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International audience; Although water has been extensively studied, not all of its unique properties have been fully understood. There is still controversy about the temperature at which hydrogen bonds are broken or weakened, producing the anomalous temperature dependence of many water properties. Different temperatures between 23 and 48 °C have been reported, but no study has scrutinized the reasons for this discrepancy. We suggest the determining role of pH in the alteration of the water anomaly temperature. We employed a luminescent europium trisbipyridine cryptate, which is highly sensitive to changes in the arrangement of water molecules and whose luminescence intensity and lifetime are not significantly influenced by variations over a broad pH range. Our results revealed an increase of the crossover temperature from circa 35 °C at pH 3.5 to circa 45 °C at pH 7 to 9, which explains the discrepancies of previous studies. The pH dependence of water anomaly temperature is an important property for a better understanding of water and water-based systems and applications.

Curie-Weiss temperature dependence of the dielectric permittivity has been shown to take place in water. A bilinear change at T*=60 oC has been reported recently. In this work an effective field st...

Good crystalline quality CaF 2 sub-micron size particles doped with neodymium ions have been produced by the co-precipitation process and their crystallinity have been further improved by thermal treatment at 500 °C. Core and surface related luminescence defect centres have been identified and the effects of Y 3+ and Yb 3+ codopants are also investigated. Core defects centres are associated with single-ion and multi-ion defect centres as observed in bulk single crystal whereas the origin of the surface or near surface defect, A′, centre has been ascertained to be derived from a single-ion centre most probably charge compensated by a hydroxyl group.

In this study, the photoluminescence properties of congruent codoped LiNbO3:Cr3+; W4+, crystals have been systematically investigated by performing photoluminescence studies at room temperature in the 0–280 kbar pressure range. In particular, we focus on the influence that hydrostatic pressure has on the 2E→ 4A2 (R-lines) transitions of Cr3+. It has been observed that the pressure dependence of the spectral position of the R-lines associated with both Cr3+ centres β and γ shows a bilinear behaviour with an abrupt slope change near 210 kbar. This change is related to the existence of a pressure-induced structural phase transition in the LiNbO3 host. The analysis of experimental results provides the Racah parameters B and C and the crystal field parameter 10Dq and their pressure and volume, through the crystal field theory and equation of state, dependences.

Recently, the paraelectric response of water was investigated in the range 0–100 °C. It showed an almost perfect Curie–Weiss behaviour up to 60 °C, but a slight change in slope of 1/ed versus T at 60 °C was overlooked. In this work, we report optical extinction measurements on metallic (gold and silver) nanoparticles dispersed in water, annealed at various temperatures in the range from 20 to 90 °C. An anomalous response at 60 °C is clearly detectable, which we associate to a subtle structural transformation in the water molecules at that temperature. This water anomaly is also manifested by means of a blue shift in the longitudinal surface plasmon resonance of the metallic nanoparticles for the solutions annealed at temperatures higher than about 60 °C. A reanalysis of 1/ed (T) for water in the whole temperature range leads us to conclude that the water molecule undergoes a subtle transformation from a low temperature (0–60 °C) configuration with a dipole moment μ1 = 2.18 D (close to the molecular dipole...

