Optical Curtain Effect: Extraordinary Optical Transmission Enhanced by Antireflection

In this paper, we employ an antireflective coating which comprises inverted π-shaped metallic grooves to manipulate the behaviour of a transverse-magnetic (TM)-polarised plane wave transmitted through a periodic nanoslit array. At normal incidence, such scheme cannot only retain the optical curtain effect in the output region but also generate the extraordinary transmission of light through the nanoslits with the total transmission efficiency as high as 90 %. Besides, we show that the spatially invariant field distribution in the output region as well as the field distribution of resonant modes around the inverted π-shaped grooves can be reproduced immaculately when the system is excited by an array of point sources beneath the inverted π-shaped grooves. Furthermore, we investigate the influence of centre groove and side-corners of the inverted π-shaped grooves on suppressing the reflection of light, respectively. Based on our work, it shows promising potential in applications of enhancing the extraction efficiency as well as controlling the beaming pattern of light emitting diodes.     

Design of Optical Filters with Flat-on-Top Transmission Lineshapes Based on Two Side-Coupled Metallic Grooves

We investigate theoretically and numerically the resonant transmission through side-coupled metallic grooves. In the framework of coupled mode theory (CMT), a single metallic groove can be considered as a lossy optical resonator and two metallic grooves coupled via tunneling effect can be treated as a second-order cascade resonator. The relationship between the transmission lineshape of the coupled grooves and the cross-coupling between the grooves is analyzed by CMT. It is found that a flat-on-top lineshape can be obtained when the cross-coupling is equal to the total decay rate of the groove mode. Predictions based on the CMT analysis are in good agreement with the simulation results based on the finite-difference time-domain technique.     

Photocurrent Enhancement in Si-Ge Photodetectors by Utilizing Surface Plasmons

A circular slit-groove surface plasmon polaritons (SPPs) launcher surrounding a photodetector is employed theoretically to enhance the photocurrent of atypical Si-Ge photodetectors. The slit and grooves are designed such that the SPPs are focused at the center of the absorption layer of the photodetector to result in additional electric current. Fabry–Perot resonance condition accurately calculates the period of the groove, slit-groove distance, photodetector radius, and slit-photodetector distance. The manipulation leads to constructive interference between the incident light impinging from the top and the SPPs propagating toward the photodetector. Simulation result shows that photocurrent increases by approximately 13-fold when the SPPs are introduced.     

Far-Field Focusing of Spiral Plasmonic Lens

In this paper, we study the nanoscale-focusing effect in the far field for a spiral plasmonic lens with a concentric annular groove by using finite-difference time domain simulation. The simulation result demonstrates that a left-hand spiral plasmonic lens can concentrate an incident right-hand circular polarization light into a focal spot at the exit surface. And this spot can be focused into far field due to constructive interference of the scattered light by the annular groove. The focal length and the focal depth can be adjusted by changing the groove radius and number of grooves within a certain range. These properties make it possible to probe the signal of spiral plasmonic lens in far field by using conventional optical devices.     

Tunneling in PSII.

With available high resolution structures of PSII and a collection of reported redox midpoint potentials for most of the cofactors, it is possible to compare the expected electron tunneling rates with experimental rates to determine which electron transfer reactions are likely to reflect simply engineered electron tunneling, and which are more sophisticated and associated with large product rearrangements or the making and breaking of bonds. Reliable reorganization energies are largely lacking in this photosystem compared to PSI and purple bacteria and contribute about an order of magnitude uncertainty in tunneling rate estimates. Nevertheless it seems clear that as in purple bacterial reaction centers and PSI, with the notable exception of the oxygen evolving center, the majority of electron transfers within PSII are electron-tunneling limited at room temperature. Tunneling simulations also suggest that the short circuit between pheophytin and the adjacent chlorophyll cation may be fast enough to challenge triplet decay as the principle means of charge recombination from Q(A)(-) at room temperature.     

