Search

Dielectric gratings that couple optical fibers and planar waveguide circuits are key for optical-to-electronic (electronic-to-optical) signal conversion, but their applicability to platforms that require broader bandwidths and higher capacity is limited by their single-wavelength response. Herein, we present the design of a quasi-periodic grating coupler with multiband fiber-to-waveguide (waveguide-to-fiber) coupling response, where the grating consists of a periodic repetition of unit cells made of alternating silicon and air grooves according to the Fibonacci sequence. Through finite-difference time-domain (FDTD) calculations, we show that this new device could be used for coupling multi-wavelength fiber modes in a single grating structure. The results were obtained for fibers operating in the wavelength range from 1000 nm to 2000 nm, but the concept can be readily extended to other frequency ranges. Moreover, the allowed modes in the grating are almost insensitive to fiber misalignments and small fabrication errors for high Fibonacci steps, which is useful when alignment of optical components is impractical. It is hoped that properly designed gratings overlapping multiple modes may lead to ultra-broadband fiber-waveguide couplers that can cope with the growing demand for higher capacity and bandwidth in optical communications.

This work describes a guiding method to select the best sensing units to compose the array of an electronic tongue. It is based on the principal component analysis (PCA), in which use is made of the PCA plot of the transposed matrix of experimental data, e.g., for different concentration of an analyte. When points in such a plot appear very close to each other, it means that one of the corresponding sensing unit can be removed from the array without hampering the discrimination capability of the e-tongue. The method was validated with electric capacitance data obtained with aqueous solutions made with ultrapure water and KCl or HCl or glucose at different concentrations.

We report on the chemiresistive gas sensing characteristics of ZnO nanorods (NRs) modified by a thin layer of nickel phthalocyanine (NiPc). Ozone detection was carried out through electrical measurements with an optimized performance at 250°C, good reproducibility and suitable concentration range (from 80 to 890 ppb) for technological applications. The hybrid NiPc:ZnO films had superior performance to pure ZnO nanorods in terms of response time and sensitivity. The response times were 22 s and 26 s, respectively, whereas the ratio of resistances under ozone and air was 3.27 for NiPc:ZnO films and 2.56 for the pure ZnO NRs. The improvement in response time is attributed to the large surface area generated with the coating of the ZnO nanorods with the NiPc layer. Significantly, images taken with field-emission scanning electron microscopy (FE-SEM) indicated that the ZnO nanorods were fully covered with NiPc. X-ray diffraction measurements (XRD) revealed a preferential growth of the nanorod-like structures along the [100] direction. In summary, a successful approach has been developed to functionalize ZnO nanorods, which is promising for detection of ppb levels of ozone gas.

In machine learning accurate predictors may be obtained by combining predictions of an ensemble of accurate and diverse predictors. Ensembles are efficiently constructed with the random subspace method (RSM) performed in the instance or in the principal components (PCs) spaces. In this paper, we extend RSM to explore the synergy in the characteristics of these two spaces, with a method referred to as RSM-IPCS. Using 24 datasets from the UCI machine learning repository, we show an enhanced performance of RSM-IPCS in comparison to the original RSM and RSM in PCs space, in terms of higher accuracy and smaller variances. Since RSM-IPCS exhibited at least a similar performance to the best method in a separate space, it opens the way for optimization of ensembles based on the combination of multiple spaces.

The ability to access embedded knowledge makes complex networks extremely promising for natural language processing, which normally requires deep knowledge representation that is not accessible with first-order statistics. In this paper, we demonstrate that features of complex networks, which have been shown to correlate with text quality, can be used to evaluate summaries. The metrics are the average degree, cluster coefficient, and the extent to which the dynamics of network growth deviates from a straight line. They were found to be much smaller for the high-quality, manual summaries, and increased for automatic summaries, thus pointing to a loss of quality, as expected. We also discuss the comparative performance of automatic summarizers.

