MEMS-based adaptive optics scanning laser ophthalmoscopy

We have developed a compact, robust adaptive optics (AO) scanning laser ophthalmoscope using a microelectromechanical (MEMS) deformable mirror (DM). Facilitated with a Shack-Hartmann wavefront sensor, the MEMS-DM-based AO operates a closed-loop modal wave aberration correction for the human eye and reduces wave aberrations in most eyes to below 0.1 microm rms. Lateral resolution is enhanced, and images reveal a clear cone mosaic near the foveal center. The significant increase in throughput allows for a confocal pinhole whose diameter is less than the Airy disc of the collection lens, thereby fully exploiting the axial resolution capabilities of the system.     

Coupling between precipitation and contact-line dynamics: multiring stains and stick-slip motion

The contact line in an evaporating drop can stay pinned to form a single ring or can shrink in a discontinuous stepwise manner and generate multiple rings. We demonstrate the latter with DNA solutions and attribute it to a pinning-depinning cycle that generates new contact lines. The new contact line recedes after depinning and is repinned at an internal precipitate ring that determines the location of the next contact line. Each precursor ring is formed when DNAs are trapped by an internal microstagnation flow and precipitation dynamics hence control this unsteady drop motion.     

Frequency-domain modal delay measurement for higher-order mode fiber based on stretched pulse interference

We propose and demonstrate a novel modal delay measurement technique for a higher-order mode fiber (HOF) based on optical frequency-domain reflectometry (OFDR) using an extremely simple, entirely passive, and ultrafast wavelength sweeping mechanism, namely, dispersion-induced optical pulse stretching. We obtained a high temporal resolution of approximately 1.12 ps, which was sufficient for discerning the four excited modes in an HOF with a length of only approximately 5 m. The results from our measurements were very consistent with those obtained by using a traditional time-domain measurement method and a conventional OFDR measurement based on a tunable CW laser. Our proposed technique can be also easily adapted to perform conventional time-domain modal delay measurements for very long HOFs.     

Phase transitions in liquid crystal 6O.4 (p-<Emphasis Type="Italic">n</Emphasis>-hexyloxybenzylidine-p′-<Emphasis Type="Italic">n</Emphasis>-butylaniline)

DSC measurements on p-n-hexyloxybenzylidine-p′-n-butylaniline (6O.4) showed that the crystalline to liquid crystalline (K-S _H) transition at 33.7°C observed in the heating cycle does not revert even when the sample is cooled down to −100°C. Hence it is inferred that a physically stable supercooled liquid crystalline phase is formed on cooling 6O.4. To investigate the K-S _H transition further the techniques of polarized microscopy and X-ray diffraction were used which concurred with the DSC results. Quasielastic neutron scattering measurements carried out to study the re-orientational motions in the ordered phases of 6O.4 (K and S _H) show that while in the crystalline phase (at RT) the re-orientational motion is found to involve only the core of the molecule, in the S _H phase (at 45°C) the dynamics involves the whole molecule and this motion is found to persist even when the sample cools back to room temperature corroborating the results of the DSC, microscopy and X-ray diffraction.      

Comparative nucleophilic reactivities in carboxylate,phosphinate, and thiophosphate esters cleavage

Nucleophilic substitution reaction of p‐nitrophenyl acetate (PNPA), p‐nitrophenyldiphenyl phosphinate, and pesticide parathion with different α‐nucleophiles [I] have been studied at 27 °C in different pH in the presence of a novel cationic surfactant. The kinetic study was performed spectrophotometrically under pseudo‐first order conditions with the α‐nucleophile in excess. The pK_a of nucleophiles have also been determined by kinetic method. In the presence of surfactant, the rate constant increased with increasing surfactant concentration up to a limiting value. This behavior has been analyzed in quantitative terms on the basis of pseudo‐phase model of micellar catalysis. Finally the nucleophilic reactivity of hydroxamate ions has been compared with other α‐nucleophiles, like oxime, hydroxybenzotriazole, and 2‐iodosobenzoic acid (IBA). The order of cleavage of electrophilic centers, that is, C?O, P?O, and P?S have also been discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

Ultrasensitive long-period fiber gratings for broadband modulators and sensors

We demonstrate long-period fiber gratings whose attenuation can be changed by 25 dB over a 48-nm spectral band, with ambient-index changes of only 2.7 x 10(-4). To achieve this, the fiber waveguide is engineered to induce coupling between the core and a highly ambient-sensitive cladding mode with identical group velocities. The device schematic allows arbitrarily high index sensitivities to be achieved, which makes it an attractive platform for realizing sensors and modulators that respond to small index changes.     

