A comparative performance analysis of stand-alone, off-grid solar-powered sodium hypochlorite generators.

Sodium hypochlorite (NaClO) is a chemical commodity widely employed as a disinfection agent in water treatment applications. Its production commonly follows electrochemical routes in an undivided reactor. Powering the process with photovoltaic (PV) electricity holds the potential to install stand-alone, independent generators and reduce the NaClO production cost. This study reports the comparative assessment of autonomous, solar-powered sodium hypochlorite generators employing different photovoltaic (PV) technologies: silicon hetero-junction (SHJ) and multi-junction (MJ) solar cells. For Si hetero-junctions, the series connection of either four or five SHJ (4SHJ and 5SHJ, respectively) cells was implemented to obtain the reaction potential required. MJ cells were illuminated by a novel planar solar concentrator that guarantees solar tracking with minimal linear displacements. The three solar-hypochlorite generators were tested under real atmospheric conditions, demonstrating solar-to-chemical conversion efficiencies (SCE) of 9.8% for 4SHJ, 14.2% for 5SHJ and 25.1% for MJ solar cells, respectively. Simulations based on weather databases allowed us to assess efficiencies throughout the entire model year and resulted in specific sodium hypochlorite yearly production rates between 7.2-28 gNaClO cm-2 (referred to the PV surface), depending on the considered PV technology, location, and deployment of electronics converters. The economic viability and competitiveness of solar hypochlorite generators have been investigated and compared with an analog disinfection system deploying ultraviolet lamps. Our study demonstrates the feasibility of off-grid, solar-hypochlorite generators, and points towards the implementation of SHJ solar cells as a reliable technology for stand-alone solar-chemical devices.

Vapor-fed microfluidic hydrogen generator.

Water-splitting devices that operate with humid air feeds are an attractive alternative for hydrogen production as the required water input can be obtained directly from ambient air. This article presents a novel proof-of-concept microfluidic platform that makes use of polymeric ion conductor (Nafion®) thin films to absorb water from air and performs the electrochemical water-splitting process. Modelling and experimental tools are used to demonstrate that these microstructured devices can achieve the delicate balance between water, gas, and ionic transport processes required for vapor-fed devices to operate continuously and at steady state, at current densities above 3 mA cm(-2). The results presented here show that factors such as the thickness of the Nafion films covering the electrodes, convection of air streams, and water content of the ionomer can significantly affect the device performance. The insights presented in this work provide important guidelines for the material requirements and device designs that can be used to create practical electrochemical hydrogen generators that work directly under ambient air.

Optofluidic-tunable color filters and spectroscopy based on liquid-crystal microflows.

The integration of color filters with microfluidics has attracted substantial attention in recent years, for on-chip absorption, fluorescence, or Raman analysis. We describe such tunable filters based on the micro-flow of liquid crystals. The filter operation is based on the wavelength-dependent liquid crystal birefringence that can be tuned by modifying the flow velocity field in the microchannel. The latter is possible both temporally and spatially by varying the inlet pressure and the channel geometry, respectively. We explored the use of these optofluidic filters for on-chip absorption spectroscopy in poly(dimethylsiloxane) microfluidic systems; by integrating the distance-dependent color filter with a dye-filled micro-channel, the absorption spectrum of a dye could be measured. Liquid crystal microflows substantially simplify the optofluidic integration, actuation and tuning of color filters for lab-on-a-chip spectroscopic applications.

Double-helix enhanced axial localization in STED nanoscopy.

Stimulated Emission Depletion (STED) microscopy enables subdiffraction resolution in the imaging plane. However, STED's lateral improvement in resolution is generally better than the enhancement in the axial direction. Here, we combine conventional STED superresolution imaging with Double Helix Point Spread Function (PSF) modulation for axial localization with a precision better than the classical Rayleigh limit. To demonstrate the capability of the method we resolve in a STED microscope sub-diffraction fluorescent bead assemblies, and localize them axially with better than 25nm precision. We also show that the same setup allows straightforward implementation of wide field phase contrast by imaging larger beads with spiral and dark field phase filtering.

Investigation of nonlinear absorption processes with femtosecond light pulses in lithium niobate crystals.