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of Physical Chemistry C, © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/doi/abs/10.1021/acs.jpcc.8b03937, Aqueous solutions of luminescent metal-ion complexes, in particular, those of lanthanide ions, can play an essential role in biomedical applications. For all of these applications, the knowledge about the influence of temperature variations within the physiological range (20-60 °C) on their optical properties becomes essential. At variance with other liquids, water is unique as it does present an anomalous temperature-dependent behavior. In particular, most water properties present remarkable physicochemical changes above a certain temperature, which ranges between 30 and 50 °C. In this work, we systematically investigate the effect of temperature on the luminescence properties of Eu3+ ions when dissolved in either H2O or D2O. An anomalous thermal behavior, manifested as a bilinear trend, with crossover at around 35 °C for H2O and 38 °C for D2O, is found in a variety of Eu3+ optical spectroscopic properties (branching ratio, luminescence lifetime, and emission band shape). The observed changes are tentatively explained here in terms of different aggregation states of H2O and D2O molecules below and above the crossover temperature. Such changes in the molecular clustering lead to a temperature-induced change in the relative concentrations of the 8-fold and 9-fold coordinated Eu3+ complexes. Finally, we have observed that the pH of the aqueous solution plays an essential role in defining the temperature at which the anomaly takes place so that the differences in the values reported in the literature for the crossover temperature are accounted for, The work was partially supported by the Ministerio de Economiá y Competitividad de España (MAT2016-75362-C3-1-R), by COST Action CM1403, by Instituto de Salud Carlos III (PI16/00812), by Comunidad Autonóma de Madrid (B2017/BMD-3867RENIM-CM), by the European Comission (NanoTBTech), and “cofinanciado con Fondos Estructurales de la Union Europea”. L.L.-P. thanks Universidad Autonomá de Madrid for the “Formacioń de Personal Investigador (FPI-UAM)” program. E.M. thanks CONACyT for the Postdoctoral Fellowship. M.B. thanks Dipartimento di Biotecnologie, Univ. Verona, for financial support

This paper presents a study of the behaviour as optical superlattices of periodically poled LiNbO3 and LiTaO3 crystals using the Scanning Near-field Optical Microscopy (SNOM) technique. The periodic ferroelectric domains were formed either by electric field poling technique or by the “off-centre Czochralski method” during crystal growth. SNOM patterns were recorded in both collective and reflective configuration, simultaneously with surface topography measurement by AFM (Atomic Force Microscopy). It has been found that regardless of the method used to create the domains, a modulation in the SNOM signal intensity is correlated with the ferroelectric domain structure. The maximum intensity of the collected or reflected SNOM light intensity is centred on the negative domains, whereas the minimum intensity is positioned on the positive domains. This result demonstrates that the modulation of the refractive index is related to the domains and not with the wall between them.

Paloma Rodríguez-Sevilla, Yuhai Zhang, Patricia Haro-González, Francisco Sanz-Rodríguez, Francisco Jaque, José García Sole, Xiaogang Liu, Daniel Jaque, "Avoiding induced heating in optical trap", Optical Trapping and Optical Micromanipulation XIV, Proc. SPIE 10347 - 1034716 (25 August 2017); doi: 10.1117/12.2276355. ne print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited., Proceedings of XIV Optical Trapping and Optical Micromanipulation Conference (San Diego, California, United States), Luminescence of a single upconverting particle (NaYF 4 :Er 3+ ,Yb 3+ ) can be used to determine the optical trap temperature due to the partial absorption of the trapping beam either by the medium (water) or the optically trapped particle itself. This fact is an important drawback can be reduced by shifting the trapping wavelength out of the water absorption band, or by using time-modulated laser trapping beams. Both approaches have been studied and the results have shown that the thermal loading due to the trapping radiation can be minimized., This work was supported by the Spanish Ministerio de Educación y Ciencia (MAT2016-75362-C3-1-R) and by COST Action 1403. P.H.G. thanks the Spanish Ministerio de Economía y Competitividad (MINECO) for the Juan de la Cierva- Incorporación program. P.R.S. thanks MINECO and the Fondo Social Europeo (FSE) for the “Promoción del talento y su Empleabilidad en I+D+i” statal program (BES-2014-069410)