Factors affecting tactile spatial acuity

Tactile spatial acuity on the fingerpad was measured using a grating orientation task. In this task, subjects are required to identify the orientation of square-wave gratings placed on the skin. Previous studies have shown that performance varies as a function of the width of the grooves in the gratings. In the present study, both groove width and the overall size and configuration of the contactors were varied. Sensitivity improved with wider grooves and with larger contactors. Additional measurements showed that the improved sensitivity is not the result of the increase in total area contacted, but rather is due to two other factors associated with larger contactors. One is the greater linear extent of the larger contactors. The other appears to be due to the reduction in the interference produced by the outer edge of the contactor. Specifically, as the contactor increases in size, the distance between the outer edge and the center portion of the grooves also increases. It was also shown that subjects are more sensitive to a single, continuous groove as compared with two grooves of the same total length but spatially discontinuous. Similarly, subjects are more sensitive to a contactor with a continuous groove than to a contactor in which just the end points of the groove are presented. The results are generally consistent with the results of peripheral, neurophysiological recordings. The results are discussed in terms of the way in which both spatial and intensive factors may affect sensitivity to grating orientation.     

Thermal quenching of luminescence of LiSr4(BO3)3:Eu2+ orange‐emitting phosphor

An orange‐emitting phosphor, Eu²⁺‐activated LiSr₄(BO₃)₃, was synthesized using the conventional solid‐state reaction. The photoluminescence excitation and emission spectra, and temperature dependence of the luminescence intensity of the phosphor were investigated. The results showed that LiSr₄(BO₃)₃:Eu²⁺ could be efficiently excited by incident light of 250–450 nm, and emits a strong orange light. With increasing temperature, the emission bands of LiSr₄(BO₃)₃:Eu²⁺ show an abnormal blue‐shift with broadening bandwidth and decreasing emission intensity. Copyright © 2013 John Wiley & Sons, Ltd.     

Mixed phospholipid-cardiotoxin monomolecular films studied by intrinsic polarized surface fluorescence

Surface fluorescence spectroscopy was used to study mixed phospholipid-cardiotoxin monomolecular films. Using polarized incident light we were able to detect the intrinsic fluorescence emission at a wavelength of 339 nm of the single Trp11 in the cardiotoxin molecule. Its fluorescence intensity increased continuously upon film compression up to a maximal value reached at 42 mN x m-1 in surface pressure. A relative polarization index can be used as an indication of the orientation of the indole ring. A transition at around 25 +/- 5 mN x m-1 in the orientation of the Trp11 relative to the plane of the interface was clearly evidenced during film compression, in agreement with our independent data showing a flipping of the whole toxin molecule (Bougis et al., (1981), Biochemistry 20, 4915-4920).     

Effects of incident light intensity and light path length on cell growth and oil accumulation in Botryococcus braunii (Chlorophyta)

Botryococcus braunii was cultured in different light path length under different incident light intensity to investigate the effect of light on alga growth as well as hydrocarbon and fatty acid accumulation. Results indicated that longer light path length required higher incident light intensity in order to meet the light requirement of algal growth and hydrocarbon accumulation during the course of cultivation. However, hydrocarbon profile was only affected by the incident light intensity and not influenced by the light path length. High incident light intensity enhanced the accumulation of hydrocarbons with longer carbon chains. Besides, the fatty acid content and profiles were significantly influenced by both incident light intensity and light path. Higher fatty acid content and higher percentage of C18 and monounsaturated fatty acid components were achieved at the higher incident light intensity and lower light path length. Taken together, these results are benefit to improve its biomass and oil productivity through the optimization of light and photobioreactor design.     

Ions around DNA: Monte Carlo estimates of distribution with improved electrostatic potentials

Coulomb's law for the electrostatic interactions between ions is modified when discontinuities in dielectric constant (relative permittivity) occur. In a DNA solution such a discontinuity occurs at the interface between the DNA molecular helix and the surrounding water. We take the modified interaction potentials from a previous report [Macromolecules (1986) 19 , 1186–1194] and use them with the Monte Carlo method to find the distribution of univalent and bivalent counterions around the DNA helix in the absence of coions (i.e., no added salt). In comparing the ion distribution with the modified potential to the distribution without, we find that the effects of the modifications to the potentials are considerable. The modifications tend to drive the ions out of the grooves of the helix, especially out of the major groove. This result comes partly from the repulsion exerted on the ions by the low-permittivity helix and partly from the concentration of the field of the phosphates at the surface of the helix, a concentration that is also caused by the discontinuity in permittivity.     