The propagation of high-power femtosecond light pulses in lithium niobate crystals (LiNb O3 ) is investigated experimentally and theoretically in collinear pump-probe transmission experiments. It is found within a wide intensity range that a strong decrease of the pump transmission coefficient at wavelength 388 nm fully complies with the model of two-photon absorption; the corresponding nonlinear absorption coefficient is beta(p) approximately = 3.5 cm/GW. Furthermore, strong pump pulses induce a considerable absorption for the probe at 776 nm. The dependence of the probe transmission coefficient on the time delay Delta t between probe and pump pulses is characterized by a narrow dip (at Delta t approximately = 0) and a long (on the picosecond time scale) lasting plateau. The dip is due to direct two-photon transitions involving pump and probe photons; the corresponding nonlinear absorption coefficient is beta(r) approximately = 0.9 cm/GW. The plateau absorption is caused by the presence of pump-excited charge carriers; the effective absorption cross section at 776 nm is sigma(r) approximately = 8 x 10(-18) cm(2). The above nonlinear absorption parameters are not strongly polarization sensitive. No specific manifestations of the relaxation of hot carriers are found for a pulse duration of approximately = 0.24 ps.

Femtosecond time-resolved absorption processes in lithium niobate crystals.

Femtosecond pump pulses are strongly attenuated in lithium niobate owing to two-photon absorption; the relevant nonlinear coefficient beta(p) ranges from approximately 3.5 cm/GW for lambda(p) = 388 nm to approximately 0.1 cm/GW for 514 nm. In collinear pump-probe experiments the probe transmission at the double pump wavelength 2lambda(p) = 776 nm is controlled by two different processes: A direct absorption process involving pump and probe photons (beta (r) = 0.9 cm/GW) leads to a pronounced short-duration transmission dip, whereas the probe absorption by pump-excited charge carriers results in a long-duration plateau. Coherent pump-probe interactions are of no importance. Hot-carrier relaxation occurs on the time scale of < or approximately equal to 0.1 ps.

Storage density of shift-multiplexed holographic memory.

The storage density of shift-multiplexed holographic memory is calculated and compared with experimentally achieved densities by use of photorefractive and write-once materials. We consider holographic selectivity as well as the recording material's dynamic range (M/#) and required diffraction efficiencies in formulating the calculations of storage densities, thereby taking into account all major factors limiting the raw storage density achievable with shift-multiplexed holographic storage systems. We show that the M/# is the key factor in limiting storage densities rather than the recording material's thickness for organic materials in which the scatter is relatively high. A storage density of 100 bits/mum(2) is experimentally demonstrated by use of a 1-mm-thick LiNbO(3) crystal as the recording medium.

Holographic memory with localized recording.

We experimentally demonstrate and characterize a memory module that features selective page erasure and readout persistence using the localized recording method in doubly doped LiNbO(3). Pages of information can be selectively erased without partially erasing the whole memory. Data pages can be written over erased pages multiple times. Information is read millions of times before refreshing is required. We quantify the optical quality of the holograms by measuring their signal-to-noise ratio for a memory size up to 100 holograms. A compact phase-conjugate readout architecture is also presented and experimentally demonstrated.

System measure for persistence in holographic recording and application to singly-doped and doubly-doped lithium niobate.

We define a measure for persistence in holographic recording. Using this measure and the known measures for dynamic range and sensitivity, we compare the performance of singly-doped and doubly-doped LiNbO(3) crystals. We show that the range of performance that can be obtained using doubly-doped crystals is much larger than that obtained using singly-doped ones.

Feasibility of intraoperative corneal topography monitoring during photorefractive keratectomy.

PURPOSE: We propose a feasibility study of new corneal topography technology with the aim of monitoring intraoperative corneal topography during excimer laser photorefractive keratectomy. The PAR system measures corneal topography with single grid projection and triangulation but requires fluorescent fluid to be deposited on the corneal surface for shape extraction. We propose and demonstrate a novel corneal topography system based on structured incoherent visible light projection and triangulation that does not require addition of fluorescent fluid. METHODS: We used a binary liquid crystal spatial light modulator to display multiple fringe patterns onto the cornea. The depth accuracy of the corneal topography system was measured using a white reflected test sphere mounted on a micrometer translation stage. The performance of the corneal topography system was tested on 5 de-epithelialized swine eyes in vitro ablated with a VISX excimer laser. RESULTS: Depth accuracy on the test sphere was 0.5+/-0.75 microm over an area of 17.6 mm2. On de-epithelialized swine corneas, ablation at the apex of the cornea treated with an excimer laser was measured without addition of fluorescein. CONCLUSIONS: This new corneal topography system achieved an adequate level of accuracy on a test sphere.