This work was supported by the Spanish Ministerio de Educación y Ciencia (MAT2013-47395-C4-1-R and MAT2016-75362-C3-1- R) and by the COST Action CM1403. L.L.-P. thanks the Universidad Autónoma de Madrid for the “Formación de Personal Investigador (FPI-UAM)” program. P.H.-G. thanks the Spanish Ministerio de Economia y Competitividad (MINECO) for the Juan de la Cierva program. The authors from the University of Belgrade acknowledge the financial support of the Ministry of Education, Science and Technological Development of the Republic of Serbia (Project Nos. 45020 and 172056). M.I.M. thanks financial support from the Spanish Ministerio de Economía y Competitividad (MICINN) Project FIS2015-69295-C3-3-P and the “María de Maeztu” Program Ref: MDM-2014-0377. The work of K.S. was supported by the Latvian National Research Program IMIS2 (Grant No. 302/2012)., Nowadays a large variety of applications are based on solid nanoparticles dispersed in liquids—so called nanofluids. The interaction between the fluid and the nanoparticles plays a decisive role in the physical properties of the nanofluid. A novel approach based on the nonradiative energy transfer between two small luminescent nanocrystals (GdVO4:Nd3+ and GdVO4:Yb3+) dispersed in water is used in this work to investigate how temperature affects both the processes of interaction between nanoparticles and the effect of the fluid on the nanoparticles. From a systematic analysis of the effect of temperature on the GdVO4:Nd3+ → GdVO4:Yb3+ interparticle energy transfer, it can be concluded that a dramatic increase in the energy transfer efficiency occurs for temperatures above 45 °C. This change is properly explained by taking into account a crossover existing in diverse water properties that occurs at about this temperature. The obtained results allow elucidation on the molecular arrangement of water molecules below and above this crossover temperature. In addition, it is observed that an energy transfer process is produced as a result of interparticle collisions that induce irreversible ion exchange between the interacting nanoparticles., Spanish Ministerio de Economia y Competitividad; Universidad Autónoma de Madrid; Spanish Ministerio de Educación y Ciencia MDM-2014-0377,FIS2015-69295-C3-3-P, 302/2012; Spanish Ministerio de Ciencia Tecnología y Telecomunicaciones MAT2013-47395-C4-1-R,MAT2016-75362-C3-1-R; Ministarstvo Prosvete, Nauke i TehnoloÅ¡kog Razvoja 45020,172056; European Cooperation in Science and Technology CM1403; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

The photoluminescence properties of Cr-doped PLZT-9/65/35 (Pb0.95La0.09Zr0.65Ti0.35O3) have been studied. The fluorescence spectra consist of two inhomogeneously broadened narrow bands centred at ∼734 and 740 nm and a broadband emission with a maximum at ∼1000 nm. The two high energy bands have been assigned to the 2 E ( 2 T 1 ) → 4 A 2 transition of two Cr3+ defect centres in octahedral site with high/intermediate crystal field strength whereas the near infrared emission is tentatively assigned to the overlapping of the broadband emission of the Cr3+ ions defect centre’s 4 T 2 → 4 A 2 transition and the emission of the Cr5+ ions’ 2 E → 2 T 2 transition.

We show how the ultrafast laser inscription of nanodimple arrays enables near-field local enhancement in high refractive-index crystals. We first demonstrate both experimentally and numerically how, when two nanodimples are separated by intermediate distances (700–1000 nm), near-field interference can take place resulting in a near-field enhancement between nanodimples. Based on this effect, near-field enhancements close to 60% are then demonstrated at the center of nanodimple arrays, and strong optical contrasts in excess of 100% are produced. The integration of these arrays into crystal substrates provides a versatile tool for new optical trapping and evanescent-field nanosensing devices.

The reflectivity and photoluminescence of cobalt-doped ZnO ceramics samples have been studied by optical spectroscopy. The result reveals, in addition to the emission bands associated with intrinsic defects, weak narrow emission peaks which are ascribed to a mixed 4 T 1 (P), 2 T 1 (G), 2 E(G) → 4 A 2 (F) transition between Co 2+ d-levels located in regular Zn 2+ sites, [Co 2+ ] Zn and a broad near infrared emission band centred at 710 nm which has been interpreted as belonging to Co 2+ ions located in a perturbed Zn 2+ site, [ Co 2 + ] Zn ∗ . This study shows that the annealing treatment at 800 °C activates the diffusion of point defects thus creating more [Co 2+ ] Zn centres.

This paper presents a study of the near-field optical profile of corrugated surfaces obtained from commercial digital versatile disk (DVD) patterns using a scanning near-field microscope (SNOM). The profile of the SNOM light intensity collected is related to the topography of the samples. The SNOM patterns consist of a periodic modulated light intensity along the corrugated surface with the same periodicity of the topography structure. For the virgin sample, the light profile has intensity peaks corresponding to the ridges of the DVD structure. However, when the corrugated surface is metalized with a thin gold film of 30 nm thick, the SNOM light intensity peaks are centred in the valley of the DVD structure. A Finite-Difference Time-Domain (FDTD) model has been employed to simulate the SNOM experimental data.

