The plasma mirror - A subpicosecond optical switch for ultrahigh power lasers

Plasma mirrors are devices capable of switching very high laser powers on subpicosecond time scales with a dynamic range of 20–30 dB. A detailed study of their performance in the near-field of the laser beam is presented, a setup relevant to improving the pulse contrast of modern ultrahigh power lasers ~TW–PW!. The conditions under which high reflectivity can be achieved and focusability of the reflected beam retained are identified. At higher intensities a region of high specular reflectivity with rapidly decreasing focusability was observed, suggesting that specular reflectivity alone is not an adequate guide to the ideal range of plasma mirror operation. It was found that to achieve high reflectivity with negligible phasefront distortion of the reflected beam the inequality csDt,lLaser must be met (cs : sound speed, Dt: time from plasma formation to the peak of the pulse!. The achievable contrast enhancement is given by the ratio of plasma mirror reflectivity to cold reflectivity.

Investigation of the temperature homogeneity of die melt flows in polymer extrusion

Polymer extrusion is fundamental to the processing of polymeric materials and melt flow temperature homogeneity is a major factor which influences product quality. Undesirable thermal conditions can cause problems such as melt degradation, dimensional instability, weaknesses in mechanical/optical/geometrical properties, and so forth. It has been revealed that melt temperature varies with time and with radial position across the die. However, the majority of polymer processes use only single-point techniques whose thermal measurements are limited to the single point at which they are fixed. Therefore, it is impossible for such techniques to determine thermal homogeneity across the melt flow. In this work, an extensive investigation was carried out into melt flow thermal behavior of the output of a single extruder with different polymers and screw geometries over a wide range of processing conditions. Melt temperature profiles of the process output were observed using a thermocouple mesh placed in the flow and results confirmed that the melt flow thermal behavior is different at different radial positions. The uniformity of temperature across the melt flow deteriorated considerably with increase in screw rotational speed while it was also shown to be dependent on process settings, screw geometry, and material properties. Moreover, it appears that the effects of the material, machine, and process settings on the quantity and quality of the process output are heavily coupled with each other and this may cause the process to be difficult to predict and variable in nature

A switch in the cellular localization of macrophage migration inhibitory factor in the rat testis after ethane dimethane sulfonate treatment.

Macrophage migration inhibitory factor (MIF), one of the first cytokines to be discovered, has recently been localized to the Leydig cells in adult rat testes. In the following study, the response of MIF to Leydig cell ablation by the Leydig cell-specific toxin ethane dimethane sulfonate (EDS) was examined in adult male rats. Testicular MIF mRNA and protein in testicular interstitial fluid measured by ELISA and western blot were only marginally reduced by EDS treatment, in spite of the fact that the Leydig cells were completely destroyed within 7 days. Immunohistochemistry using an affinity-purified anti-mouse MIF antibody localized MIF exclusively to the Leydig cells in control testes. At 7 days post-EDS treatment, there were no MIF immunopositive Leydig cells in the interstitium, although distinct MIF immunostaining was observed in the seminiferous tubules, principally in Sertoli cells and residual cytoplasm, and some spermatogonia. A few peritubular and perivascular cells were also labelled at this time, which possibly represented mesenchymal Leydig cell precursors. At 14 and 21 days, Sertoli cell MIF immunoreactivity was observed in only a few tubule cross-sections, while some peritubular and perivascular mesenchymal cells and the re-populating immature Leydig cells were intensely labeled. At 28 days after EDS-treatment, the MIF immunostaining pattern was identical to that of untreated and control testes. The switch in the compartmentalization of MIF protein at 7 days after EDS-treatment was confirmed by western blot analysis of interstitial tissue and seminiferous tubules separated by mechanical dissection. These data establish that Leydig cell-depleted testes continue to produce MIF, and suggest the existence of a mechanism of compensatory cytokine production involving the Sertoli cells. This represents the first demonstration of a hitherto unsuspected pattern of cellular interaction between the Leydig cells and the seminiferous tubules which is consistent with an essential role for MIF in male testicular function.

