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Porous platinum is a frequently used catalyst material in electrosynthesis and a robust broadband absorber in thermoelectrics. Pore size distribution and localization determine its properties by a large extent. However, the pore formation mechanism during the growth of the material remains unclear. In this work we elucidate the mechanism underlying electrochemical growth of nanoporous platinum layers and its control by ionic concentration and current density during electrolysis. The electrode kinetics and reduction steps of PtCl4 on platinum electrodes are investigated by cyclic voltammetry and impedance measurements. Cyclic voltammograms show three reduction steps: two steps relate to the platinum cation reduction, and one step relates to the hydrogen reduction. Hydrogen is not involved in the reduction of PtCl4, however it enables the formation of nanopores in the layers. These findings contribute to the understanding of electrochemical growth of nanoporous platinum layers in isopropanol with thickness of 100 nm to 500 nm. Porous platinum is a frequently used electrocatalyst and thermoelectric material, but the growth mechanism of nanopores in platinum layers is still not fully understood. Here, the authors show that hydrogen is not involved in the reduction process of PtCl4, however it enables the formation of nanopores.

We describe the preparation and properties of bilayers of graphene‐ and multi‐walled carbon nanotubes (MWCNTs) as an alternative to conventionally used platinum‐based counter electrode for dye‐sensitized solar cells (DSSC). The counter electrodes were prepared by a simple and easy‐to‐implement double self‐assembly process. The preparation allows for controlling the surface roughness of electrode in a layer‐by‐layer deposition. Annealing under N2 atmosphere improves the electrode's conductivity and the catalytic activity of graphene and MWCNTs to reduce the I3 − species within the electrolyte of the DSSC. The performance of different counter‐electrodes is compared for ZnO photoanode‐based DSSCs. Bilayer electrodes show higher power conversion efficiencies than monolayer graphene electrodes or monolayer MWCNTs electrodes. The bilayer graphene (bottom)/MWCNTs (top) counter electrode‐based DSSC exhibits a maximum power conversion efficiency of 4.1 % exceeding the efficiency of a reference DSSC with a thin film platinum counter electrode (efficiency of 3.4 %). In addition, the double self‐assembled counter electrodes are mechanically stable, which enables their recycling for DSSCs fabrication without significant loss of the solar cell performance., The straightforward, low‐temperature, and cheap production of metal‐free counter electrodes for dye‐sensitized solar cells (DSSCs) is presented. The self‐assembly of carbon materials such as graphene flakes and multi‐walled carbon nanotubes (MWCNTs) leads to high‐performing counter electrodes. Recycling of counter electrodes in DSSCs is demonstrated and compared for different electrolyte compositions.

We propose and experimentally demonstrate the implementation of lithography-free nanoapertures on optical fibers. By sputtering metallic nanofilms onto the end face of step index fibers that contai...

In this study, laser-treated polycrystalline Si (pc-Si) wafers, fabricated by wire sawing of hot-pressed ingots sintered from Si powder, have been investigated. As-cut wafers and those with high-quality thin Si layers deposited on top of them by e-beam have been subjected to laser irradiation to clarify typical trends of structural modifications caused by laser treatments. Moreover, possibility to use laser-treated Si powder-based substrates for fabrication of advanced Si structures has been analysed. It is established that (i) Si powder-based wafers with thicknesses ~180 μm can be fully (from the front to back side) or partly (subsurface region) remelted by a diode laser and grain sizes in laser-treated regions can be increased; (ii) a high-quality top layer can be fabricated by crystallization of an additional a-Si layer deposited by e-beam evaporation on top of the pc-Si; and (iii) silicon nanowires can be formed by metal-assisted wet chemical etching (MAWCE) of polished Si powder-based wafers and as-cut wafers irradiated with medium laser power, while a surface texturing on the as-cut pc-Si wafers occur, and no nanowires can form in the region subject to a liquid phase crystallization (LPC) caused by high-power laser treatments.

Scientific reports 9(1), 16110 (2019). doi:10.1038/s41598-019-52397-x, Published by Macmillan Publishers Limited, part of Springer Nature, [London]

Transparent conductive hybrid thin films based on silver nanowires (AgNW), zinc oxide (ZnO) and graphene were prepared using solution processing methods. For this study, the hexagonal (wurtzite) ZnO particles with a flower-like structure were synthesized by sol-gel reaction using zinc acetate dihydrate as a precursor at room temperature and pH value of 10. Ethanol dispersions of ZnO and commercial AgNW were deposited by spincoating method on glass substrates with fixed parameters to obtain AgNW-ZnO and ZnO-AgNW films, then their surfaces were covered with monolayer graphene flakes (MGFs) using a double self-assembly (DSA) process. The hybrid ZnO-AgNW-MGFs and AgNW-ZnO-MGFs films were characterized by XRD, SEM, UV?vis, four-point probe conductivity measurement, thermal (85 degrees C for 120 h) and long-term (ambient condition for 120 days) stability analyzes. The transmittances of AgNW-ZnO and ZnO-AgNW films were identical at 550 nm (80 %) and their sheet resistances (R-s) values were measured as 20.6 and 21.6 Omega/sq, respectively. After the MGFs deposition, the transmittances of hybrid films reduced to 72.5 and 76.8 %, while their R-s values decreased to 17.6 and 21.4 Omega/sq, respectively. Graphene flakes coating both enhanced the conductivity of hybrid films by improving the connection between silver nanowires and increased the thermal and long-term ambient stability of the films due to its barrier property against oxidation of silver nanowires. Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK)