The thermoluminescence (TL) sensitivity to ionising (beta source) and non-ionising (UV) radiation on KCl:Eu 2+ single crystals has been investigated. The different shapes of the TL glow curves allow us to detect specific peaks (over 220–250 °C) due to UV exposure that exhibit a negligible contribution associated with ionising radiation. The UV-induced TL emission could be deconvoluted into five groups of components peaked at about 120, 150, 210, 250 and 330 °C assuming first order kinetic processes. Dose saturation and linearity region have been determined for a wavelength of 254.7 nm. The effect of several cycles of UV irradiation processes on the linearity of the high energy ultraviolet KCl:Eu 2+ dosimeter has been also studied to determine the potential reusability.

This work shows a study of the periodic ferroelectric domains formed in LiNbO3 crystals doped with rare earths by means of scanning near field optical microscopy (SNOM) technique. It has been observed periodic structures associated with ferroelectric domains with an unexpected high value of the optical contrast working under reflectance SNOM mode. From Raman-Nath diffraction patterns, a refractive index modulation of Δ n ∼ 1 0 − 4 has been calculated. These results were correlated with the ferroelectric periodic domains obtained by the SNOM technique. A light waveguide effects along the ferroelectric domains is suggested to explain the high reflectance contrast observed in SNOM experiments.

The KCl:Eu2+ system response to UV-C was investigated by analyzing the optically stimulated luminescence (OSL) and thermo-luminescence (TL) signal produced by ultraviolet light exposure at room temperature. It was found that after UV-C irradiation, OSL was produced on a wide band of visible wavelengths with decay time that varied by several orders of magnitude depending on the Eu2+ aggregation state. In spite of the low intensity of solar UV-C reaching the Earth's surface in Madrid (40o N, 700 m a.s.l.), it was possible to measure the UV-C radiation dose at 6:48 solar time by using the TL response of the KCl:Eu2+ system and differentiate it from the ambient beta radiation dose.

The effect that femtosecond laser filamentation has on the refractive index of Nd:YAG ceramics, and which leads to the formation of waveguide lasers, has been studied by micro-spectroscopy imaging, beam propagation experiments and calculations. From the analysis of the Nd3+ luminescence and Raman images, two main types of laser induced modifications have been found to contribute to the refractive-index change: (i) a lattice defect contribution localized along the self-focusing volume of the laser pulses, in which lattice damage causes a refractive-index decrease, and (ii) a lattice strain-field contribution around and inside the filaments, in which the pressure-driven variation of the inter-atomic distances causes refractive-index variations. Scanning near-field optical-transmission and end-coupling experiments, in combination with beam propagation calculations, have been used to quantitatively determine the corresponding contribution of each effect to the refractive-index field of double-filament waveguides. Results indicate that the strain-field induced refractive-index increment is the main mechanism leading to waveguiding, whereas the damage-induced refractive-index reduction at filaments leads to a stronger mode confinement.

This paper reports on the spectroscopy properties, absorption and luminescence, of Cr3+ ions in singly doped, ZnO-codoped, and Zn in-diffused LiNbO3:Cr crystals. In addition to the broad absorption, inter-ionic transitions ascribed to Cr3+ ions located in Li+ and Nb5+ sites; [Cr]Li and [Cr]Nb centres two absorption bands at higher energy are reported and ascribed to the charge transfer transitions of the Cr3+ ions of the two defect centres. The charge transfer transitions are used as optical probe to study the role of the Zn ions in the Zn in-diffused LiNbO3:Cr samples. It has been observed that the Zn-in-diffused processes created [Cr]Nb centres in the diffusion zone. The location of the diffused Zn2+ ions is considered to be in Li+ site, displacing the Cr3+ ions from the Li+ sites, [Cr]Li, to the Nb5+ positions, [Cr]Nb.

