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.     

Photothermal deflection microscopy for imaging sub-micronic defects in optical materials

Photothermal deflection is widely used to study defects in optical coatings and role of these defects in laser damage. Because defects responsible for laser damage are assumed to be nanometer-sized and lowly absorbing, both high spatial resolution and high sensitivity are required to detect them. In this work we theoretically and experimentally explore the capability of collinear photothermal deflection to give micronic resolution by reduction of the pump beam diameter. Thanks to a model describing temperature distribution and photothermal deflection, we have studied the effects of pump beam focusing on photothermal deflection. Then, we have developed a high resolution, high sensitivity microscope based on the photothermal deflection of a transmitted probe beam. The setup is characterized and the theoretical predictions are checked. We present a test of lateral spatial resolution obtained on specially prepared absorbing resolution targets and show that a lateral spatial resolution of 1 μm is reached on non-isolated defects. In case of single defects, we expect that 10 nm sized defects could be detected.     

Characterization by X-ray diffraction and electron microscopy of GaInAs and GaAsN single layers and quantum wells grown on GaAs

Experiments were performed on thick GaInAs and GaAsN layers and on GaInAs quantum wells grown on (1 1 1)B and (0 0 1) GaAs substrates. The aim of this work is to develop an experimental procedure in order to evaluate the chemical compositions and relaxation state of the samples at global as well as nanometre scale. Chemical analyses (EDS, RBS, etc.), X-ray diffraction (reciprocal space map, sin²ψ, etc.) and XTEM were carried out.The validity of the sin²ψ method on the above mentioned thin layers has been tested. Good accuracy is obtained for In and N composition but more work has to be done in order to optimize the determination of the relaxation state. Coupling TEM observations to these calculations gives valuable information on the relaxation mechanisms (misfit dislocations, stacking faults, microtwins, etc.).     

The synchrotron infrared beamline SISSI at ELETTRA

The infrared synchrotron radiation emitted from a bending magnet of the third generation storage ring ELETTRA is studied in detail by ray-tracing techniques. Constant magnetic-field and edge infrared emissions are taken into account in the project of the bending vacuum chamber and on the beamline design. Moreover the gain in flux and brightness with respect to a conventional source of the infrared radiation are calculated and discussed.     

Coherent gradient sensing microscopy (micro-CGS): A microscale curvature detection technique

Micro-CGS, a full-field, real-time, micro-interferometric technique for quantitative imaging is introduced. Micro-CGS extends the capabilities of the large-scale shearing interferometric technique of Coherent Gradient Sensing (CGS) in the micro-scale. The experimental setup is detailed and resulting interferograms are shown. A digital image processing program is created to compute specimen curvature from recorded fringe patterns. The experimental method is validated by the successful measurement of the curvature of 30 μm glass microspheres.     

Heavy ion induced damage in MgF 2 crystals

MgF 2 single crystals were irradiated with 22 Ne and 238 U ions in the energy range 31-2642 v MeV. The ion-induced damage was studied by optical spectroscopy, scanning force microscopy, and volume expansion measurements. The efficiency of damage creation depends on the stopping power (d E /d x ) and the fluence of the ions. Out-of-plane swelling and the formation of hillocks on the crystal surface require a critical d E /d x of approximately 3 v keV/nm. The efficiency for creation of simple defects (color centers) and of more complex damage (swelling, hillock formation) in MgF 2 is compared with the corresponding efficiency in LiF.     

Development and Infrared Applications of Chalcogenide Glass Optical Fibers

Chalcogenide glass fibers based on sulphide, selenide, telluride, and their rare earth doped compositions are being actively pursued both at the Naval Research Laboratory in Washington, D.C. (NRL) and worldwide. Great strides have been made in reducing optical losses using improved chem ical purification techniques, but further improvements are needed in both purification and fiberization technology to attain the theoretical optical losses. Despite this, current singlemode and multimode chalcogenide glass fibers are enabling numerous applications. Some of these applications include laser power delivery, chemical sensing, imaging, scanning near field microscopy spectroscopy, fiber infrared (IR) sources lasers, amplifiers, and optical switches. The authors assert that the research and development of chalcogenide glasses will grow in the foreseeable future, especially with respect to improvements the optical quality of the fibers and the performance of the fibers in existing future applications.     

NMR Microscopy and Image Processing for the Characterization of Flexible Polyurethane Foam

NMR tomography in material science? Three-dimensional images with a spacial resolution of 10 μm are now possible for porous, liquid-containing materials with NMR microscopy. They can be processed further with image evaluation methods and used, for example, for the characterization of open-cell foams (see illustration).     

基于相移技术的显微数字全息重构细胞相位

介绍了用显微镜物镜、压电陶瓷和CCD建立的一套测量细胞相位的显微数字全息光路,基于相移技术,给出了重构相位的理论分析,并用洋葱磷片叶细胞作为测试样品,完成了测量细胞相位的实验.结果表明：该系统可以完成细胞相位重构,系统分辨率不低于1 μm.     

AFM研究双链两亲性分子的自组织现象

通过AFM研究了双链两亲性分子双十八烷基-N-[4-（对-NN-二甲氨基苯基偶氮）吡啶丁酰基]-L-天冬氨酸酯溴化物（DDPA）LB膜在气固及气液界面的结构．在被水化后,DDPA的单层膜自发组织成双层和囊泡．在增加成像用力时,形成的双层很容易被压缩．在用轻敲模式（tappingmode）在溶液中成像时,观察到了囊泡被AFM针尖压扁成双层的过程．本文还提出了自组织过程的可能机理．