Multi-metallic nanoparticles are useful for applications in biophotonics or catalysis. Metal compounds alter the properties of pure metallic nanoparticles and thus expand the particle performance compared to particles consisting of the pure phase of metals. We investigated the formation of bi-metallic Pd/Au nanoparticle systems fabricated by UV excimer laser exposure at 248 nm in the ns range and the structural, the morphological as well as the optical properties of these alloy nanoparticles. Laser parameters like energy and pulse number or the thickness proportions of the individual layers to the total layer thickness were varied. Due to the volume percentage of the metals, the total layer thickness and the used laser parameters, the particle size and size distribution could be tuned. The corresponding nanoparticle formation is able to influence the plasmon resonance in position and in shape. Based on x-ray diffraction measurements and energy dispersive x-ray analysis an alloy formation was observed. Using x-ray diffraction, the formation of additional reflexes of palladium-gold alloys between the reflexes of the pure metals could be detected. Furthermore, the position and intensity was adjustable by the percentage composition of metals. The energy dispersive x-ray analysis showed the spatial presence of both elements in each particle.

Different morphologies of TiO2 photoelectrodes for dye sensitized solar cells were obtained by using TiO2 gels dried in normal conditions (TiO2 amb) and in CO2 atmosphere at high pressure (TiO2 press). After a subsequent calcination, the powders were processed as pastes and drop-casted on conductive glass in order to prepare photoanodes for dye sensitized solar cells. N719 commercial dye was used as sensitizer in all the experiments. The structure and morphology of the processed TiO2 materials were investigated via X-ray diffraction, scanning electron microscopy, transmission electron microscopy and N2 adsorption/desorption measurements. The dye adsorption capacity of the photoanodes was tested using ultraviolet-visible absorption spectroscopy. The photovoltaic performances of the dye sensitized solar cells were investigated using current/voltage curves (I/V), open circuit photovoltage decay measurements and electrochemical impedance spectroscopy. The conversion efficiency (η) and short circuit density (Jsc) for TiO2 amb were 1.90 % and 5 mA/cm2 respectively, while the TiO2 press cell exhibited a 40 and 50 % increase in Jsc and η values respectively. This was correlated with increased dye loading capacities due to a broader distribution of pore size towards the mesopore region.

Several series of preform and fiber samples of Yb2O3-Al2O3-SiO2 glass with finely-graded composition steps were prepared by MCVD and solution doping under well-defined conditions using both oxidizing and reducing atmospheres during preform collapse. Their optical properties, including absorption and emission behavior in the NIR/VIS/UV region, have been characterized and correlated with the detailed glass composition. The results present an overview of the property spectrum, which should contribute to the further development of laser fibers and the discussion and control of disturbing effects such as photodarkening.

The laser parameters, energy density and number of pulses, influence the UV laser-induced formation of gold nanoparticles on glass substrates. But also the deposition rate and the thickness of the gold layers used are important criteria for nanoparticle production. This is due to the fact that the resulting microstructures and the optical properties of the gold layers change by variation of sputtering power and deposition time based on different growth conditions of the films. The laser power of the KrF excimer laser used was varied from 50 to 500 mJ/cm2, the pulse number from 1 to 10 pulses and the film thickness from 20 nm to 60 nm as well. Low deposition rates produce layers with higher transparency for the same film thickness. The significant [111] texture of the films with grain sizes from 11 nm to 25 nm in dependent on the deposition rate, evaluated by x-ray diffraction and electron backscatter diffraction, changes after laser exposure. Particle size distributions were evaluated by scanning electron microscopy. Uni- or bi-modal distributions were observed dependent on deposition rate and laser parameters. The position and the width of nanoparticle plasmon resonances vary accordingly.

We report on the fabrication of spherical Au spheres by pulsed laser treatment using a KrF excimer laser (248 nm, 25 ns) under ambient conditions as a fast and high throughput fabrication technique. The presented experiments were realized using initial Au layers of 100 nm thickness deposited on optically transparent and low cost Borofloat glass or single-crystalline SrTiO3 substrates, respectively. High (111)-orientation and smoothness (RMS ≈ 1 nm) are the properties of the deposited Au layers before laser treatment. After laser treatment, spheres with size distribution ranging from hundreds of nanometers up to several micrometers were produced. Single-particle scattering spectra with distinct plasmonic resonance peaks are presented to reveal the critical role of optimal irradiation parameters in the process of laser induced particle self-assembly. The variation of irradiation parameters like fluence and number of laser pulses influences the melting, dewetting and solidification process of the Au layers and thus the formation of extremely well shaped spherical particles. The gold layers on Borofloat glass and SrTiO3 are found to show a slightly different behavior under laser treatment. We also discuss the effect of substrates.

A sensing structure based on a cleaved silica microsphere is proposed for temperature sensing. The microsphere was cleaved using focused ion beam milling. The asymmetry in the structure introduced by the cut generates not only new cavities but also random interferometric reflections inside the microsphere. These two spectral components can be separated using low-pass and high-pass filters, respectively. The sensor response to temperature can be extracted from the cavities’ component using a correlation method. The device achieved a temperature sensitivity of −10.8 ± 0.2 pm/°C between 30 °C and 80 °C. The same effect is impossible to be obtained in a normal uncleaved microsphere. The random interferometric component did not provide any information on temperature using the same analysis. However, when changing the temperature, a new and completely distinct reflection spectrum with no apparent correlation with others at different temperatures was achieved.