Near-field-optical-microscopy has been used to study the micro-modifications caused by femtosecond laser pulses focused at the surface and in the volume of lithium niobate crystals. We have found experimental evidence of the existence, close to femtosecond ablation craters, of periodic modifications in the surface reflectivity. In addition, the potential application of near-field-optical microscopy for the spatial location of permanent modifications caused by femtosecond pulses focused inside lithium niobate crystals has been also demonstrated.

In this work, Scanning Near Field Microscopy (SNOM) observation and characterization of quasi-periodic ferroelectric domains in rare earth doped LiNbO 3 , Nd doped Ba 2 NaNb 5 O 15 , and LiTaO 3 crystals are presented. The formation of ferroelectric domains has been induced either during the crystal growth procedure (LiNbO 3 and Ba 2 NaNb 5 O 15 ) or by applying pulsed electric field (LiTaO3). SNOM reflection, transmission and collection modes have been used indistinctly to characterize the ferroelectric domain structure. In the three crystals under study, an unexpected optical contrast associated to the presence of ferroelectric domains has been observed. In particular, working in the SNOM collection mode the optical contrasts reached a value closed to ∼ 80% in Ba 2 NaNb 5 O 15 and LiTaO 3 crystals. These high optical contrast values are explained considering that the ferroelectric domains pattern operates as a periodic array of planar waveguides (optical super-lattices). The invoked light-pipe mechanism...

Lithium niobate (LiNbO3) crystals doped with chromium ions show a clear green colouring reflecting the absorption profile of the dominating [Cr](Li) defect centres. A significant change in its colouration takes place when it is co-doped with other valency impurities such as Mg2+, Sc3+ and W6+, above a certain threshold concentration. This concentration singularity has been attributed to the formation of [Cr](Nb) centres coexisting with the [Cr](Li) centres. In this work, we extended the investigation on the effect of co-dopant ions in Cr:LiNbO3 to tetravalent cation such as GeO2. A singularity in the relative intensity of the (4)A(2)-> T-4(1) and (4)A(2)-> T-4(2) absorption band was observed for a concentration of similar to 1.5 mol%, compared with 4.5 mol% for Mg2+. The photoluminescence emission spectra also reveal a new emission band, at a lower energy than the [Cr](Li) centre, corresponding to this threshold concentration. A charge compensation model is proposed to explain the role of cation impurities and results are compared with those of other valence impurities.

In this paper we report on experimental evidence of laser light confinement between ferroelectric domains in the neodymium doped Ba2NaNb5O15 ferroelectric crystal. High contrast near field optical microscopy images have been obtained as a consequence of the periodic modulation in the refractive index caused by the presence of 2D antiparallel ferroelectric domains.

Single particle spectroscopy in the form of three-dimensional optical manipulation of an upconverting nanoparticle is here used for non-invasive thermal sensing at the cellular level. In particular, a single infrared 980 nm laser beam is used as a three-dimensional optical tweezer and, simultaneously, as an optical excitation source for a single NaYF4:Er3+,Yb3+ upconverting particle. Real time analysis of the thermosensitive green emission of Er3+ ions obtained after Yb3+ excitation provides thermal sensing during optical manipulation. Thus, three-dimensional particle scanning allows for the measurement of thermal gradients in the surroundings of individual cancer cells subjected to a plasmonic-mediated photothermal therapy. It is found that such thermal gradients extends for distances larger than 10 microns, avoiding real single cell photothermal treatments under in vitro conditions. This work introduces to the scientific community a novel and simple approach for high resolution thermal sensing at the cellular level that could constitute a powerful tool for a better understanding of cell dynamics during thermal treatments, This work was supported by the Spanish Ministerio de Educación y Ciencia (MAT2013–47395-C4–1-R) and by Banco Santander for “Proyectos de Cooperación Interuniversitaria” (2015/ASIA/06). P.H.G thanks the Spanish Ministerio de Economía y Competitividad (MINECO) for the Juan de la Cierva program. P.R.S thanks the Spanish Ministerio de Economía y Competitividad (MINECO) for the “Promoción del talento y su Empleabilidad en I+D+i” statal program

The simulation of light propagation using standard beam propagation techniques, in periodically inverted domains in Ba2NaNb5O12 crystals is presented. The poled domains pattern gives rise to periodic modulation of the refractive index in the crystal, which can be observed with diffraction experiments and simulated by one-dimensional beam propagation methods. The micro-domains are characterized by Scanning Near Field Microscopy, using both transmission and collection modes. In particular, under the collection mode, micro- pictures have been obtained with unexpected high optical contrast between positive and negative domains of up to 80%. Using a beam simulation method, the results are explained considering that the ferroelectric domains operate as a periodic array of planar waveguides.