The methacrylic copolymer functionalized with the azo chromophore 4-[N-ethyl-N-(2-hydroxiethyl)]- amino-2'- chloro-4-nitroazobenzene (MMADR13), in its polyelectrolyte form, can be used to fabricate thin films by the layer-by-layer (LbL) technique just if one alternates this anionic polyelectrolyte with a cationic polyelectrolyte such as poly(allylamine hydrochloride) (PAH). Since PAH does not present any particular optical functionality, the main final film feature will came from the side chain DR13 azo-chromophore group due to its large nonlinear optical properties and photoisomerization capabilities. This work reports the electrooptic activity of MMADR13/DR13 LBL films, which arises from the high hiperpolarizability about the azo side chain group.

Adsorbed layers of poly(o-ethoxyaniline) (POEA) produced with the layer-by-layer technique have been studied with atomic force spectroscopy (AFS). The films were prepared under different conditions (pH and solution concentration), and measurements were carried out in air and in a liquid cell. Both the doping level and fabrication conditions affect the POEA film morphology and the interactions of the atomic force microscopy (AFM) tip with the polymer molecules. For example, a mapping of the forces indicates the presence of conducting islands surrounded by a less conductive matrix in POEA and PANI films, with the conducting islands being characterized by the presence of double-layer forces. The conducting islands are further visualized via transmission electron microscopy (TEM) pictures. At pH 5, POEA appears completely dedoped where PANI can preserve some of its conducting character, and this is confirmed in electron paramagnetic resonance (EPR) measurements.

Poly(p-phenylene vinylene) (PPV) derivatives are well known for their applications in polymer light emitting diodes (PLEDs). These derivatives are highly susceptible to photooxidation though, which is mainly caused by the scission of the vinyl double bond on the polymer backbone. In this work, we show that Langmuir-Blodgett (LB) films are less degraded than cast films of a PPV derivative (OC/sub 1/OC/sub 6/-PPV). Both films had similar thickness (/spl sim/50 nm) to allow for a more realistic comparison. Photodegradation experiments were carried out by illuminating the films with white light from a halogen lamp (50W, 12V), placed at a fixed distance from the sample. The decay was monitored by UV-Vis and FTIR spectroscopies. The results showed that cast films are completely degraded in ca. 300 min, while LB took longer times, ca. 1000 min, i.e. 3 times the values for the cast films. The degradation process occurs in at least two stages, the rates of which were calculated assuming that the reaction follows a first order kinetics. The characteristic times for the first stage were 3.6/spl times/10/sup -2/and 1.3/spl times/10/sup -3/ min/sup -1/ for cast and LB films, respectively. For the second stage the characteristic times were 5.6/spl times/10/sup -2/ and 5.0/spl times/10/sup -3/min/sup -1/ . The differences can be attributed to the more compact morphology in the LB than in the cast films. With a compact morphology the diffusion of oxygen in the LB film is hampered and this causes a delay in the degradation process.

The phase transition from the non-polar /spl alpha/-phase to the polar /spl beta/-phase of poly(vinylidene fluoride) (PVDF) has been investigated using micro-Raman spectroscopy, which is advantageous for being a non-destructive technique. Films of a-PVDF were subjected to stretching under controlled rates and at 80 /spl deg/C, the transition to /spl beta/-PVDF being monitored by the decrease in the Raman band at 794 cm/sup -1/ characteristic of the /spl alpha/-phase, with the concomitant increase in the 839 cm/sup -1/ band characteristic of the /spl beta/-phase. Poling with negative corona discharge was found to affect the a-PVDF morphology improving the Raman bands related to this crystalline phase. This effect is minimized for films stretched to higher ratios. Significantly, corona-induced effects could not be observed with the other experimental techniques, viz, X-ray diffraction and infrared spectroscopy.