We report on germania and alumina dopant profile shift effects at preparation of compact optical fibers using packaging methods (Stack-and-Draw method, Rod-in-Tube (RiT) technique). The sintering of package hollow volume by viscous flow results in a shift of the core-pitch ratio in all-solid microstructured fibers. The ratio is increased by about 5% in the case of a hexagonal package. The shift by diffusion effects of both dopants is simulated for typical slow speed drawing parameters. Thermodynamic approximations of surface dissociation of germania doped silica suggest the need of an adequate undoped silica barrier layer to prevent an undesired bubble formation at fiber drawing. In contrast, alumina doping does not estimate critical dissociation effects with vaporous aluminium oxide components. We report guide values of diffusion length of germania and alumina for the drawing process by kinetic approximation. The germania diffusion involves a small core enlargement, typically in the sub-micrometer scale. Though, the alumina diffusion enlarges it by a few micrometers. A drawn pure alumina preform core rod transforms to an amorphous aluminosilicate core with a molar alumina concentration of only about 50% and a non-gaussian concentration profile.

usc The small dimensions of optical fiber sensors are of particular interest to biological applications, given the ability to penetrate relatively inaccessible regions. However, conventional optical fibers are larger than biological material such as cells, and thus there is a need to further miniaturize fiber sensors. Here we present the fabrication of ultra-small Fabry-Perot cavities written into optical micro-fibers using focused ion beam milling. We have fabricated cavities as small as 2.8 μm and demonstrated their use for sensing of both bulk refractive index and thin-layer coatings. In order to achieve sensitive measurements we interrogate at visible wavelengths with a broadband detection system, thereby reducing the free spectral range of the interferometer relative to the measurement bandwidth, increasing the number of interference fringes, and allowing for the implementation of the Fourier shift method. Refereed/Peer-reviewed

The mesoporous silica particle embedded graphene oxide (GO) hybrid system is a promising platform for electrochemical biosensing owing to its large 2-dimensional structure, fast electron transfer kinetics, improved hydrophilic nature and surplus functional groups. Here, GO sheets were synthesized by Hummer's improved method and sub-micron sized homogeneous mesoporous silica (SiO2) particles were prepared by Stober's method. The SiO2 particles were embedded on the GO surfaces and were optimized with different concentrations for better applicability and hydrophilicity. Micro-structural and spectroscopic characterization of as-synthesized materials was carried out to confirm the successful synthesis as well as the functionalities required for biosensing. Scanning electron microscopy investigations suggest that the average size of the SiO2 particles decorated on the GO surface is ∼500 nm. Raman investigation provides information regarding the increase in defects and disorder on the GO surface with the increase in the SiO2 content. The optimized GO–SiO2 (GOS) composite electrode was prepared by the electrophoretic deposition technique and was used for the attachment of urease and glutamate dehydrogenase enzymes for urea detection employing the cyclic voltammetry method. The reproducibility, specificity and stability of the fabricated biosensor were found to be excellent for the urea sensing. Such an easy and cost effective material based GOS urea sensor showed a high sensitivity (2.6 μA mM−1 cm−2) and a good detection limit (14 mg dL−1).

Graphene and its derivatives have many superior electrical, thermal, mechanical, chemical, and structural properties, and promise for many applications. One of the issues for scalable applications is the lack of a simple, reliable method that allows the deposit of a well-ordered monolayer using low-cost graphene flakes onto target substrates with different surface properties. Another issue is the adhesion of the deposited graphene thin film, which has not been well investigated yet. Following our former finding of a double self-assembly (DSA) process for efficient deposition of a monolayer of graphene flakes (MGFs), in this work we demonstrate that the DSA process can be applied even on very challenging samples including highly hydrophobic polytetrafluoroethylene (PTFE), flexible textiles, complex 3D objects, and thin glass fibers. Additionally, we tested adhesion of the graphene flakes on the flat glass substrate by scotch tape peel test of the MGFs. The results show that the graphene flakes adhere quite well on the flat glass substrate and most of the graphene flakes stay on the glass. These findings may trigger many large-scale applications of low-cost graphene feedstocks and other 2D materials.

Silicon nanowire (SiNW) based solar cells are a promising candidate for third generation solar cells with high efficiency, which will further reduce the cost of the PV modules, making it competitive with conventional energy sources. In this paper the wet chemical etching process for generating silicon nanowires is investigated on different low-cost substrate materials. These nanowires are useful for a radial pn-junction solar cell with core–shell configuration, which allows for using absorber materials with low-carrier lifetime. In this work, wet chemical nanowire etching is applied to edge-defined film-fed growth (EFG) mc-Si wafers, mc-Si wafers made from metallurgical grade Si, multicrystalline (mc-Si) thin films deposited on glass, and thin crystallized films deposited on mc-Si wafers produced from metallurgical grade Si. A proof of concept for using low-cost materials in PV is demonstrated for SiNWs prepared on mc-Si wafer produced from highly doped MG-Si.