Localized holographic recording in doubly doped lithium niobate.

Persistent holograms are recorded locally with red light in a LiNbO>(3) crystal doped with Mg and Fe. Selective erasure is realized by use of a focused UV sensitizing light. We demonstrate the recording of 50 localized images as well as selective erasure in a 4 mm x 4 mm x 4 mm crystal. A comparison of the total recording time for M holograms obtained with the conventional distributed-volume recording and the localized methods is presented.

Sensitivity improvement in two-center holographic recording.

Persistent holograms are recorded with green light in LiNbO(3) crystals doped with Mn and Fe. The recording sensitivity is 20 times better than that obtained by recording with red light. Partial loss of persistence is caused by using green light for recording.

Holographic recording in a photopolymer by optically induced detachment of chromophores.

We demonstrate holographic recording in a new photopolymer system. The recording material is created by copolymerization of an optically inert monomer, methyl methacrylate, and a second monomer that is optically sensitive. On exposure of the recording material to light, a portion of the optically sensitive component detaches from the polymer matrix and causes hologram amplification through diffusion of the free molecules. We measured postrecording grating amplifications as high as 170% by this process. The recorded holograms are persistent at room temperature under continuous illumination at the recording wavelength.

Diffraction efficiency of localized holograms in doubly doped LiNbO(3) crystals.

The diffraction efficiency of M holograms superimposed in the volume of the recording medium is proportional to 1/M(2). We present a method, based on nondestructive localized holograms in a doubly doped LiNbO(3) crystal, that allows us to also record M holograms in the same volume without an exposure schedule or a diffraction efficiency that has 1/M dependence. We compare experimentally the final diffraction efficiency obtained with the localized and distributed recording methods.

Liquid-crystal blazed-grating beam deflector.

A transmission-type nonmechanical multiple-angle beam-steering device that uses liquid-crystal blazed grating has been developed. Sixteen steering angles with a contrast ratio of 18 has been demonstrated. A detailed analysis of the liquid-crystal and poly(methyl methacrylate) blazed-grating deflector was carried out to provide guidance during the deflector's development. A manufacturing offset compensation technique is proposed to improve the device's performance greatly. A hybrid approach utilizing electrically generated blazed grating combined with the cascading approach described here yields in excess of 500 deflecting angles.

Quasi-Periodic Variability and the Inner Radii of Thin Accretion Disks in Galactic Black Hole Systems.

We calculate upper bounds on the inner radii of geometrically thin accretion disks in galactic black hole systems by relating their rapid variability properties to those of neutron stars. We infer that the inner disk radii do not exhibit large excursions between different spectral states, in contrast with the concept that the disk retreats significantly during the soft-to-hard-state transition. We find that, in the hard state, the accretion disks extend down to radii less, similar6-25 GM/c2 and discuss the implications of our results for models of black hole X-ray spectra.

Multiplexing holograms in LiNbO3:Fe:Mn crystals.

Persistent holograms are recorded with red light in lithium niobate crystals doped with manganese and iron. Different erasure mechanisms are investigated, and a recording schedule for multiplexing holograms with equal diffraction efficiencies is proposed. To test the recording schedule experimentally, we multiplex 50 plane-wave holograms with the proposed recording schedule.

Pixel size limit in holographic memories.

The bandwidth of holographic recording in LiNbO(3) (Fe doped) in the 90 degrees geometry is studied theoretically and experimentally. The wide holographic bandwidth of LiNbO(3) makes it possible to record submicrometer pixels and reconstruct them by phase conjugation in a holographic memory system. This approach reduces the system cost and increases the system storage density. We demonstrate the recording and the phase-conjugate reconstruction of various pixel sizes down to 1 mumx1 mum . The signal-noise ratio and the bit-error rate are examined.

Thermal fixing of 10,000 Holograms in LiNbO3:Fe.

We discuss thermal fixing as a solution to the volatility problem in holographic storage systems that use photorefractive materials such as LiNbO(3):Fe. We present a systematic study to characterize the effect of thermal fixing on the error performance of a large-scale holographic memory. We introduce a novel, to our knowledge, incremental fixing schedule to improve the overall system fixing efficiency. We thermally fixed 10,000 holograms in a 90 degrees -geometry setup by using this new schedule. All the fixed holograms were retrieved with no errors.