-photon photoionization by singlet–singlet annihilation in polycrystalline,thin perylene films

Two-photon photoionization processes in thin, polycrystalline, α- and β-crystalline perylene films have been investigated for photon energies at the optical absorption threshold. The kinetic energy of the emitted electrons is found to be essentially independent of the photon energy, indicating that geometrically relaxed excited states are involved. The high ionization yield at such low photon energies is therefore attributed rather to the annihilation of adiabatically relaxed singlet excitons than to the direct two-photon ionization process. This leads to hole final states different from those obtained in one-photon, vertical ionization processes.     

Relativistic second-harmonic generation in a laser-produced plasma

Summary In this paper the relativistic second-harmonic generation of a high-power laser radiation in a laser-produced plasma has been studied theoretically in the presence of a self-generated magnetic field. The relativistic Vlasov equation has been employed for the nonlinear response of the electrons in the hot magnetized plasma. It is observed that the power conversion efficiency of the generated second harmonic wave is much higher for relativistic calculations than that for nonrelativistic calculations. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

The small axial charge of the N(1535)-resonance

There is a natural cancellation between the contributions of the qqq and qqqq components to the axial charge of the N(1535)-resonance. While the probability of the former is larger than that of the latter, its coefficient in the axial charge expression is exceptionally small. The magnitude of two of the corresponding coefficients of the qqqq components is in contrast large and has the opposite sign. This result provides a phenomenological illustration of the recent unquenched lattice calculation result that the axial charge of the N(1535)-resonance is very small, if not vanishing (T.T. Takahashi, T. Kunihiro, arXiv: 0801.4707 [hep-lat]). The result sets an upper limit on the magnitude of the probability of qqqq components as well.     

Energy analysis of a toroidal magnetohydrodynamic plasma with fluid motion

Summary The relaxation process in an ideal magnetohydrodynamic (MHD) plasma with fluid velocity and enclosed in a toroidal vessel has been discussed. The expressions for the field parameters and the energy state of the system have been derived. The expression for the minimum energy state of the system has been deduced. An analysis of the conservation of energy of the system in the presence of weak and strong magnetic fields has also been presented. The author of this paper has agreed to not receive the proofs for correction. This work was commenced and partly completed during author's short stay at I.C.T.P.-Trieste, Italy, in 1988.     

Pb1－xMnxSe薄膜的光学特性

采用分子束外延的方法在BaF₂(111)衬底上制备出了高质量的Pb_1-xMn_xSe(0≤x≤0.0681)薄膜.X射线衍射结果表明，Pb₁-xMn_xSe薄膜为立方相NaCl型结构，没有观察到MnSe相分离现象，薄膜的取向为平行于衬底(111)晶面.晶格常数随着Mn含量的增加逐渐减小，Mn含量由Vegard公式得到.通 关键词： 1-xMn_xSe外延薄膜')" href="#">Pb_1-xMn_xSe外延薄膜 透射光谱 带隙 折射率     

A new lateral distribution function for electrons in extensive air showers (EAS) detected near sea level

Summary A detailed analysis of Extensive Air Showers in the size range 10⁴–10⁶ particles detected near sea level has yielded a new distribution function for the radial distribution of EAS electrons. The goodness-of-fit criteria applied to the present and already existing similar distribution functions confirm that the present function is appropriate in EAS at radial distances beyond 20 m from the shower axis.