Fibroblasts on micromachined substrata orient hierarchically to grooves of different dimensions

Contact guidance was studied by light, scanning (SEM) and transmission electron microscopy (TEM) in cultures of human gingival fibroblasts cultured on grooved surfaces. The grooves were originally produced in silicon wafers by micromachining, a process which is based on the methods used to fabricate microelectronic components, and the grooved surfaces were then replicated in Epon. Micromachining enables precise control of groove depth, groove spacing, and groove shape to be obtained. In silicon wafers with appropriate crystal orientation, a second smaller set of grooves, called the minor grooves, is found on the floor of the major grooves. The minor grooves are oriented at a 54 degree angle to the major grooves, so that cells cultured on such surfaces are concurrently exposed to grooves of different dimensions which direct cell migration in different directions. Marked fibroblast alignment with the major grooves was observed both within the grooves and in the intervening flat ridges between the grooves. In addition, shallow and closely spaced grooves in epon or titanium-coated polymer or silicon were also capable of orienting fibroblasts. Although the minor grooves were able to orient fibroblasts in the absence of any other orienting influence, when fibroblasts were concurrently exposed to major and minor grooves the cells aligned themselves with the major grooves. TEM showed that the cellular filamentous cytoskeletal elements reflected the orientation of the cell as a whole. Fibroblasts on grooved substrata appeared to have more filopodia and to round up more frequently than fibroblasts cultured on flat substrata. It is suggested that both the mechanical properties of the cytoskeleton as well as the durability of the cellular attachment to groove edges may play a role in the contact guidance effected by grooved surfaces produced by micromachining.     

Plasmon Resonances in V-Shaped Gold Nanostructures

Using numerical simulations, we examine the change in plasmon resonance behavior in gold nanorod structures that have a V shape. The reduction in symmetry compared to linear rods causes two different longitudinal-type resonances to appear in a single structure, and the relative intensity and hybridization of these can be controlled by varying the angle of the arms of the ??V.?? The resonances may also be selectively excited by controlling the polarization of the incident light, thereby providing a convenient way to control a nanoscale optical electric field using far-field parameters. For example, the wavelength at which a strong resonance occurs in the V-shaped structures studied can be switched between 630 and 900?nm by a 90° rotation of the polarization of the incident light. Due to the symmetry of the targets, there will be three types of special near-field location; a location at which the electric field intensity is enhanced by either resonance, a location at which the electric field intensity is enhanced by the 630?nm resonance but not by the 890?nm resonance, and a location at which the electric field intensity is enhanced by the 890?nm resonance but not by the 630?nm one.     

Mode of reversible binding of neocarzinostatin chromophore to DNA: evidence for binding via the minor groove

Two general approaches have been taken to understand the mechanism of the reversible binding of the nonprotein chromophore of neocarzinostatin to DNA: (1) measurement of the relative affinity of the chromophore for various DNAs that have one or both grooves blocked by bulky groups and (2) studies on the influence of adenine-thymine residue-specific, minor groove binding agents such as the antibiotics netropsin and distamycin on the chromophore-DNA interaction. Experiments using synthetic DNAs containing halogen group (Br, I) substituents in the major groove or natural DNAs with glucosyl moieties projecting into the major groove show that obstruction of the major groove does not decrease the binding stoichiometry or the binding constant for the DNA-chromophore interaction. Chemical methylation of bases in both grooves of calf thymus DNA, resulting in 13% methylation of N-7 of guanine in the major groove and 7% methylation of N-3 of adenine in the minor groove, decreases the binding affinity and increases the size of the binding site for neocarzinostatin chromophore. Similar results were obtained whether binding parameters were determined directly by spectroscopic measurements or indirectly by measuring the ability of the DNA to protect the chromophore against degradation. On the other hand, netropsin and distamycin compete with neocarzinostatin chromophore for binding to the minor groove of DNA, as shown by their decrease in the ability of poly(dA-dT) to protect the chromophore against degradation and their reduction in chromophore-induced DNA damage as measured by thymine release.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chromatic shifts in the fluorescence emitted by murine thymocytes stained with Hoechst 33342.