We are reporting on SiO2-Al2O3-La2O3 glasses—with and without Yb2O3—suitable for nonlinear and fiber laser applications. We will also be presenting successful supercontinuum generation and fiber laser operation around 1060 nm in step-index fibers. We optimized the glass compositions in terms of thermal and optical requirements for both a high La2O3 and Yb2O3 concentration and good compatibility with a silica cladding. We also produced bulk samples, unstructured fibers, and structured fibers with a SiO2-Al2O3-La2O3 core and silica cladding for this purpose.

usc Refereed/Peer-reviewed Focused ion beam technology is combined with dynamic chemical etching to create microcavities in tapered optical fiber tips, resulting in fiber probes for temperature and refractive index sensing. Dynamic chemical etching uses hydrofluoric acid and a syringe pump to etch standard optical fibers into cone structures called tapered fiber tips where the length, shape, and cone angle can be precisely controlled. On these tips, focused ion beam is used to mill several different types of Fabry-Perot microcavities. Two main cavity types are initially compared and then combined to form a third, complex cavity structure. In the first case, a gap is milled on the tapered fiber tip which allows the external medium to penetrate the light guiding region and thus presents sensitivity to external refractive index changes. In the second, two slots that function as mirrors are milled on the tip creating a silica cavity that is only sensitive to temperature changes. Finally, both cavities are combined on a single tapered fiber tip, resulting in a multi-cavity structure capable of discriminating between temperature and refractive index variations. This dual characterization is performed with the aid of a fast Fourier transform method to separate the contributions of each cavity and thus of temperature and refractive index. Ultimately, a tapered optical fiber tip probe with sub-standard dimensions containing a multi-cavity structure is projected, fabricated, characterized and applied as a sensing element for simultaneous temperature and refractive index discrimination.

Refereed/Peer-reviewed Optical fibers are promising tools for performing biological and biomedical sensing due to their small cross section and potential for multiplexing. In particular, fabricating ultra-small sensing devices is of increasing interest for measuring biological material such as cells. A promising direction is the use of interferometric techniques combined with optical fiber post-processing. In this work we present recent progress in the development of Fabry-Perot micro-cavities written into optical fiber tapers using focused ion beam (FIB) milling. We first demonstrate that FIB milled optical fiber microcavities are sensitive enough to measure polyelectrolyte layer deposition. We then present new results on the fabrication and optical characterization of serially-multiplexed dual cavity micro-sensors. Two cavities were written serially along the fiber with two different cavity lengths, producing a total of four reflecting surfaces and thus six possible interferometric pairs/cavities. By using fast Fourier transform it is possible to obtain de-multiplexed measurements for each cavity. This will be particularly important for bioassays where positive and negative controls are required to be measured within close spatial proximity.

Currently, two types of direct methods to characterize and identify single virions are available: electron microscopy (EM) and scanning probe techniques, especially atomic force microscopy (AFM). AFM in particular provides morphologic information even of the ultrastructure of viral specimens without the need to cultivate the virus and to invasively alter the sample prior to the measurements. Thus, AFM can play a critical role as a frontline method in diagnostic virology. Interestingly, varying morphological parameters for virions of the same type can be found in the literature, depending on whether AFM or EM was employed and according to the respective experimental conditions during the AFM measurements. Here, an inter-methodological proof of principle is presented, in which the same single virions of herpes simplex virus 1 were probed by AFM previously and after they were measured by scanning electron microscopy (SEM). Sophisticated chemometric analyses then allowed a calculation of morphological parameters of the ensemble of single virions and a comparison thereof. A distinct decrease in the virions' dimensions was found during as well as after the SEM analyses and could be attributed to the sample preparation for the SEM measurements. Graphical abstract The herpes simplex virus is investigated with scanning electron and atomic force microscopy in view of varying dimensions.

usc Refereed/Peer-reviewed Optical fiber micro-tips are promising devices for sensing applications in small volume and difficult to access locations, such as biological and biomedical settings. The tapered fiber tips are prepared by dynamic chemical etching, reducing the size from 125 μm to just a few μm. Focused ion beam milling is then used to create cavity structures on the tapered fiber tips. Two different Fabry-Perot micro-cavities have been prepared and characterized: a solid silica cavity created by milling two thin slots and a gap cavity. A third multi-cavity structure is fabricated by combining the concepts of solid silica cavity and gap cavity. This micro-tip structure is analyzed using a fast Fourier transform method to demultiplex the signals of each cavity. Simultaneous measurement of temperature and external refractive index is then demonstrated, presenting sensitivities of-15.8 pm/K and-1316 nm/RIU, respectively.

usc Refereed/Peer-reviewed The small dimensions of optical fiber sensors are of interest to biological applications, given the ability to penetrate relatively inaccessible regions. However, conventional optical fibers are larger than biological material such as cells, and thus there is a need for further miniaturization. Here we present the fabrication of ultra-small Fabry-Perot cavities written into optical micro-fibers using focused ion beam (FIB) milling. We have fabricated cavities as small as 2.8 μm and demonstrated their use for measuring refractive index. In order to achieve sensitive measurements we interrogate at visible wavelengths, thereby reducing the free spectral range of the interferometer (relative to infra-red interrogation), increasing the number of interference fringes, and allowing for the implementation of the Fourier shift method.

usc Refereed/Peer-reviewed We demonstrate the use of focused ion beam milling to machine optical structures directly into the core of microstructured optical fibers. The particular fiber used was exposed-core microstructured optical fiber, which allowed direct access to the optically guiding core. Two different designs of Fabry-Perot cavity were fabricated and optically characterized. The first cavity was formed by completely removing a section of the fiber core, while the second cavity consisted of a shallow slot milled into the core, leaving the majority of the core intact. This work highlights the possibility of machining complex optical devices directly onto the core of microstructured optical fibers using focused ion beam milling for applications including environmental, chemical, and biological sensing.

We describe a novel, fully symmetrical deposition method, based on a sputtering technique and use of a gold ring target for deposition of a gold layer uniform around the taper waist. With such a circular symmetric coating, the plasmon resonance effect in the fiber taper becomes completely independent of the polarization of the illuminating light. We have measured the complex refractive index of such sputtered gold layers. On this basis, model calculations have been performed to describe the plasmon resonance as a function of taper waist diameter, gold layer thickness, analyte interaction length, and analyte refractive index. Optimized parameters especially for measurement in aqueous solutions are derived from these theoretical calculations. Experimental results are shown to be in good agreement with the theoretical analysis.