The photoconductivity of LiNbO3 crystals codoped with Cr2O3 and different concentrations of MgO from 0 up to 6 mol % is reported. The spectral excitation of photocurrent presents an intense band corresponding to the Cr3+ induced blue absorption edge, but illuminating at the 4 A 2→4 T 1 ionic transition band a weak photocurrent is also excited. The photoconductivity is interpreted in terms of Cr3+ ionization and the formation of NbNb 4+ small polarons as charge carriers. Photoconductivity below about 140 K is suppressed. The analysis of the photocurrent temperature dependence results in a 0.14±0.02 eV binding energy of the polaron at low temperature. Photoconductivity induced by excitation in the intraionic Cr3+ can be explained by a tunnelling process between the excited Cr3+ and the polaron state at the next-nearest-neighbour NbNb 5+.

This paper reports on the photo-luminescence spectroscopic results of congruent strontium-barium-mobate (SBN) crystals doped with Cr2O, at cryogenic temperature (20 K). The experimental results reveal the need of re-assignment of the Cr3+ ions defect centres in this material. For first time, a broad emission band in the near infrared region centred at ca. 950 nm is reported. This emission band has micro-seconds decaytime constant and a band-width full-width at half-maximum (FWHM) larger than 1700 cm(-1) and has been ascribed to the vibronically assisted T-4(2) -> (4)A(2) transition. A much narrower emission band centred at ca. 764 nm with milli-seconds decaytime constant and a FWHM band-width of ca. 170 cm(-1) is correlated to the E-2 -> (4)A(2) radiative transition (R-line). (c) 2006 Elsevier B.V. All rights reserved.

This paper reports the photo-luminescence spectroscopic results of Strontium–Barium–Niobate, Srx,Ba1� xNb2O5 (SBN, x = 0.61 for near congruent composition) crystals doped with Cr2O, at cryogenic temperature (20 K). The experimental results reveal the need of reassignment of the Cr 3+ ions defect centres in this material. For first time, a broad emission band in the near infrared region centred at ca. 950 nm is reported. This emission band has micro-seconds decaytime constant and a FWHM band-width > 1700 cm � 1 and has been ascribed to the vibronically assisted 4 T2 ! 4 A2 transition. A much narrower emission band centred at ca. 764 nm with milli-seconds decaytime constant and a FWHM band-width of ca. 170 cm � 1 is correlated to the 2 E ! 4 A2 radiative transition (R-line).

The dependence of the refractive indices, absorption edge and optical damage in congruent HfO2-doped LiNbO3 crystal are reported. A small amount of Cr3+ ions (0.1 mol%) was introduced in the doped crystals as optical probe. The experimental data indicates that the extraordinary and ordinary indices depend on the Hf4+ ions concentration showing a singularly for a dopant level of ~ 2.7 mol%. The dependence of the absorption edge also shows a transition region at this Hf4+concentration. The distortion of the transmitted light through samples doped with different concentrations of Hf4+ ions indicates that an inhibition of optical damages is achieved at above 3 mol% of dopant. A charge compensation mechanism is proposed to explain the role of the Hf4+ ions for the observed experimental result.