BACKGROUND: Many methods in flow cytometry rely on staining DNA with a fluorescent dye to gauge DNA content. From the relative intensity of the fluorescence signature, one can then infer position in cell cycle, amount of DNA (i.e., for sperm selection), or, as in the case of flow karyotyping, to distinguish individual chromosomes. This work examines the staining of murine thymocytes with a common DNA dye, Hoechst 33342, to investigate nonlinearities in the florescence intensity as well as chromatic shifts. METHODS: Murine thymocytes were stained with Hoechst 33342 and measured in a flow cytometer at two fluorescence emission bands. In other measurements, cells were stained at different dye concentrations, and then centrifuged. The supernatant was then used for a second round of staining to test the amount of dye uptake. Finally, to test for resonant energy transfer, we measured fluorescence anisotropy at two different wavelengths. RESULTS: The fluorescence of cells stained with Hoechst 33342 is a nonlinear process that shows an overall decrease in intensity with increased dye uptake, and spectral shift to the red. Along with the spectral shift of the fluorescence to the longer wavelengths, we document decreases in the fluorescence anisotropy that may indicate resonant energy transfer. CONCLUSIONS: At low concentrations, Hoechst 33342 binds to the minor groove of DNA and shows an increase in fluorescence and a blue shift upon binding. At higher concentrations, at which the dye molecules can no longer bind without overlapping, the blue fluorescence decreases and the red fluorescence increases until there is approximately one dye molecule per DNA base pair. The ratio of the blue fluorescence to the red fluorescence is an accurate indicator of the cellular dye concentration.     

Intense green‐, red‐emitting Tb3+, Tb3+/Bi3+‐doped and Sm3+, Sm3+/La3+‐doped Ca2Al2SiO7 phosphors

This paper focuses on an optical study of a Tb³⁺/Bi³⁺‐doped and Sm³⁺/La³⁺‐ doped Ca₂Al₂SiO₇ phosphor synthesized using combustion methods. Here, Ca₂Al₂SiO₇:Sm³⁺ showed a red emission band under visible light excitation but, when it co‐doped with La³⁺ ions, the emission intensity was further enhanced. Ca₂Al₂SiO₇:Tb³⁺ shows the characteristic green emission band under near‐ultraviolet light excitation wavelengths, co‐doping with Bi³⁺ ions produced enhanced photoluminescence intensity with better colour tunable properties. The phosphor exhibited better phase purity and crystallinity, confirmed by X‐ray diffraction. Binding energies of Ca(2p), Al(2p), Si(2p), O(1s) were studied using X‐ray photoelectron spectroscopy. The reported phosphor may be a promising visible light excited red phosphor for light‐emitting diodes and energy conversion devices.     

An Efficient Large-Area Grating Coupler for Surface Plasmon Polaritons

We report the design, fabrication, and characterization of a periodic grating of shallow rectangular grooves in a metallic film with the goal of maximizing the coupling efficiency of an extended plane wave (PW) of visible or near-infrared light into a single surface plasmon polariton (SPP) mode on a flat metal surface. A PW-to-SPP power conversion factor >45% is demonstrated at a wavelength of 780?nm, which exceeds by an order of magnitude the experimental performance of SPP grating couplers reported to date at any wavelength. Conversion efficiency is maximized by matching the dissipative SPP losses along the grating surface to the local coupling strength. This critical coupling condition is experimentally achieved by tailoring the groove depth and width using a focused ion beam.     

Amyloid from a histochemical perspective. A review of the structure,properties and types of amyloid,and a proposed staining mechanism for Congo red staining