A variety of samples made via different routes were investigated. Samples are nanostructured (average grain sizes are about 20 nm). The advantage of high-pressure (HP)-manufactured (2 GPa, 800-1050°C, 1 h) MgB2 bulk is the possibility to get almost theoretically dense (1-2% porosity) material with very high critical current densities reaching at 20 K, in 0-1 T jc = 1.2 1.0 · 106 A/cm2 (with 10% SiC doping) and jc = 9.2 - 7.3 105 A/cm2 (without doping). Mechanical properties are also very high: fracture toughness up to 4.4 ± 0.04 MPa · m0.5 and 7.6 ± 2.0 MPa · m0.5 at 148.8 N load for MgB2 undoped and doped with 10% Ta, respectively. The HP-synthesized material at moderate temperature (2 GPa, 600°C, 1 h) from B with high amount of impurity C (3.15%) and H (0.87%) has jc = 103 A/cm2 in 8 T fleld at 20 K, highest irreversibility fields (at 18.4 K Hirr = 15 T) and upper critical fields (at 22 K HC2 = 15 T) but 17% porosity. HP materials with stoichiometry near MgB12 can have Tc = 37 K and jc = 6 · 104 A/cm2 at 0 T and Hirr = 5 T at 20 K. The spark plasma synthesized (SPS) material (50 MPa, 600-1050°C 1.3 h, without additions), demonstrated at 20 K, in 0-1 T jc = 4.5-4 105 A/cm2. Dispersed inclusions of higher magnesium borides, which are usually present in MgB2 structure and obviously create new pinning centers can be revealed by Raman spectroscopy (for the first time a spectrum of MgB2 was obtained). Tests of quench behavior, losses on MgB2 rings and material thermal conductivity show promising properties for fault current limiters. Due to high critical fields, the material can be used for magnets.

Critical current density (jc) of high-pressure (2 GPa) manufactured MgB2-based superconductors depends on the amount and distribution of higher borides (MgB12) in MgB2 matrix, which in turn are determined by the nature of the initial components first of all B or MgB2 and the temperature of sintering or synthesis. Ti and Ta additions can improve jc by promoting the higher boride formation via impurity hydrogen absorption, thus preventing MgH2 detrimental for jc being formed, which possibly increases the MgB12 nucleation barrier. SiC (0.2-0.8 mum) addition increases jc of MgB2, allowing us to get jc = 106 A/cm2 at 20 K in the 1 T field: pinning is increased by SiC and higher boride grains and there is no notable interaction between SiC and MgB2 . As the synthesis temperature increases from 800 to 1050degC, Ti and SiC additions may affect the oxygen segregation and formation of Mg-B-O inclusions enriched with oxygen as compared to the amount of oxygen in the MgB2 matrix, which can also promote an increase in pinning. Materials high-pressure synthesized from Mg and B taken in 1:4, 1:6, 1:7, 1:8, 1:10, 1:12, 1:20 ratios were superconductive with Tc of about 37 K. High jc (7middot104 - 2middot104 A/cm2 in zero field at 10-30 K, respectively) showed materials with the matrix composition near MgB12 stoichiometry, they have doubled microhardness of MgB2.

Additions of Zr can increase critical current density ( j c ) of high-pressure synthesized MgB 2 (HPS-MgB 2 ) in the same manner as additions of Ta or Ti, i.e. due to the absorption of impurity hydrogen (to form ZrH 2 ). The formation in HPS-MgB 2 of ZrB 2 phase at higher synthesis temperatures (about 950 °C) does not result in the j c increase. Some increase in j c of HPS-MgB 2 at 10 K in the fields higher than 8 T was observed when nano-SiC was added. The additions of Zr, Ta or Ti can prevent the harmful MgH 2 impurity phase from appearing and may prevent hydrogen from being introduced into the material structure and besides, their presence in HPS-MgB 2 promotes the formation of a higher amount of Mg–B (most likely MgB 2 ) inclusions in the Mg–B–O material “matrix” that in turn leads to the increase of j c in magnetic fields. The high level of superconductive (SC) and mechanical characteristics attained for HPS-MgB 2 and the possibility to manufacture large samples make its application in the superconductive electromotors, generators, pumps, etc., very promising.

We report on the fluorine incorporation in powder based materials for the fabrication of Al and Al/Yb co-doped silica glasses. The achieved maximum Fluorine concentration of 1.55 mol% SiF 4 corresponds to a refractive index decrease of -8 x 10 -3 . Simultaneously, the Tg of the doped material is reduced by about 200 K compared to pure silica. Moreover, the fluorine doping is also eminently suitable for the direct refractive index adjustment in active doped silica glass materials (e.g. Al/Yb or Al/Tm). The index matching with pure silica is possible to date up to 2.7 mol% Al 2 O 3 and 0.1 mol% Yb 2 O 3 . The additional influence on the blue shift of the UV transmission will also be discussed.

