Large area double p–i–n heterostructure for signal multiplexing and demultiplexing in the visible range

Results on the use of a double a-SiC:H p–i–n heterostructure for signal multiplexing and demultiplexing applications in the visible range, are presented.Modulated monochromatic beams together (multiplexing mode), or a single polychromatic beam (demultiplexing mode) impinge in the device and are absorbed, accordingly to their wavelength, giving rise to a time and wavelength dependent electrical field modulation.Red, green and blue pulsed input channels are transmitted together, each one with a specific transmission rate. The combined optical signal is analyzed by reading out, under different applied voltages, the generated photocurrent. Results show that in the multiplexing mode the output signal is balanced by the wavelength and transmission rate of each input channel, keeping the memory of the incoming optical carriers. In the demultiplexing mode the photocurrent is controlled by the applied voltage allowing regaining the transmitted information. An electrical model gives insight into the device operation.     

Optical properties of amorphous carbons and their applications and perspectives in photonics

Amorphous carbon exhibits a wide variety of optical properties and, thus, offers substantial opportunities for various applications in photonics. The main optical properties, which should be taken into account for the design of new photonic devices, are the refractive index n, the fundamental gap E_g and the E₀₄ gap. In this work, the optical properties of the various forms of amorphous carbon films grown by plasma-enhanced chemical vapor deposition, pulsed laser deposition, sputtering and vacuum cathodic arc deposition and the crucial structural and chemical factors that determine n, E_g, and E₀₄ are reviewed. The knowledge of the optical properties of such films is exploited in order to design and implement various photonic devices such as: 1) anti-reflection (AR) coatings for various uses including photovoltaic modules, 2) interferometric sensors and indicators based on carbon-based AR layers, and 3) laser patterning of amorphous carbons and study of its photosensitivity for holographic applications.     

An integrated system for optical imaging of ice cores

An end-to-end system for optical image acquisition and data processing for ice cores has been developed for the United States National Ice Core Laboratory (NICL). The components of this system include highly integrated, automated methods for image capture in the cold-room environment and subsequent analysis by scientists. These components seamlessly manage the various aspects of physical scanning, metadata capture, image processing tests for data quality assurance, database integration and file management, processing of raw data to standard products, data distribution, and image processing and annotation tools for end-users in the ice core science community. The system has been tested operationally on cores retrieved from the West Antarctic Ice Sheet Divide drilling project during the core processing lines at NICL in 2006 and 2007.     

Liquid phase epitaxy growth of bismuth-substituted yttrium iron garnet thin films for magneto-optical applications

The novel Bi-substituted rare-earth iron garnet films were grown by the modified liquid phase epitaxy (LPE) technique for use as a 45° Faraday rotator in optical isolators. First, single crystals of Y₃Fe₅O₁₂ (YIG), with a lattice constant of 1.2378 nm, were grown by means of the Czochralski method. Using the seed crystal of YIG instead of the conventional non-magnetic garnet of Gd₃Ga₅O₁₂ as a substrate, a film of BiYbIG was grown by means of the LPE method from Bi₂O₃–B₂O₃ fluxes. The structural, magnetic and magneto-optical properties of BiYbIG LPE film/YIG crystal composite have been investigated using directional X-ray diffraction, electron probe microanalysis, vibrating sample magnetometer and near-infrared transmission spectrometry. The saturation magnetization 4πM_s has been estimated to be approximately 1200 G. The Faraday rotation spectrum was measured by the method of rotating analyzer ellipsometry with the wavelength varying from 800 to 1700 nm. The resultant Bi_0.37Yb_2.63Fe₅O₁₂ LPE film/YIG crystal composite showed an increased Faraday rotation coefficient due to doping Bi³⁺ ions on the dodecahedral sites of the magnetic garnet without increasing absorption loss, therefore a good magneto-optic figure of merit, defined by the ratio of Faraday rotation and optical absorption loss, has been achieved of 21.5 deg/dB and 30.2 deg/dB at 1300 nm and 1550 nm wavelengths, respectively, at room temperature. Since Yb³⁺ ions and Y³⁺ ions provide the opposite contributes to the wideband and temperature characteristic of Faraday rotation, the values of Faraday rotation wavelength and temperature coefficients were reduced to 0.06%/nm and 0.007 deg/°C at 1550 nm wavelength, respectively.     