Polyfluorenes are promising materials for the emitting layer of polymer light emitting devices (PLEDs) with blue emission. In this work, we report on PLEDs fabricated with Langmuir-Blodgett (LB) films of a polyfluorene derivative, namely poly(9,9-di-hexylfluorenediyl vinylene-alt-1,4-phenylenevinylene) (PDHF-PV). Y-type LB films were transferred onto ITO substrates at a surface pressure of 35 mN m/sup -1/ and with dipping speed of 3 mm min/sup -1/1. A thin aluminum layer was evaporated on top of the LB film, thus yielding a sandwich structure (ITO/PDHF-PV(LB)/Al). Current-voltage (I vs V) measurements indicate that the device displays a classical behavior of a rectifying diode. The threshold value is approximately 5 V, and the onset for visible light emission occurs at ca. 10 V. From the a.c. electrical responses we infer that the active layer has a typical behavior of PLEDs where the real component of ac conductivity obeys a power-law with the frequency. Cole-Cole plots (Im(Z) vs. Re(Z)) for the device exhibit a series of semicircles, the diameter of which decreases with increasing forward bias. This PLED structure is modeled by a parallel resistance and capacitance combination, representing the dominant mechanisms of charge transport and polarization in the organic layer, in series with a resistance representing the ITO contact. Overall, the results presented here demonstrate the feasibility of LEDs made with LB films of PDHF-PV.

The interaction between poly(o-ethoxyaniline) (POEA) adsorbed onto solid substrates and humic substances (HS) and Cu/sup 2+/ ions has been investigated with UV-VIS spectroscopy and atomic force microscopy. Both HS and Cu/sup 2+/ cause the POEA film to be further doped, and alter the film morphology. This interaction was exploited in a sensor array made of nanostructured films of POEA, sulfonated lignin and HS, which could detect small concentrations of HS and Cu/sup 2+/ in water.

Layer-by-layer (LbL) films have been produced with poly(o-ethoxyaniline) (POEA), chitosan and chitosan-poly(methacrylic acid) (CS-PMAA) nanoparticles. Because the adsorption of LbL films depends on ionic interactions and H-bonding, optimized conditions had to be established for the growth of multilayer films. Unusually thick films were obtained for POEA and CS-PMAA, thus demonstrating the importance of using chitosan in the form of nanoparticles

The authors present DSC studies where heating and cooling rates and also thermal treatments were varied systematically. The results for 60/40 P(VDFTrFE) are presented in this paper which confirm the importance of thermal history and also show that samples with various configurations in terms of ferroelectric phases can be obtained in a somewhat controlled way via thermal treatment.

Atomic force microscopy (AFM) was employed to monitor the buildup of layer-by-layer films of poly(o-methoxyaniline) (POMA) and poly(ethenesulfonic acid) (PVS) onto several substrates. Using the contact mode of an AFM, we produced a furrow on the film, which was made possible because the film material was less stiff than the substrate and the silicon tip. Film thickness could then be measured using the tapping mode. The thickness and adsorbed amount of POMA/PVS increased linearly after several bilayers had been deposited, while the roughness increased with the number of bilayers until reaching a saturation value. This behavior was attributed to the ability of these polymers to aggregate, which was confirmed by an analysis of grain sizes.

Surface potential measurements are presented for corona charged P(VDF-TrFE) (polyvinylidene fluoride and trifluorethylene) samples in a constant-current corona triode. The surface potential curves display three distinct regions, characteristic of ferroelectric polymers. The results are interpreted by neglecting the intrinsic conductivity and assuming a dynamics for the electrical polarization which is usually employed in switching measurements of ferroelectric materials, but which is adapted for the constant corona charging condition. >

Electric discharges in small air gaps (10-300 mu m) were investigated by measuring the charge transferred to a dielectric foil which filled part of the air gap and was subjected to an impulsive high voltage. The Teflon FEP samples were charged (thus forming electrets) if the peak impulse voltage exceeded a threshold value. Measurements were performed in order to investigate the importance of the metallic electrode and the voltage impulse duration. Also discussed are the mechanisms for triggering the discharge in the air gap and the processes for charge transfer to the dielectric. >