We report on the magneto-optical (MO) properties of heavily Tb3+-doped GeO2-B2O3-Al2O3-Ga2O3 glasses towards fiber-integrated paramagnetic MO devices. For a Tb3+ ion concentration of up to 9.7 × 1021 cm−3, the reported glass exhibits an absolute negative Faraday rotation of ~120 rad/T/m at 632.8 nm. The optimum spectral ratio between Verdet constant and light transmittance over the spectral window of 400–1500 nm is found for a Tb3+ concentration of ~6.5 × 1021 cm−3. For this glass, the crystallization stability, expressed as the difference between glass transition temperature and onset temperature of melt crystallization exceeds 100 K, which is a prerequisite for fiber drawing. In addition, a high activation energy of crystallization is achieved at this composition. Optical absorption occurs in the NUV and blue spectral region, accompanied by Tb3+ photoluminescence. In the heavily doped materials, a UV/blue-to-green photo-conversion gain of ~43% is achieved. The lifetime of photoluminescence is ~2.2 ms at a stimulated emission cross-section σem of ~1.1 × 10−21 cm2 for ~ 5.0 × 1021 cm−3 Tb3+. This results in an optical gain parameter σem*τ of ~2.5 × 10−24 cm2s, what could be of interest for implementation of a Tb3+ fiber laser.

A combination of focused ion beam milling and chemical etching is proposed for the creation of Fabry-Perot cavities in microwires. Both simple cavities and cantilevers are created on 15 μm-diameter microwires and characterized in temperature. The cantilever structure shows sensitivity to vibration and is capable of measuring frequencies in the range 1 Hz – 40 kHz.

We report on a Yb-doped large-core fiber laser at 976 nm prepared by powder-sinter technology. Wavelength-locked single-mode operation is achieved with 10 W out of a standard single-mode fiber.

A new approach for silica laser materials is presented. The technology is based on reactive powder doping, sintering and vitrification and yields in homogeneous active doped bulk material with an excellent RI homogeneity of about 10-4.

XLMA fibers based on Yb-doped bulk silica possess an excellent refractive index and doping level homogeneity [1]. To achieve the highest optical-to-optical efficiency and long-term operation without degradation we simulated the effect of the brightness conversion factor of different core dopant compositions of such XLMA fibers. We also investigated the beam quality of a multi-kW single XLMA fiber laser system and its long-term stability. The current state-of-the-art XLMA single fiber laser has 5 kW maximum output power and a degradation rate of about 0.5 % / 500 h at 4 kW measured over a period of 1700 h. Several application tests demonstrate the excellent performance of the XLMA fiber laser.

We report on SiO 2 -Al 2 O 3 -La 2 O 3 glasses – with and without Yb 2 O 3 – suitable for nonlinear and fiber laser applications. We also present successful supercontinuum generation and fiber laser operation around 1060 nm in step-index fibers. We have optimized the glass compositions in terms of thermal and optical requirements for both a high La 2 O 3 (24 mol%) and Yb 2 O 3 (6 mol%) concentration. The aluminum concentration was adjusted to about 21 mol% Al 2 O 3 to increase the solubility of lanthanum and ytterbium in the glass beyond possible MCVD based techniques. The glasses have been characterized by dilatometrical methods to find transition temperatures from 860 to 880°C and thermal expansion coefficients between 4.1 and 7.0 × 10 -6 K -1 . Structured step index fibers with a SiO 2 -Al 2 O 3 -La 2 O 3 core and silica cladding have been realized showing a fiber loss minimum of about 500 dB/km at 1200 nm wavelength. The chromatic dispersion could be adjusted to shift the zero dispersion wavelength (ZDW) close to the pump wavelength of 1550 nm in a supercontinuum generation setup. First fiber laser experiments show an efficiency of about 41 % with a remarkably reduced photodarkening compared to MCVD based fibers.

Silicon nanowire-based solar cells received increasing attention due to their enhanced light harvesting properties and the potential to use low-cost materials to produce solar cells comparable with those on costly monocrystalline counterparts. It is essential to improve the performance of nanowire solar cells by suppressing surface recombination. A multiple core (crystalline silicon nanowires)–shell silicon nanowire-based heterojunction solar cell has been fabricated to deal with this problem. To this end, an ultrathin passivating Al2O3 tunnel layer was deposited on the highly doped p-type a-Si:H emitter prior to a transparent conducting oxide by atomic layer deposition (ALD). Both open circuit voltage and current density increase significantly due to the insertion of the ultrathin Al2O3 layer. An efficiency of 10.0% has been reached by using this multiple core–shell structure.

Josephson junctions generate, when subjected to microwave irradiation, voltages with a very high precision and are used in metrology applications. So-called PJVS (programmable Josephson voltage-standards) are capable of generating both AC and DC voltages of up to 10 V. Our work addresses a full fabrication scenario for 10 V PJVS arrays driven at 70 GHz to be used in low microwave-power conditions as in, but not limited to GUNN diodes or cryocooler applications. Nb x Si1−x in its function as a barrier material was characterised with AFM, RBS and reflectometry in order to establish a reliable technological foundation. A 10 V PJVS array driven with microwave power below 50 mW is further presented, which was achieved by optimising the fabrication technology regarding the degree of homogeneity of the Josephson junctions composition and thickness. Control over these parameters is crucial in choosing a stable and well-suited characteristic voltage (I c R n product) and critical current density j c. With this, a low-power operation of a PJVS array is possible without the need for liquid helium cooling, which is currently limiting the availability of PJVS based metrology.