Synthesis of carboxyl-capped and bright YVO4:Eu,Bi nanoparticles and their applications in immunochromatographic test strip assay

Carboxyl-capped YVO₄:Eu,Bi nanoparticles with average diameter of ∼10 nm were synthesized via a copolymer of phosphono and carboxylic acid mediated hydrothermal method. Under a 350 nm ultraviolet light excitation, the YVO₄:Eu,Bi NPs exhibit sharp and bright red emission peaked at 615 nm and with highest quantum yield of ∼43%. Furthermore, the nanoparticles show good water/buffer stability and feasible bioconjugation benefiting from the carboxylic groups on their surface. Based on these kind optical and surface properties of the YVO₄:Eu,Bi nanoparticles, an immunochromatographic test strip assay for quantitative determination of human IgG was achieved. This protocol can be extended to other rare-earth nanoparticles with the purpose of developing bioprobes for desired applications.     

Thermally stable chromophores for nonlinear optical applications

Three thermal organic second-order nonlinear optical chromophores were synthesized. The decomposition temperature was determined by DSC, and the absorption spectra was measured. The second-order polarizabilities at zero energy and the dispersion of second-order polarizabilities were measured by solvatochromic method.     

Highly conducting and transparent tin-doped CdO thin films for optoelectronic applications

Highly conducting and transparent thin films of tin-doped cadmium oxide were deposited on quartz substrate using pulsed laser deposition technique. The effect of growth temperature on structural, optical and electrical properties was studied. These films are highly transparent (78-89%) in visible region, and transmittance of the films depends on growth temperature. It is observed that resistivity increases with growth temperature after attaining minimum at 150 °C, while carrier concentration continuously decreases with temperature. The lowest resistivity of 1.96 × 10^− 5 Ω cm and carrier concentration of 5.52 × 10²¹ cm³ is observed for the film grown at 150 °C. These highly conducting and transparent tin-doped CdO thin films grown via pulsed laser deposition could be an excellent candidate for future optoelectronic applications.     

Fe@Ag core-shell nanoparticles with both sensitive plasmonic properties and tunable magnetism

Monodisperse Fe@Ag core-shell nanoparticles with relatively uniform Fe cores and Ag shells have been successfully fabricated by a seed mediated method in a two-step reducing process, and then characterized by electron microscopy techniques (HRTEM, EDX), X-ray diffraction (XRD), UV-vis spectroscopy,and magnetometry. The results demonstrate unique optical and magnetic properties for Fe@Ag core-shell nanoparticles. The surface plasmon resonance of Fe@Ag core-shell nanoparticles is red shifted as compared with that of pure colloidal nano-silver, while the plasmon band of Fe@Ag core-shell nanoparticles with thinner Ag shells is shifted to a longer wavelength. Fe@Ag core-shell nanoparticles have a narrow plasmon band and therefore sensitive plasmonic properties. The magnetism of Fe@Ag nanoparticles can be tuned from superparamagnetic to ferromagnetic by modifying the proportion between Fe and Ag contents. The multifunctional Fe@Ag core-shell nanoparticles have potential in optoelectronic, spintronic, and biomedicine applications.     

Doping effects on the optical properties of evaporated a-Si:H films

Thin films of a-Si:H are deposited on substrates at 300°C by a conventional thermal evaporation technique. The electrical conductivity of these films is modified by the addition of antimony giving n-type films. The optical properties of the films are investigated using spectrophotometric measurements of the transmittance and reflectance in the wavelength range 200–3000 nm. Both the refractive index n and the absorption coefficient increase when the Sb content is increased. The absorption edge shifts to lower energies for doped films. The optical gap E_g is evaluated using three different plots for comparison, namely; (hν)^1/2, (/hν)^1/2 and (hν)^1/3. The value of E_g decreases with doping for the three expressions. The Urbach parameter E₀ is calculated and found to increase with doping from 74 meV for the undoped film to 183 meV for concentrations of 9.4 at.% Sb.     