Amyloid is a diverse group of unrelated peptides or proteins that have positive functionality or are associated with various pathologies. Despite vast differences, all amyloids share several features that together uniquely define the group. 1) All amyloids possess a characteristic cross-ß pattern with X-ray diffraction typical of ß-sheet secondary protein structures. 2) All amyloids are birefringent and dichroic under polarizing microscopy after staining with Congo red, which indicates a crystalline-like (ordered) structure. 3) All amyloids cause a spectral shift in the peak wavelength of Congo red with conventional light microscopy due to perturbation of π electrons of the dye. 4) All amyloids show heightened intensity of fluorescence with Congo red, which suggests an unusual degree of packing of the dye onto the substrate. The ß portion of amyloid molecules, the only logical substrate for specific Congo red staining under histochemical conditions, consists of a stack of ß-sheets laminated by hydrophilic and hydrophobic interactions between adjacent pairs. Only the first and last ß-sheets are accessible to dyes. Each sheet is composed of numerous identical peptides running across the width of the sheet and arranged in parallel with side chains in register over the length of the fibril. Two sets of grooves are bordered by side chains. X grooves run perpendicular to the long axis of the fibril; these grooves are short (the width of the sheet) and number in the hundreds or thousands. Y grooves are parallel with the long axis. Each groove runs the entire length of the fibril, but there are very few of them. While Congo red is capable of ionic bonding with proteins via two sulfonic acid groups, physical constraints on the staining solution preclude ionic interactions. Hydrogen bonding between dye amine groups and peptide carbonyls is the most likely primary bonding mechanism, because all ß-sheets possess backbone carbonyls. Various amino acid residues may form secondary bonds to the dye via any of three van der Waals forces. It is possible that Congo red binds within the Y grooves, but that would not produce the characteristic staining features that are the diagnostic hallmarks of amyloid. Binding in the X grooves would produce a tightly packed series of dye molecules over the entire length of the fibril. This would account for the signature staining of amyloid by Congo red: dichroic birefringence, enhanced intensity of fluorescence and a shift in visible absorption wavelength.     

Synthesis and characterization of pure and Li+ activated Alq3 complexes for green and blue organic light emitting diodes and display devices

Pure and Li⁺‐doped Alq₃ complexes were synthesized by simple precipitation method at room temperature, maintaining the stoichiometric ratio. These complexes were characterized by X‐ray diffraction, ultraviolet‐visible absorption and Fourier transform infrared and photoluminescence (PL) spectra. X‐ray diffraction analysis reveals the crystalline nature of the synthesized complexes, while Fourier transform infrared spectroscopy confirm the molecular structure, the completion of quinoline ring formation and presence of quinoline structure in the metal complex. Ultraviolet‐visible and PL spectra revealed that Li⁺ activated Alq₃ complexes exhibit the highest intensity in comparison to pure Alq₃ phosphor. Thus, Li⁺ enhances PL emission intensity when doped into Alq₃ phosphor. The excitation spectra lie in the range of 383–456 nm. All the synthesized complexes other than Liq give green emission, while Liq gives blue emission with enhanced intensity. Thus, he synthesized phosphors are the best suitable candidates for green‐ and blue‐emitting organic light emitting diode, PL liquid‐crystal display and solid‐state lighting applications. Copyright © 2013 John Wiley & Sons, Ltd.     

The use of an internal standard for semiquantitative analysis of low temperature (77°K) fluorescence of photosynthetic cells

A simple method is described which permits quantitative estimation of chlorophyll fluorescence emission intensity relative to the chlorophyll concentration in sample material at 77°K. A fluorescent standard excited by light of a similar wavelength as that of the sample material but emitting at a slightly shorter wavelength is mixed with the sample in known proportions. The fluorescence yield of the various peaks of the sample are normalized to the fluorescence yield of the internal standard. The spectra are recorded using an inexpensive attachment consisting of a Dewar holder, mounted instead of the light source of any double beam or split beam spectrophotometer. A glass rod is immersed in the sample standard mixture and then placed in the liquid nitrogen containing Dewar. Exciting light is provided by a light pipe mounted at 90° to the sample holder, connected with a tungsten-hallogen lamp provided with an appropriate filter. This method has been found to be particularly useful for the quantitation of chlorophyll fluorescence emission at 77°K of photosynthetic cells or chlorophyll containing membrane fractions. For this purpose, phycocyanin was found to be a suitable internal standard.     

A Facile Strategy for All-Optical Controlling Platform by Using Plasmonic Perfect Absorbers

Dual-band light absorption with the maximal absorptivity up to 99.7% and the minimal spectral bandwidth down to 3 nm is obtained in the plasmonic absorbers consisting of triple-layer plasmonic crystal-nonlinear medium cavity-metal substrate structure, where the intercalated dielectric material is chosen to be a Kerr medium cavity. Efficient all-optical controlling with high spectral intensity change ratios and detecting signal-to-noise is achieved for the system after a slight increase of pumping intensity. These impressive results mainly result from the strong plasmonic resonant field confinement in the middle nonlinear Kerr medium cavity and the near-perfect relative intensity change response by the ultra-sharp anti-reflection spectrum. This work can lay a foundation for advanced all-optical devices by exploiting light perfect absorption behavior and resonant optical field enhancement.