Precipitation, microstructure and mechanical properties of low nickel maraging steel

A maraging steel with a composition of Fe–12·94Ni–1·61Al–1·01Mo–0·23Nb (wt-%) was investigated. Optical, scanning electron and transmission electron microscopy and X-ray diffraction analysis were employed to study the microstructure of the steel after different aging periods at temperatures of 450–600°C. Hardness and Charpy impact toughness of the steel were measured. The study of microstructure and mechanical properties showed that nanosized precipitates were formed homogeneously during the aging process, which resulted in high hardness. As the aging time is prolonged, precipitates grow and hardness increases. Fractography of the as forged steel has shown mixed ductile and brittle fracture and has indicated that the steel has good toughness. Relationships among heat treatment, microstructure and mechanical properties are discussed. Further experiments using tensile testing and impact testing for aged steel were carried out.

Mechanical fatigue and spectrum analysis of small-satellite

Mechanical fatigue due to environmental loads and spectrum analysis due to launch loads of the primary structure of a low cost, low-earth orbit small satellite intended for earth observation missions are presented. The payload of the satellite under consideration is a precise optical unit to image the earth’s surface having a mass of 45 kg. 3-D Finite Element Model for the satellite structure is generated by applying substructure method. Modal analysis is required to determine natural frequencies of the satellite and define its mode shape. Then, ranking of mode shapes according to specific constraint is performed. Harmonic analysis at resonance frequencies with the highest ranking is done and cumulative fatigue damage analysis is performed. Spectrum analysis is performed for Small Sat structure to verify the satellite structure reliability under all dynamic random vibration loads applied during transportation and launch cases.

Optical Cooling Using the Dipole Force

The term `laser cooling' is applied to the use of optical means to cool the motional energies of either atoms and molecules, or micromirrors. In the literature, these two strands are kept largely separate; both, however suffer from severe limitations. Laser cooling of atoms and molecules largely relies on the internal level structure of the species being cooled. As a result, only a small number of elements and a tiny number of molecules can be cooled this way. In the case of micromirrors, the problem lies in the engineering of micromirrors that need to satisfy a large number of constraints---these include a high mechanical Q-factor, high reflectivity and very good optical quality, weak coupling to the substrate, etc.---in order to enable efficient cooling. During the course of this thesis, I will draw these two sides of laser cooling closer together by means of a single, generically applicable scattering theory that can be used to explain the interaction between light and matter at a very general level. I use this `transfer matrix' formalism to explore the use of the retarded dipole--dipole interaction as a means of both enhancing the efficiency of micromirror cooling systems and rendering the laser cooling of atoms and molecules less species selective. In particular, I identify the `external cavity cooling' mechanism, whereby the use of an optical memory in the form of a resonant element (such as a cavity), outside which the object to be cooled sits, can potentially lead to the construction of fully integrated optomechanical systems and even two-dimensional arrays of translationally cold atoms, molecules or even micromirrors.

Statistical-mechanical description of classical test-particle dynamics in the presence of an external force field: modelling noise and damping from first principles

Aiming to establish a rigorous link between macroscopic random motion (described e.g. by Langevin-type theories) and microscopic dynamics, we have undertaken a kinetic-theoretical study of the dynamics of a classical test-particle weakly coupled to a large heat-bath in thermal equilibrium. Both subsystems are subject to an external force field. From the (time-non-local) generalized master equation a Fokker-Planck-type equation follows as a "quasi-Markovian" approximation. The kinetic operator thus defined is shown to be ill-defined; in specific, it does not preserve the positivity of the test-particle distribution function f(x, v; t). Adopting an alternative approach, previously introduced for quantum open systems, is proposed to lead to a correct kinetic operator, which yields all the expected properties. A set of explicit expressions for the diffusion and drift coefficients are obtained, allowing for modelling macroscopic diffusion and dynamical friction phenomena, in terms of an external field and intrinsic physical parameters.

Effects of sintering temperature on morphology and mechanical characteristics of 3D printed porous titanium used as dental implant

Porous titanium samples were manufactured using the 3D printing and sintering method in order to determine the effects of final sintering temperature on morphology and mechanical properties. Cylindrical samples were printed and split into groups according to a final sintering temperature (FST). Irregular geometry samples were also printed and split into groups according to their FST. The cylindrical samples were used to determine part shrinkage, in compressive tests to provide stress-strain data, in microCT scans to provide internal morphology data and for optical microscopy to determine surface morphology. All of the samples were used in microhardness testing to establish the hardness. Below 1100 C FST, shrinkage was in the region of 20% but increased to approximately 30% by a FST of 1300 C. Porosity varied from a maximum of approximately 65% at the surface to the region of 30% internally. Between 97 and 99% of the internal porosity is interconnected. Average pore size varied between 24 µm at the surface and 19 µm internally. Sample hardness increased to in excess of 300 HV0.05 with increasing FST while samples with an FST of below 1250 C produced an elastic-brittle stress/strain curve and samples above this displayed elastic-plastic behaviour. Yield strength increased significantly through the range of sintering temperatures while the Young's modulus remained fairly consistent. © 2013 Elsevier B.V.