Structural, optical and electrical properties of In doped CdO thin films for optoelectronic applications

Thin films of indium doped cadmium oxide were deposited on quartz substrate using pulsed laser deposition technique. The effect of growth temperature and partial oxygen pressure on structural, optical and electrical properties was studied. We find that the optical transparency of the films largely depends on the growth temperature, while partial oxygen pressure has virtually no effect on the transparency of the films. Electrical properties are found to be sensitive to both the growth temperature and oxygen pressure. It is observed that conductivity and carrier concentration decreases with temperature. The film grown at 200 °C under an oxygen pressure of 5.0 × 10^− 4 mbar shows high mobility (155 cm²/V s), high carrier concentration (1.41 × 10²¹ cm³), and low resistivity (2.86 × 10^− 5 Ω cm).     

Biotemplating of BaFBr:Eu for X-ray storage phosphor applications

The design of hierarchically patterned novel structures by replicating the cellular tissue of wood has recently attained increasing interest. X-ray storage phosphor BaFBr:Eu²⁺ is manufactured via vacuum assisted repeated infiltration of wood tissue (Pinus sylvestris). A submicrometer precipitate is formed via wet chemical reaction of NH₄F, BaBr₂·2H₂O and EuCl₃·6H₂O in methanol. According to scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX), the original wood cell walls are filled with the precipitate and completely transformed into BaFBr struts after sintering at 800 °C. The optical properties of the biomorphous phosphor microstructure are determined by photoluminescence spectroscopy (PL) at room temperature, photo-stimulated luminescence spectroscopy (PSL) and cathodoluminescence spectroscopy (CL) in the SEM. A broadening of the PSL peak is observed and ascribed to the incorporation of calcium impurities present in the pine wood tissue. The potential of biotemplates for generating highly oriented and optically isolated μm- and sub-μm matrix of X-ray storage phosphor material is illustrated.     

Annealing effect on structures and luminescence of amorphous SiN films

The microstructures and chemical bonding configurations of amorphous silicon nitride films with various compositions are investigated. Room temperature photoluminescence is observed which depended on the film concentrations. The post-annealing treatment at moderate temperature region of 700-900 °C is performed and the annealing effect on the structures and luminescence is studied. It is found that the structural rearrangements occurred after thermal annealing due to the effusion of hydrogen from the films. The luminescence is also changed after annealing and the possible originations are briefly discussed.     

Thermal compression wafer bonding of tungsten applied to fabrication of small-period tungsten woodpile structures

In this paper, we report on the thermal compression bonding of tungsten at very high temperatures and pressures, and the realization of a 3D tungsten woodpile structure using this method. The structure is fabricated by holographic patterning followed by dry etching of tungsten-on-Si (for the base) and tungsten-on-oxide-on-Si (for subsequent layers). The patterned layers are then wafer-bonded together at a pressure of about 50 MPa and a temperature about 700 °C for several hours. The substrate with the oxide-on-Si is then removed by a combination of lapping (for the bulk of the Si substrate), selective dry etching of Si with SF₆ (for the remaining few microns of Si), and buffered oxide etch for selective removal of the oxide, leaving the two layers of tungsten firmly bonded. The process is repeated for additional layers. Bonding strength for the tungsten layers is between 2 and 9 MPa, depending on bonding conditions. This is a potential pathway for manufacturable fabrication of three-dimensional small-period (~ 500 nm) tungsten woodpile structures, which may have particular applications for high-efficacy incandescent filaments.     

Direct patterning of polymer-based photo luminescent structures with a mask

We report a simple method to directly pattern polymer-based photo luminescent material, i.e. a prepatterned mask is placed a close distance above it. The final structure is a positive replica of the lateral structures in the mask with submicrometer resolution. The comparison of luminescence efficiency before and after patterning indicates almost no degradation in optical property of the material during the experiments. The mechanism of pattern formation is also discussed.     