The emission intensity of the R-line of Cr3þ ions in near stoichiometric LiNbO3crystals codoped with MgO has been studied at a low temperature under excitation of the 4A2-4T1 transition. The R-line emission intensity associated with Cr3þ ions in the Nb5þ site, ½CrNb centre, was observed to decrease with the illumination time whereas the Cr3þ ions located in the Li site, ½CrLi centres, are optically stable. This selective bleaching is explained by considering the photoionization of Cr3þ ions in Nb5þ sites and the formation of Nb4þ

The optical spectroscopy of Cr3+ ions in near stoichiometric LiNbO3 crystals, ‘pure’ and co-doped with MgO, has been investigated. In the near stoichiometric LiNbO3:Cr (0.2%) optical absorption and emission spectra show the usual features of broad bands and narrow lines typical of congruent samples with the an additional of a narrow emission lines centred at 738 nm and has been ascribed to the zero-phonon line of the 4T2→4A2 transition. The concentration equilibrium between the two type of centres is strongly displaced towards the [Cr]Nb centre permitting thus to obtain with accuracy the parameters of the broad bands. The R-lines associated with the [Cr]Nb centre is only observable in low-temperature emission spectrum. The presence of Fano anti-resonance lines has been observed to be more pronounced in the near stoichiometric than in congruent samples.

A comparison of the optical properties such as optical absorption band edge, ordinary and extraordinary refractive indices, powder second harmonic generation and the non-linear coefficient, d33, of congruent LiNbO3 crystals doped with di-, tri- and hexa-valent cations is reported. It has been found that the referred parameters show a clear concentration singularity depending on the valency of the cation. A charge compensation mechanism is proposed to explain the role of the cation. The experimental data of no and ne are described by a generalised Sellmeier equation considering the different ions sites in the crystal.

The photoluminescence property of pure and MgO co-doped near stoichiometric LiNbO3:Cr crystals has been investigated. It has been found that the presence of low Mg ion content in the crystals induces the coexistence of two types of Cr3+ sites; Cr3+ ion substituting Li+ and Nb5+ sites. In MgO co-doped samples, four narrow emission bands have been found and three of them are correlated with the broad absorption bands associated to the Cr3+ ions located in Nb5+ site ([Cr]Nb centres). The observation of the zero-phonon line at 13 540 cm−1 only in pure LiNbO3:Cr crystals of near stoichiometric composition suggests it could be used as a probe to monitor the composition of LiNbO3 crystals.

In this work near-field optical measurements of a corrugated grating coated with a 30 nm thick gold film are presented. The grating was made using the polycarbonate corrugated substrate of a commercially available recordable CD as template. This has been proved to be a versatile and low cost technique in producing large 1.6 μm period gratings. The study was carried out using a Scanning Near-Field Optical Microscope (SNOM) working in both collection and reflection modes at two different wavelengths, 532 nm and 633 nm. The results illustrate that the intensity patterns of near-field images are strongly polarization-dependent, even showing different periodicity of the localized fields for orthogonal polarization states. When electric field of the light is polarized parallel to the grooves, the periodicity of the SNOM images is coincident with the grating period, whereas when the light is polarized perpendicular to the grooves the SNOM pattern shows a periodicity twice that of the corresponding topography of the grating. Numerical simulations of the SNOM data based on a two-dimensional Finite Difference Time-Domain (2D-FDTD) model have been realized. The results of the simulations are in good agreement with the experimental data, emphasizing the need of performing numerical simulation for the correct interpretation of SNOM data, This work has been partially supported by Ministerio de Ciencia e Innovacion (project TEC2010-21574-C02-01) and Comunidad de Madrid (P2009/TIC-1476), Spain. E. Camarillo G. recognizes to DGAPA-UNAM support for a sabbatical year at UAM

The optical spectroscopy of Cr3+ ions doped into near-stoichiometric LiNbO3 crystals, pure and co-doped with MgO, has been investigated. In the near-stoichiometric LiNbO3:Cr(0.2 mol%):Mg(2 mol%) crystal, the optical spectra resemble those previously observed for congruent LiNbO3:Cr:MgO samples when the total MgO content exceeds the 4.6 mol% threshold. The coexistence of two types of Cr3+ centre ([Cr]Li and [Cr]Nb) characterized the optical and luminescence spectra of this sample. The concentration equilibrium between the two types of centre is strongly displaced towards the [Cr3+ ] Nb centre, permitting us to obtain with accuracy the parameters of the broad bands. The R-line associated with the [Cr]Nb centre is only observable in the low-temperature emission spectrum. The Fano anti-resonance lines present have been observed to be more pronounced for the near-stoichiometric samples than for congruent ones.