Structural and electrical properties of poly(vinylidene fluoride-trifluorethylene)P(VDF-TrFE) films, obtained either from casting a solution or by compression moulding from a molten phase, have been reported in a number of studies. Even though results may differ to a considerable extent when samples are prepared with different methods, few studies address these differences whose origins may lie in distinct thermal treatments. In the present study we undertake a careful comparison of experimental DSC and FTIR results on 60/40, 70/30 and 80/20 P(VDF-TrFE) copolymer samples, fabricated using these two methods. The DSC results for the 60/40 and 70/30 copolymer show two phase transitions, with the samples behaving as a paraelectric material only after the 2nd transition. It is shown that the main differences between melt-solidified and cast films arise from the thermal treatment suffered by the former samples.

Ultra-thin films comprising alternating layers of polyacids, poly(styrene sulfonate) (PSS) or poly(vinyl sulfonic)-acids (PVS) and doped poly(o-methoxyaniline) (POMA) were produced using the self-assembly (SA) technique. The combination of POMA and polyacids is of particular interest, for the polyacid may prevent the POMA from undoping when the film is exposed to neutral or slightly basic solutions. Doped poly(o-alkoxyanilines) are suitable systems for SA because of their high solubility in water. The UV-visible absorbance spectra show a peak ranging from 585 nm to 800 nm depending on the doping level. The absorbance at the peak increases linearly with the number of deposited bi-layers, which indicates that equal amounts of material are adsorbed in each deposition procedure.

The recent developments on the main strategies for fabricating Langmuir-Blodgett (LB) films from conducting polymers are presented. Because this is a brief report, we concentrate on our latest results on films produced from parent polyaniline and poly(alkoxyanilines). It is found that the use of a pre-doped polymer on a subphase containing a doping agent enhanced significantly the quality of the Langmuir monolayer. When transferred onto a solid substrate the LB films are already in the doped state as confirmed by UV-Vis spectroscopy. LB films obtained with pre-doped poly(alkoxyanilines) exhibit electrochromism, with color changes from yellow to dark blue as the potential in an electrochemical cell is varied from -0.1 V to 0.7 V (versus SCE). In addition, good quality LB films were obtained with pre-doped, high molecular weight parent polyaniline.

The nucleation and growth model, which is usually applied to switching phenomena, is adapted for explaining surface potential measurements on the P(VDF-TrFE) (polyvinylidene fluoride-trifluoroethylene) copolymer obtained in a constant current corona triode. It is shown that the growth is one-dimensional and that the nucleation rate is unimportant, probably because surface potential measurements take much longer than the switching ones. The surface potential data can therefore be accounted for by a growth model in which the velocity of growth varies exponentially with the electric field. Since hysteresis loops can be obtained from surface potential measurements, it is suggested that similar mechanisms can be used when treating switching and hysteresis phenomena, provided that account is taken of the difference in the time scale of the measurements. >

The plate current for negative corona in a corona triode is observed to become positive and saturate at a value I/sub s/ when the grid-to-plate distance is less than 15 mm and the grid is biased positively. The saturation current, I/sub s/, is strongly dependent on the point-to-grid distance, which appears to indicate that the current is linked to those high energy hot electrons which survive attachment and are capable of ionizing molecules below the grid. Also investigated are the dependence of the saturation current on a few parameters and the current density distribution over the plate area. The relevance to the effects of corona treatment on the surface charge decay of polymer samples is discussed. >

The fabrication of Langmuir and Langmuir-Blodgett (LB) films from high-molecular-weight, doped parent polyaniline is reported. Parent polyaniline was rendered soluble by doping it with functionalized protonic acids (e.g. camphorsulfonic and toluenesulfonic acids) whose counter-ion induces solubility in low boiling point solvents such as chloroform. Condensed monolayers with high collapse pressures (above 30 mN/m) were obtained when an acidic subphase was used. The monolayers could be readily transferred onto hydrophilic substrates in the form of Z-type Langmuir-Blodgett films, and most importantly with transfer ratios close to 1. Interestingly, the films are readily obtained in the doped form and are electroactive.