Organic against inorganic electrodes grown onto polymer substrates for flexible organic electronics applications

One of the most challenging topics in the area of organic electronic devices is the growth of transparent electrodes onto flexible polymeric substrates that will be characterized by enhanced conductivity in combination with high optical transparency. An essential aspect for these materials is their synthesis and/or microstructure which define the transparency, the stability and the interfacial chemistry which in turn determine the performance and stability of the organic electronic devices, such as organic light emitting diodes, organic photovoltaics, etc.In this work, we will discuss the latest advances in the growth of organic (e.g. PEDOT:PSS) and inorganic (e.g. zinc oxide-ZnO, indium tin oxide-ITO) conductive materials and their deposition onto flexible polymeric substrates. We will compare the optical, structural, nano-mechanical and nano-topographical properties of the inorganic and organic materials and we investigate the effect of their structure on their properties and functionality. In the case of the organic conductive materials, we will discuss the effects of PEDOT:PSS weight ratios and the various spin speeds on their optical and electrical properties. Furthermore, in the case of ZnO the growth mechanisms, interface phenomena, crystallinity and optical properties of ZnO thin films grown onto polymer and hybrid (inorganic-organic) flexible substrates will be also discussed.     

Low temperature sol-gel metal oxide and fluoride layer stacks for optical applications

MgF₂ and TiO₂ single layers and layer stacks were produced by a spin-coating sol-gel process. The final temperature treatment was carried out at 100 °C. The layers were deposited onto silicon and fused silica substrates and were analysed by means of atomic force microscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, ellipsometry, and UV-vis transmission spectroscopy. MgF₂ and TiO₂ single layers have morphological and optical properties comparable with physical vapour deposited layers. By using spectroscopic mapping ellipsometry, a good inter- and intra-sample homogeneity was confirmed. Multiple deposition steps result in a linear increase of layer thickness. Various films were deposited with thicknesses between 25 nm and 350 nm.It was shown that the low temperature sol-gel process results in films of optical quality. Anti-reflective and high reflective layer stacks consisting of MgF₂ and TiO₂ were designed and can be produced now by a sol-gel process, whereas the MgF₂ layers in the layer stacks contains also traces of MgF_2-2xO_x.     

Low temperature remote plasma sputtering of indium tin oxide for flexible display applications

Tin doped indium oxide (ITO) has been directly deposited onto a variety of flexible materials by a reactive sputtering technique that utilises a remotely generated, high density plasma. This technique, known as high target utilisation sputtering (HiTUS), allows for the high rate deposition of good quality ITO films onto polymeric materials with no substrate heating or post deposition annealing. Coatings with a resistivity of 3.8 × 10^− 4 Ωcm and an average visible transmission of greater than 90% have been deposited onto PEN and PET substrate materials at a deposition rate of 70 nm/min. The electrical and optical properties are retained when the coatings are flexed through a 1.0 cm bend radius, making them of interest for flexible display applications.     

Dilute Al–Mn alloys for superconductor device applications

We discuss results on the superconducting and electron-transport properties of Mn-doped Al produced by sputter deposition. The critical temperature of Al has been systematically reduced to below 50 mK by doping with 1000–3000 ppm Mn. Values of the parameter are in the range of 450–500, indicating sharp normal-to-superconductor transitions. This material is thus of significant interest for both transition-edge sensors operating in the 100 mK regime and superconductor/insulator/superconductor and superconductor/insulator/normal devices, in the latter case where appropriately doped Al–Mn replaces the normal metal.     

Synthesis and characterization of ZnO tetrapods for optical and antibacterial applications

ZnO tetrapods have been grown by sublimation process employing thermal evaporation at 950 °C on quartz substrate. The grown ZnO tetrapods exhibited noteworthy microstructure, phase formation, emission bands and antibacterial performance. A detailed micro- and nano- analysis supported with analytical measurements elucidated the faceted tetrapod growth with hexagonal wurzite crystal structure of ZnO. Each arm of the tetrapod revealed a facetted hexagonal cross-section with uniform length and diameter. The photolumiscence spectra showed engrossing optical properties with pre-dominant green emission completely overwhelming the near band edge (NBE) UV peak. These fascinating tetrapod objects responded well for antibacterial activity against Escherichia coli and Staphylococcus aureus.