High performance particle detectors are needed for fundamental research in high energy physics in the exploration of the Higgs boson, dark matter, anti-matter, gravitational waves and proof of the standard model, which will extend the understanding of our Universe. Future particle detectors should have ultrahigh radiation hardness, low power consumption, high spatial resolution and fast signal response. Unfortunately, some of these properties are counter-influencing for the conventional silicon drift detectors (SDDs), so that they cannot be optimized simultaneously. In this paper, the main issues of conventional SDDs have been analyzed, and a novel core–shell detector design based on micro- and nano-structures etched into Si-wafers is proposed. It is expected to simultaneously reach ultrahigh radiation hardness, low power consumption, fast signal response and high spatial resolution down to the sub-micrometer range, which will probably meet the requirements for the most powerful particle accelerators in the near future. A prototype core–shell detector was fabricated using modern silicon nanotechnology and the functionality was tested using electron-beam-induced current measurements. Such a high performance detector will open many new applications in extreme radiation environments such as high energy physics, astrophysics, high resolution (bio-) imaging and crystallography, which will push these fields beyond their current boundaries.

Silica-based fiber tips are used in a variety of spectroscopic, micro- or nanoscopic optical sensor applications. The efficiency of such measurement systems is dependent on the tip geometry and the diameter of the tapered fiber end. In the present study we investigated the preparation of geometrically predefined, nanoscaled fiber tips by taking advantage of the dopant concentration profiles of highly doped step-index fibers. For this purpose, a gas phase etching process using hydrofluoric acid (HF) vapor was applied. The shaping of the fiber tips was based on very different etching rates as a result of the doping characteristics of specific optical fibers. Technological studies on the influence of the etching gas atmosphere on the temporal tip shaping and the final geometry were performed using undoped and doped silica fibers. The influence of the doping characteristics was investigated in phosphorus-, germanium-, fluorine- and boron-doped glass fibers. Narrow exposed as well as protected internal fiber tips in various shapes and tip radiuses down to less than 15 nm were achieved and characterized geometrically and topologically. For investigations of surface plasmon resonance effects the fiber tips were coated with nanometer-sized silver layers by means of vapour deposition and finally subjected to an annealing treatment.

Optical fibers for high power lasers and amplifiers are fabricated on the basis of quartz glass which has outstanding properties concerning high fiber strength, high power hardness and low optical losses compared with other glasses such as heavy metal fluoride or oxide glasses. It is well known, however, that the host properties of pure silica regarding the active rare earth ions are insufficient and the laser medium has to be improved by the incorporation of codopants. Here we present new investigations of material and fiber properties for phosphorus/aluminium codoping, with regard to the realization of efficient rare earth doped cw and pulsed high power fiber devices. The diffusion behaviour in the complex systems shows characteristic interaction effects, which influence the dopant concentration and their spatial distribution. The refractive index in the codoped systems and the basic attenuation deviate remarkably from additivity relations. The absorption spectrum in the VIS/NIR region depends on codopant concentration and on preparation conditions, with influence on the fluorescence properties of the rare earths and the laser efficiency.

The MgB2 - based material high-pressure synthesized at 2 GPa and 800 C for 1 h from Mg and B (taken in the stoichiometry ratio of MgB2) with addition of 2-10 wt% of Ti demonstrated the critical current density (jc) higher than 100 kA/cm^2 at 33 K in 0 T field and at 20 K up to 3 T. At 20 K the critical current density higher than 10 kA/cm^2 was observed up to 5 T field. In the magnetic field up to 2 T high-pressure synthesized MgB2 (with 10 % of Ti) at 20 K behaves in the same manner as Nb3Sn at 4.2 K. In XRD patterns of magnesium diboride with the Ti added, we observed no evidence of titanium diboride or unreacted titanium and only one compound with titanium was identified, namely, titanium dihydride TiH2 (or more strictly TiH1.924). The sample with the highest critical current density and irreversible field in the temperature range of 25-10 K contained some amount of pure Mg that was rather homogeneously dispersed in the material. The critical currents and irreversible fields of magnesium diboride synthesized at high pressure with Ti added are higher than those in the case where Ta has been added., Comment: 17 pages, 6 figures

A short-term treatment, sintering or synthesis of high-temperature superconductors (HTS) under high pressures allow us to retain oxygen in the structure of Y123-type superconductors or to suppress the evaporation of Mg during synthesis of MgB 2 and to attain practically theoretical density materials. Besides, we can essentially reduce the time of the processes due to an increase in the mass transfer because of better contacts between grains and a larger contact area. Treatment under optimal pressure-temperature-time conditions improves SC and mechanical properties of HTS. The available high pressure apparatuses (HPA) have working volume of up to 100 cm 3 (or up to 60 mm in diameter). A HPA with 200-cm 3 working volume is under the construction. The present paper considers the peculiarities of high-pressure (1 to 5 GPa) treatment of melt-textured bulk superconductors MT-MeBCO (Me = Y, Nd, Sm) with and without addition of silver, sintering of YBa 2 Cu 3 O 7-δ ceramics from YBa 2 Cu 3 O 7-δ powder as well as synthesis of MgB 2 from a mixture of boron and magnesium.

Utilizing measurements on a lanthano-aluminosilicate core optical fiber, the specific effects of lanthana (Lasub2/subOsub3/sub) on the Brillouin characteristics of silica-based oxide glass optical fibers are described. Lanthana is an interesting species to investigate since it possesses a wide transparency window covering the common fiber laser and telecom system wavelengths. As might be expected, it is found that the properties of lanthana are very similar to those of ytterbia (Ybsub2/subOsub3/sub), namely, low acoustic velocity, wide Brillouin spectral width, and a negative photoelastic constant, with the latter two properties affording significant reductions to the Brillouin gain coefficient. However, lanthana possesses thermo-acoustic and strain-acoustic coefficients (acoustic velocity versus temperature or strain, TAC and SAC, respectively) with signs that are opposed to those of ytterbia. The lanthano-aluminosilicate (SAL) fiber utilized in this study is Brillouin-athermal (no dependence of the Brillouin frequency on temperature), but not atensic (is dependent upon the strain), which is believed to be, to the best of our knowledge, the first demonstration of such a glass fiber utilizing a compositional engineering approach.