Computational Insights into the Role of External and Local Electric Fields in Macrocyclic Chemical and Biological Systems

The investigation of the role of the electric field in systems of widespread interest employing computational techniques is an emerging area of research. The outcome of applying an oriented external electric field (OEEF) on the geometric and electronic properties of the chemically unique π-conjugated cyclic carbon ring compounds has been explored with density functional theory (DFT). Distinct changes in the structural and electronic features of such ring compounds are observed upon the application of OEEFs. Importantly, the calculations indicate that a mixed aliphatic-aromatic conjugated ring converts from a singlet to a triplet after the application of an OEEF, suggesting potential applications in optoelectronics for such molecules, without the need for photochemically induced change in the spin state. Furthermore, the influence of built-in local electric fields (LEFs) present in naturally occurring macrocyclic systems such as valinomycin has also been explored. Static and ab initio molecular dynamics (AIMD) calculations indicate that LEFs are the primary driving factor in determining the energetically favoured position of counter anions such as chloride (Cl⁻) in the potassium (K⁺) and sodium (Na⁺) coordinated valinomycin macrocycle structures: they exist inside the cage in the case of K⁺ sequestration by valinomycin and outside for Na⁺. This divergence has been proposed to be the determining factor for the selectivity of the valinomycin macrocycle for binding a K⁺ cation over Na⁺.     

Plasmonic giant quantum dots: hybrid nanostructures for truly simultaneous optical imaging,photothermal effect and thermometry

Hybrid semiconductor–metal nanoscale constructs are of both fundamental and practical interest. Semiconductor nanocrystals are active emitters of photons when stimulated optically, while the interaction of light with nanosized metal objects results in scattering and ohmic damping due to absorption. In a combined structure, the properties of both components can be realized together. At the same time, metal–semiconductor coupling may intervene to modify absorption and/or emission processes taking place in the semiconductor, resulting in a range of effects from photoluminescence quenching to enhancement. We show here that photostable ‘giant’ quantum dots when placed at the center of an ultrathin gold shell retain their key optical property of bright and blinking-free photoluminescence, while the metal shell imparts efficient photothermal transduction. The latter is despite the highly compact total particle size (40–60 nm “inorganic” diameter and <100 nm hydrodynamic diameter) and the very thin nature of the optically transparent Au shell. Importantly, the sensitivity of the quantum dot emission to local temperature provides a novel internal thermometer for recording temperature during infrared irradiation-induced photothermal heating.     

从下落的水滴中收集能量

两个世纪前,考古学家在古巴比伦废墟中的挖掘,得到了一个有趣的发现——追溯到公元前约1800年,用楔形文字书写的粘土碑。这些碑文包括对迷信行为的描述,神职人员通过倾倒在水上的油形成的图样,预言未来。这篇古文代表了已知最早的关于疏水效应的参考文献。现在两个独立的现代科学家团队已经使用这个想法,开发了非常有效的没有移动部件的纳米发电机。使纳米发电机有效的关键技术是利用疏水效应。     

Electrifying the nitrogen cycle: An electrochemical endeavor

The advent of the Haber–Bosch (HB) process has made accessible all forms of nitrogenous derivatives (ammonia, nitrates, nitrites, and amines), revolutionizing the chemical industry. However, the depleting natural resources and growing environmental concerns demand a shift toward an energy-abated and environment-friendly alternative to the HB process, hence the shift in the recent research focus. The electrochemical reduction pathway offers one of the most alluring prospects as a carbon-free, environment-friendly, energy-abated alternative to the HB process. In this context, we provide a holistic overview of the electrochemical pathways for nitrogen fixation into its activated forms and categorically discuss the energetical and technological challenges that researchers encounter, simultaneously highlighting commendable contributions in the field. We also propose research targets that can serve as a benchmark to measure progress toward the practical realization of the electrochemical nitrogen reduction reaction.