Enhancing non-classicality in mechanical systems

We study the effects of post-selection measurements on both the non-classicality of the state of a mechanical oscillator and the entanglement between two mechanical systems that are part of a distributed optomechanical network. We address the cases of both Gaussian and non-Gaussian measurements, identifying in which cases simple photon counting and Geiger-like measurements are effective in distilling a strongly non-classical mechanical state and enhancing the purely mechanical entanglement between two elements of the network.

Driven optomechanical systems for mechanical entanglement distribution

We consider the distribution of entanglement from a multimode optical driving source to a network of remote and independent optomechanical systems. By focusing on the tripartite case, we analyse the effects that the features of the optical input states have on the degree and sharing structure of the distributed, fully mechanical, entanglement. This study, which is conducted looking at the mechanical steady state, highlights the structure of the entanglement distributed among the nodes and determines the relative efficiency between bipartite and tripartite entanglement transfer. We discuss a few open points, some of which are directed towards the bypassing of such limitations.

A conserved spider silk domain acts as a molecular switch that controls fibre assembly

A huge variety of proteins are able to form fibrillar structures(1), especially at high protein concentrations. Hence, it is surprising that spider silk proteins can be stored in a soluble form at high concentrations and transformed into extremely stable fibres on demand(2,3). Silk proteins are reminiscent of amphiphilic block copolymers containing stretches of polyalanine and glycine-rich polar elements forming a repetitive core flanked by highly conserved non-repetitive amino-terminal(4,5) and carboxy-terminal(6) domains. The N-terminal domain comprises a secretion signal, but further functions remain unassigned. The C-terminal domain was implicated in the control of solubility and fibre formation(7) initiated by changes in ionic composition(8,9) and mechanical stimuli known to align the repetitive sequence elements and promote beta-sheet formation(10-14). However, despite recent structural data(15), little is known about this remarkable behaviour in molecular detail. Here we present the solution structure of the C-terminal domain of a spider dragline silk protein and provide evidence that the structural state of this domain is essential for controlled switching between the storage and assembly forms of silk proteins. In addition, the C-terminal domain also has a role in the alignment of secondary structural features formed by the repetitive elements in the backbone of spider silk proteins, which is known to be important for the mechanical properties of the fibre.

Thermo-Mechanical Properties of Poly ε-Caprolactone/Poly L-Lactic Acid Blends: Addition of Nalidixic Acid and Polyethylene Glycol Additives Journal of the Mechanical Behavior of Biomedical Materials

The search for ideal biomaterials is still on-going for tissue regeneration. In this study, blends of Poly ε-caprolactone (PCL) with Poly l-lactic acid (PLLA), Nalidixic Acid (NA) and Polyethylene glycol (PEG) were prepared. Mechanical and thermal properties of the blends were investigated by tensile and flexural analysis, DSC, TGA, WXRD, MFI, BET, SEM and hot stage optical microscopy. Results showed that the loading of PLLA caused a significant decrease in tensile strength and almost total eradication of the elongation at break of PCL matrix, especially after PEG and NA addition. Increased stiffness was also noted with additional NA, PEG and PLLA, resulting in an increase in the flexural modulus of the blends.
Isothermal degradation indicated that bulk PCL, PLLA and the blends were thermally stable at 200°C for the duration of 2h making extrusion of the blends at this temperature viable. Morphological study showed that increasing the PLLA content and addition of the very low viscosity PEG and powder NA decreased the Melt Flow Indexer and increased the viscosity.
At the higher temperature the PLLA begins to soften and eventually melts allowing for increased flow and, coupling this with, the natural increase in MFI caused by temperature is enhanced further. The PEG and NA addition increased dramatically the pore volume which is important for cell growth and flow transport of nutrients and metabolic waste.

A chemically unlocked binary molecular switch

A highly luminescent and sensitive terbium complex of a ligand comprising of a phthalimide group appended to a DO3A moiety is an active pH sensor that is conditional on its previous pH. © 2012 The Royal Society of Chemistry.