Ultrafast femtosecond all-optical modulation through nondegenerate two-photon absorption in silicon-on-insulator waveguides

We present numerical investigations of ultrafast femtosecond (with time duration of 100 fs at 1/e intensity point) all-optical modulation of a pump-probe wave arrangement by using nondegenerate two-photon absorption (TPA), namely cross absorption, inside silicon-on-insulator (SOI) optical waveguides. Our results show that when a pump pulse with femtosecond duration and a continuous probe wave are co-propagating along the SOI, the probe wave can be modulated inversely by the ultrafast pump pulse, whose modulation depth depends strongly on the system parameters such as the waveguide length, the peak power and initial chirp of the pump wave, the group velocity dispersion (GVD), etc.; this means that the modulation depth can be improved by an appropriate increase of the waveguide length, the pump peak, and the initial chirp, in addition, which has a larger value for the probe wavelength in the normal dispersion regime compared with the case of abnormal dispersion when the center wavelength of the pump wave is located at the zero-dispersion wavelength.     

Femtosecond pulse generation using stimulated Raman scattering in integrated silicon optical waveguide device

To generate ultrafast femtosecond optical pulses, we propose a model of an integrated device consisting of a Mach-Zehnder interferometer (MZI) with two symmetric 3 dB directional couplers and a straight waveguide based on the single-mode silicon-on-insulator (SOI) optical waveguide. The principle of pulse generation in the presented device is based on the strong stimulated Raman scattering (SRS) in silicon; the center wavelength of the pulse generated is tunable by changing the center wavelength of the co-propagating pump pulse. Numerical results show that, when a continuous wave (CW) with a weak power at 1670 nm wavelength and a pump pulse with a high peak power at 1550 nm wavelength are co-propagating, a narrow femtosecond pulse with a pulse width (full width at half maximum, FWHM) of ∼60 fs (FWHM of the pump pulse is 166.5 fs) can be achieved in the device proposed. In addition, when the waveguide length, pump peak power, and pump-pulse width are fixed, the properties of generated femtosecond pulse depend strongly on the incident chirp of the pump pulse and the CW power.     

Magnetization dynamics: ultra-fast and ultra-small

Ultrafast magnetic processes are of great scientific interest but also form the basis of high density magnetic recording applications. We demonstrate the uniqueness of time resolved, high resolution magnetic X-ray microscopy, and show that the motion of a magnetic vortex core can be imaged. The vortex core direction is hidden to most experimental techniques, but has a decisive influence on the dynamics of the magnetic structure.We imaged the switching of a ferromagnetic nanostructure by a spin polarized current pulse using time resolved X-ray microscopy. As opposed to the common uniform switching process due to Néel and Stoner–Wohlfarth, the magnetization in spin injection devices does not switch uniformly, but involves the motion of a magnetic vortex. To cite this article: Y. Acremann, C. R. Physique 9 (2008).     

Synthetic Gene Fragments in Genetic Engineering—The Renaissance of Chemistry in Molecular Biology

Chemistry and biology paved the way for a new scientific discipline, molecular genetics. The breathtaking progress in deciphering the genetic machinery of a living being within the last 20 years may be attributed mainly to the experimental skill and ingenuity of molecular biologists. Chemists lost interest in the investigation of biological problems. The advances in genetic engineering exemplify the new demands chemists are confronted with in contemporary science. Following a short introduction on the genetics of bacteria, the articles deals with the molecular basis of gene cloning, which is subdivided into in vitro recombination of nucleic acids and in vivo transformation of cells. Next, the criteria for the selection of passenger DNA, vectors, and host cells are discussed. Recombination and transformation are easy-to-learn experimental techniques, whereas the search for the positive clone requires detailed knowledge of microbiology and the ability to handle of a battery of modern biochemical techniques. Finally, the automated chemical synthesis of gene fragments serves to demonstrate that chemistry is starting to make its comeback into molecular biology.     

核磁共振技术及应用

核磁共振分析技术是利用物理原理，通过对核磁共振谱线特征参数的测定来分析物质的分子结构与性质。它不破坏被测样品的内部结构，是一种无损检测方法。 本文重点简介了核磁共振技术在医疗方面的应用－核磁振成像，它已成为医学诊断中一种重要手段。     

Thermodynamics of diffused phase transitions in dirty ferroelectrics

A thermodynamic approach of ferroelectric transition in which the polarization P goes to zero asymptotically when the temperature T increases is presented. The second derivatives of the free energy: $\text{dP}\text{dT}$, the susceptibility, and the specific heat exhibit maxima, but at different temperatures.     

基于平面螺旋电感的微型ICP激发源

随着微电子机械系统(MEMS)技术的发展,基于MEMS的光谱仪分光系统和光电检测系统研究报道日益增多,但是微型激发源的研究报道相对较少。文章介绍了一种基于表面微机械加工技术的微型电感耦合等离子体(ICP)激发源,其RF功耗在数瓦以下、氩气消耗量远低于常规ICP激发源。阐述了这种激发源的结构、加工工艺流程及性能指标。同时还介绍了作者设计制作的基于PCB工艺的微型ICP激发源,采用了平面螺旋电感线圈和平面梳状交指电容,在100 Pa氩气气压下用13.56 MHz,3.5 W射频功率激励点燃了等离子体火炬,给出了装置的外观微型ICP火炬的照片。最后展望了该微型的ICP激发源在光谱仪中的应用前景。     

光敏开关及其在化学生物学中的应用*

近年来随着生物医学技术的发展,人们需要越来越细致地在分子水平上研究各种生命过程。为了能够实现实时原位地观察活细胞或组织中的生命化学过程,需要使用以物理方法来选择性激活的分子探针。以共聚焦激光技术为基础的光敏开关能很好地解决这一问题。迄今,发展和用于光敏开关的光敏剂已成为化学生物学研究的重要方向。本文重点总结了各种可应用于共聚焦激光系统的单、双光子光敏基团（单光子的光敏基团主要有：硝基苯类、香豆素类等;双光子光敏基团主要有：香豆素类、喹啉类及吲哚衍生物类）以及这几类光敏基团在化学生物学中的应用。