Frequency upconversion properties of Tm3+ doped TeO2-ZnO glasses containing silver nanoparticles

Frequency upconversion (UC) properties of Tm3+ doped TeO2-ZnO glasses containing silver nanoparticles (NPs) were investigated. Infrared-to-visible and infrared-to-infrared UC processes associated to the Tm3+ ions were studied by exciting the samples with a cw 1050 nm ytterbium laser. The luminescence intensity as a function of laser intensity was also measured using a pulsed 1047 nm Nd3+:YVO laser in order to determine the number of photons participating in the UC processes. Enhancement of the UC signals for samples heat-treated during various time intervals is attributed to the growth of the local field in the vicinity of the NPs. PL enhancement by one-order of magnitude was observed in the whole spectrum of the samples heat-treated during 48 h. On the other hand PL quenching was observed for the samples heat-treated more than 48 h. (c) 2011 Elsevier B.V. All rights reserved.

Origin of the high upconversion green luminescence efficiency in -NaYF4:2% Er3+,20% Yb3+

Site-selective spectroscopy in hexagonal beta-NaYF4:Er3+,Yb3+ has revealed different environments for Er3+ ions (multisite formation). The low-temperature S-4(3/2) -> (I15/2Er3+)-I-4 green emission depends on the excitation wavelength associated with the F-4(7/2) Er3+ level. We have studied the effect of hydrostatic pressure on the green, red, and blue Er3+ emission upon NIR excitation at similar to 980 nm, in order to establish the role played by energy resonance conditions and the multiple Er3+ sites due to the disordered structure for the upconversion (UC) process (energy tuning). The variation of photoluminescence spectra and lifetimes as a function of pressure and temperature reveals that the origin of the high green UC efficiency of the beta-NaYF4:Er3+,Yb3+ compound is mainly due to the multisite distribution, and the low phonon energy of the host lattice.

Optimizing infrared to near infrared upconversion quantum yield of β-NaYF4:Er3+ in fluoropolymer matrix for photovoltaic devices

The present study reports for the first time the optimization of the infrared (1523 nm) to near-infrared (980 nm) upconversion quantum yield (UC-QY) of hexagonal trivalent erbium doped sodium yttrium fluoride (β-NaYF4:Er3+) in a perfluorocyclobutane (PFCB) host matrix under monochromatic excitation. Maximum internal and external UC-QYs of 8.4% ± 0.8% and 6.5% ± 0.7%, respectively, have been achieved for 1523 nm excitation of 970 ± 43 Wm−2 for an optimum Er3+ concentration of 25 mol% and a phosphor concentration of 84.9 w/w% in the matrix. These results correspond to normalized internal and external efficiencies of 0.86 ± 0.12 cm2 W−1 and 0.67 ± 0.10 cm2 W−1, respectively. These are the highest values ever reported for β-NaYF4:Er3+ under monochromatic excitation. The special characteristics of both the UC phosphor β-NaYF4:Er3+ and the PFCB matrix give rise to this outstanding property. Detailed power and time dependent luminescence measurements reveal energy transfer upconversion as the dominant UC mechanism.

Highly Efficient IR to NIR Upconversion in Gd2O2S: Er3+ for Photovoltaic Applications

Upconversion (UC) is a promising option to enhance the efficiency of solar cells by conversion of sub-bandgap infrared photons to higher energy photons that can be utilized by the solar cell. The UC quantum yield is a key parameter for a successful application. Here the UC luminescence properties of Er3+-doped Gd2O2S are investigated by means of luminescence spectroscopy, quantum yield measurements, and excited state dynamics experiments. Excitation into the maximum of the 4I15/2 → 4I13/2 Er3+ absorption band around 1500 nm induces very efficient UC emission from different Er3+ excited states with energies above the silicon bandgap, in particular, the emission originating from the 4I11/2 state around 1000 nm. Concentration dependent studies reveal that the highest UC quantum yield is realized for a 10% Er3+-doping concentration. The UC luminescence is compared to the well-known Er3+-doped β-NaYF4 UC material for which the highest UC quantum yield has been reported for 25% Er3+. The UC internal quantum yields were measured in this work for Gd2O2S: 10%Er3+ and β-NaYF4: 25%Er3+ to be 12 ± 1% and 8.9 ± 0.7%, respectively, under monochromatic excitation around 1500 nm at a power of 700 W/m2. The UC quantum yield reported here for Gd2O2S: 10%Er3+ is the highest value achieved so far under monochromatic excitation into the 4I13/2 Er3+ level. Power dependence and lifetime measurements were performed to understand the mechanisms responsible for the efficient UC luminescence. We show that the main process yielding 4I11/2 UC emission is energy transfer UC.