Focused ion beam technology is combined with chemical etching of specifically designed fibers to create Fabry-Perot interferometers. Hydrofluoric acid is used to etch special fibers and create microwires with diameters of 15 μm. These microwires are then milled with a focused ion beam to create two different structures: an indented Fabry-Perot structure and a cantilever Fabry-Perot structure that are characterized in terms of temperature. The cantilever structure is also sensitive to vibrations and is capable of measuring frequencies in the range 1 Hz - 40 kHz.

A facile synthetic route to NiOx nanostructures using various amphiphilic polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymers as templates was investigated. The synthesis targets NiOx nanostructures with a large surface area in order to allow an efficient functionalization, e.g., through loading with dyes to enable photo-induced hole injection for use in dye-sensitized solar cells or in (photo-)catalytic systems. The complete synthetic process to NiOx contains several steps: (i) the dissolution of the diblock copolymer, (ii) the subsequent addition of Ni2+, followed by the formation of core–corona micelles and eventually, (iii) further addition of Ni2+, resulting in the formation of a macroscopic precipitate. In all cases, (iv) deposition onto different substrates and calcination yielded NiOx films. All intermediates were thoroughly investigated using scanning or transmission electron microscopy, dynamic light scattering, and UV-vis spectroscopy. In contrast to the well-established synthetic route via the commercially available Pluronic F108 triblock copolymer, in our case a variety of different morphologies was found, i.e. spherical particles, toroid structures, or networks. Furthermore, the obtained BET area of about 50 m2 g−1 is comparable to the value for conventionally obtained NiOx surfaces. First dye sensitization experiments with coumarine 343 confirm that the dye binds to the surface, which is a prerequisite for using the material as a photo-electrode. The presented route to porous NiOx is easy and provides superior control over the morphology of the intermediates involved in nanostructure formation.

Silica-based fiber tips are used in a variety of spectroscopic, micro- or nano-scopic optical sensor applications and photonic micro-devices. The miniaturization of optical sensor systems and the technical implementation using optical fibers can provide new sensor designs with improved properties and functionality for new applications. The selective-etching of specifically doped silica fibers is a promising method in order to form complex photonic micro structures at the end or within fibers such as tips and cavities in various shapes useful for the all-fiber sensor and imaging applications. In the present study, we investigated the preparation of geometrically predefined, nanoscaled fiber tips by taking advantage of the dopant concentration profiles of highly doped step-index fibers. For this purpose, a gas phase etching process using hydrofluoric acid (HF) vapor was applied. The shaping of the fiber tips was based on very different etching rates as a result of the doping characteristics of specific optical fibers. Technological studies on the influence of the etching gas atmosphere on the temporal tip shaping and the final geometry were performed using undoped and doped silica fibers. The influence of the doping characteristics was investigated in phosphorus-, germanium-, fluorine- and boron-doped glass fibers. Narrow exposed as well as protected internal fiber tips in various shapes and tip radiuses down to less than 15 nm were achieved and characterized geometrically and topologically. For investigations into surface plasmon resonance effects, the fiber tips were coated with nanometer-sized silver layers by means of vapour deposition and finally subjected to an annealing treatment.

Focused ion beam milling has been employed to create micro-structured features onto an exposed-core microstructured optical fiber. Here we detail results for fabricating and characterizing Fabry-Perot cavities using this method.

Surface plasmon polaritons (SPPs) enable extreme device miniaturization via deeply sub-wavelength electromagnetic field confinements [1]. Conventional coupling techniques typically require phase matching or mode-matching, which is challenging at sub-wavelength dimensions. Here, we introduce and experimentally demonstrate a novel near-field hybrid fiber [2, 3] nanoprobe for the efficient excitation of short-range (SR) SPPs on a metallic nanowire which has a sub-10 nm nanotip at its end. A device schematic is shown in Fig. 1(a): a gold nanowire (NW) (radius: ∼300 nm, length: 80 μm) is incorporated into a (few-mode) step-index fiber with a central nanochannel, and protrudes from the endface forming a nanotip (Fig. 1(a) inset). Finite element calculations (Fig. 1(b)) show that a doughnut-shape mode is transmitted, but only a radially polarized (RP) mode induces an accumulation of energy at the apex of the nanotip via a hybrid mode, which contains energy on the wire surface. The experimental characterization relies on launching broadband azimuthally polarized (AP) [3] or RP (500–750 nm) light into the sample and analysing the far-field scattered and transmitted power. Figure 1(c) shows a typical side-scattered image (perpendicular to the fiber axis), showing the SR-SPPs excited on the tip. For an RP input mode the scattered intensity is maximal, and for an azimuthally polarized (AP) input it is mininal, with the transmitted power being independent on the input polarization. We analysed the scattered power as a function of the angle of a linear polarizer (LP) inserted between sample and camera (Fig. 1(e)) for a RP input, and observed a clear maximum when the LP is parallel to the fiber axis, confirming a longitudinally polarization state of the light emitted by the nanotip, characteristic of a plasmonic mode on a nanotip. This hybrid plasmonic fiber device can both deliver and collect broadband subwavelength radiation to and from the apex of a nanotip; in collection mode, light would efficiently couple from the plasmonic mode into the dielectric core. This device will find applications in near-field microscopy and nanophotonics, and can potentially offer significantly improved resolution compared to current SNOM tips [5].