Perylene-based profluorescent nitroxides for the rapid monitoring of polyester degradation upon weathering: An assessment

A profluorescent nitroxide possessing an isoindoline nitroxide moiety linked to a perylene fluorophore was developed to monitor radical mediated degradation of melamine-formaldehyde crosslinked polyester coil coatings in an industry standard accelerated weathering tester. Trapping of polyester-derived radicals (most likely C-radicals) that are generated during polymer degradation leads to fluorescent closed-shell alkoxy amines, which was used to obtain time-dependent degradation profiles to assess the relative stability of different polyesters towards weathering. The nitroxide probe couples excellent thermal stability and satisfactory photostability with high sensitivity and enables detection of free radical damage in polyesters under conditions that mimic exposure to the environment on a time scale of hours rather than months or years required by other testing methods. There are indications that the profluorescent nitroxide undergoes partial photo-degradation in the absence of polymer-derived radicals. Unexpectedly, it was also found that UV-induced fragmentation of the NO–C bond in closed-shell alkoxy amines leads to regeneration of the profluorescent nitroxide and the respective C-radical. The maximum fluorescence intensity that could be achieved with a given probe concentration is therefore not only determined by the amount of polyester radicals formed during accelerated weathering, but also by the light-driven side reactions of the profluorescent nitroxide and the corresponding alkoxy amine radical trapping products. Studies to determine the optimum probe concentration in the polymer matrix revealed that aggregation and re-absorption effects lowered the fluorescence intensity at higher concentrations of the profluorescent nitroxide, but too low probe concentrations, where these effects would be avoided, were not sufficient to trap the amount of polyester radicals formed upon weathering. The optimized experimental conditions were used to assess the impact of temperature and UV irradiance on polymer degradation during accelerated weathering.

Using tau polarization to probe the stau co-annihilation region of the minimal supergravity model at the LHC

The minimal supergravity model predicts the polarization of the tau coming from the stau to bino decay in the co-annihilation region to +1. This can be exploited to extract this soft tau signal at LHC and also to measure the tiny mass differences between the stau and the bi lightest superparticle. Moreover, this strategy will be applicable for a wider class of bino lightest superparticle models, where the lighter stau has a right component at least of similar size as the left.

Duality and energy averaging for e+e- annihilation in potential models

Analytical expressions for the corrections to duality are obtained for nonsingular potentials, and are found to be small numerically. An alternative consistent way of energy smoothing, developed by Strutinsky, is elucidated. This may be of use even when potential models are not valid.

Highly efficient spin-conversion effect leading to energy up-converted electroluminescence in singlet fission photovoltaics

Free charge generation in donor-acceptor (D-A) based organic photovoltaic diodes (OPV) progresses through formation of charge-transfer (CT) and charge-separated (CS) states and excitation decay to the triplet level is considered as a terminal loss. On the other hand a direct excitation decay to the triplet state is beneficial for multiexciton harvesting in singlet fission photovoltaics (SF-PV) and the formation of CT-state is considered as a limiting factor for multiple triplet harvesting. These two extremes when present in a D-A system are expected to provide important insights into the mechanism of free charge generation and spin-character of bimolecular recombination in OPVs. Herein, we present the complete cycle of events linked to spin conversion in the model OPV system of rubrene/C60. By tracking the spectral evolution of photocurrent generation at short-circuit and close to open-circuit conditions we are able to capture spectral changes to photocurrent that reveal the triplet character of CT-state. Furthermore, we unveil an energy up-conversion effect that sets in as a consequence of triplet population build-up where triplet-triplet annihilation (TTA) process effectively regenerates the singlet excitation. This detailed balance is shown to enable a rare event of photon emission just above the open-circuit voltage (VOC) in OPVs.

Squeezed vacuum as an eigenstate of two-photon annihilation operator

We introduce the inverse annihilation and creation operators a-1 and a(dagger-1) by their actions on the number states. We show that the squeezed vacuum exp(1/2xia(dagger2)]\0] and squeezed first number state exp[1.2xia(dagger2)]\n = 1] are respectively the eigenstates of the operators (a(dagger-1)a) and (aa(dagger-1)) with the eigenvalue xi.

Boson Inverse Operators and a New Family of Two-photon Annihilation Operators

We introduce the inverse of the Hermitian operator (acircacirc†) and express the Boson inverse operators acirc-1 and acirc†-1 in terms of the operators acirc, acirc† and (acircacirc†)-1. We show that these Boson inverse operators may be realized by Susskind-Glogower phase operators. In this way, we find a new two-photon annihilation operator and denote it as acirc2(acircacirc†)-1. We show that the eigenstates of this operator have interesting non-classical properties. We find that the eigenstates of the operators (acircacirc†)-1 acirc2, acirc(acircacirc†)-1 acirc and acirc2(acircacirc†)-1 have many similar properties and thus they constitute a family of two-photon annihilation operators.

Flavoured co-annihilation

In minimal supergravity (mSUGRA) or CMSSM, one of the main co-annihilating partners of the neutralino is the lightest stau, (tau) over tilde (1). In the presence of flavour violation in the right-handed sector, the co-annihilating partner would be a flavour mixed state. The flavour effect is two-fold: (a) It changes the mass of (tau) over tilde (1) thus modifying the parameter space of the co-annihilation and (b) flavour violating scatterings could now contribute to the cross-sections in the early Universe. In fact, it is shown that for large enough delta similar to 0.2, these processes would constitute the dominant channels in co-annihilation regions. The amount of flavour mixing permissible is constrained by flavour violating tau -> mu or tau -> e processes. For Delta(RR) mass insertions, the constraints from flavour violation are not strong enough in some regions of the parameter space due to partial cancellations in the amplitudes. In mSUGRA, the regions with cancelations within LFV amplitudes do not overlap with the regions of co-annihilations. In non-universal Higgs model (NUHM), however, these regions do overlap leading to significant flavoured co-annihilations. At the LHC and other colliders, these regions can constitute for interesting signals.

Annihilation of vortex dipoles in an oblate Bose-Einstein condensate

We theoretically explore the annihilation of vortex dipoles, generated when an obstacle moves through an oblate Bose-Einstein condensate, and examine the energetics of the annihilation event. We show that the grey soliton, which results from the vortex dipole annihilation, is lower in energy than the vortex dipole. We also investigate the annihilation events numerically and observe that annihilation occurs only when the vortex dipole overtakes the obstacle and comes closer than the coherence length. Furthermore, we find that noise reduces the probability of annihilation events. This may explain the lack of annihilation events in experimental realizations.

Confinement induced stochastic sensing of charged coronene and perylene aggregates in alpha-hemolysin nanochannels

Biological nanopores provide optimum dimensions and an optimal environment to study early aggregation kinetics of charged polyaromatic molecules in the nano-confined regime. It is expected that probing early stages of nucleation will enable us to design a strategy for supramolecular assembly and biocrystallization processes. Specifically, we have studied translocation dynamics of coronene and perylene based salts, through the alpha-hemolysin (alpha-HL) protein nanopore. The characteristic blocking events in the time-series signal are a function of concentration and bias voltage. We argue that different blocking events arise due to different aggregation processes as captured by all atomistic molecular dynamics (MD) simulations. These confinement induced aggregations of polyaromatic chromophores during the different stages of translocation are correlated with the spatial symmetry and charge distribution of the molecules.

Quark and gluon distribution functions in a viscous quark-gluon plasma medium and dilepton production via q(q)over-bar annihilation

Viscous modifications to the thermal distributions of quark-antiquarks and gluons have been studied in a quasiparticle description of the quark-gluon-plasma medium created in relativistic heavy-ion collision experiments. The model is described in terms of quasipartons that encode the hot QCD medium effects in their respective effective fugacities. Both shear and bulk viscosities have been taken in to account in the analysis, and the modifications to thermal distributions have been obtained by modifying the energy-momentum tensor in view of the nontrivial dispersion relations for the gluons and quarks. The interactions encoded in the equation of state induce significant modifications to the thermal distributions. As an implication, the dilepton production rate in the q (q) over bar annihilation process has been investigated. The equation of state is found to have a significant impact on the dilepton production rate along with the viscosities.

Influencia de la fertilización nitrogenada y densidad de siembra en el rendimiento agronómico e industrial de tomate (Lycopersicum esculentum Mill) variedades UC-82 y topacio en el Valle de Sébaco, Matagalpa

El presente estudio se realizó en la Estación Experimental " Raúl González" del Valle de Sebaco en el período del 5 de junio al 16 de septiembre de 1992. Se evaluaron las variedades, UC-82 y Topacio, las densidades de siembra de 27,777 y 50,000 plantas/ha y cuatro niveles de nitrógeno, O Kg/ha, 75 Kg/ha, 150 Kg/ha y 225 Kg/ha. Se desarrolló el estudio con tres factores y se estableció un diseño de Bloques Completos al Azar, evaluándose los parámetros de crecimiento del cultivo, así como, lo relacionado al rendimiento agronómico e industrial. Los datos obtenidos se sometieron al análisis de varianza y a la prueba de SNK al 5%, Los resultados en cuanto a crecimiento y desarrollo fueron los siguientes: La mayor altura la presentó la variedad UC-82 en combinación con la densidad de 50,OOO plantas/ha y con la aplicación de 75 Kg/ha de Nitrógeno; e1 mayor número de hijos se obtuvo con la misma variedad y densidad y la aplicación de 225 Kg/ha de Nitrógeno. En cuanto a floración y fructificación los mejores resultados se alcanzaron con la variedad UC-82, la densidad de 27,111 plantas/ha y con 75 Kg/ha de Nitrógeno. Respecto al rendimiento agronómico, el mejor tratamiento fue la variedad uc-82, con la densidad de 50,000 plantas/ha y una aplicación de 75 Kg/ha de Nitrógeno. En el rendimiento industrial la variedad Topacio, con la densidad de 50,000 plantas/ha y 225 Kg/ha de Nitrógeno reporta los mejores resultados.