The dependence of the optical absorption band edge, ordinary and extraordinary refractive index, powder second harmonic generation and the non-linear coefficient, d 33 , of congruent LiNbO 3 crystals doped with WO 3 are reported. It has been found that the referred parameters show a clear singularity for a concentration in the range of 1.5–2%. In comparison with previous works on congruent LiNbO 3 crystals doped with divalent and trivalent ions a charge compensation mechanism is proposed to explain the role of the cation. The experimental data of n o and n e are described by a generalised Sellmeier equation considering the different ions sites in the sample.

Infrared optical absorption has been used to study OH − impurities into congruent co-doped LiNbO 3 :Cr 3+ :ZnO crystals doped with different Zn 2+ concentration. The OH − IR absorption spectra present three bands that can be associated with different OH − complex centres available in the lattice. For crystals with lower Zn 2+ concentrations ( −1 ) is reported which is associated with the OH − unperturbed vibration. For crystals with higher Zn 2+ concentrations (>4.7%), two new bands associated with OH − vibration in distortion environment are reported. These bands are centred at 2827 nm (3537 cm −1 ) and 2847 nm (3512 cm −1 ) and can be associated with OH − –Zn 2+ and Cr 3+ (Li + )–OH − –Zn 2+ (Int.) complex centres, respectively. Electron paramagnetic resonance (EPR) has been used to identify the Cr 3+ centres in the lattice of the doped LiNbO 3 :ZnO crystals.

A sudden transient increase of the solar ultraviolet (UV) radiation was detected with a ground-based photometer in Madrid (Spain, 41°N, 3°W) on 25 November 1996 (Cordoba et al., 2000. UV-B irradiance at Madrid during 1996, 1997 and 1998. J. Geophys. Res. 105 (D4) (2000) 4903–4906). The data obtained by the TOMS satellite instrument revealed that this event was related to a reduction of the total ozone column values. This low ozone episode has been analysed. The ozone decrease was accompanied by a reduction of air temperature in the lower stratosphere. The anti-correlation between total ozone and air parcel height along the three-dimensional isentropic back-trajectories suggests that the adiabatic uplift of air contributed significantly to both the ozone and the temperature decreases observed. The uplift could be caused by the propagation into the lower stratosphere of the high-pressure cell located over the mid-Atlantic region.

This paper reports the optical spectroscopy (absorption and luminescence) and electron paramagnetic resonance (EPR) of codoped Sc2O3 LiNbO3:Cr crystals (Sc3+ ions concentration was varied from 0 to 7%). Two kinds of Cr3+ ions: Cr3+ Li and Cr3+ Nb centres were observed with optical and magnetic characteristics very similar to that reported in MgO and ZnO codoped LiNbO3:Cr crystals. However in Sc2O3 codoped samples the Cr3+ Li centre is predominant in all the Sc2O3 concentration ranges examined. At variance with codoped MgO and ZnO, only a partial Cr3+ centre redistribution takes place for a Sc3+ ions content around 3%. This critical value is discussed considering a charge compensation model.

The fluorescence (emission and excitation) spectrum of Mn 2+ ion in NaCl has been investigated for freshly quenched samples (diluted Mn 2+ ions) and as grown samples (Mn 2+ ions forming aggregates and/or precipitates). Two main emission bands are generally observed in as grown samples, peaking at 505 nm and around 610 nm. Both emission bands are related to different manganese precipitates. Quenched samples show only the red emission but peaking at 605 nm and with a different excitation spectrum to that of the as grown crystals. Thus, this crystal is suitable to operate as a double color (red and green) emitting phosphor where the red and green relative intensities can be controlled by a thermal treatment.

The continuous depletion of the ozone layer causes an increase in the ultraviolet-B (UV-B) reaching the Earth’s surface. In this paper the behaviour of the KCl:Eu 2+ under solar UV irradiation has been investigated. Considering the thermoluminescence (TL) excitation spectra, the geometrical solar radiation parameters (solar zenith angle related to the light path) and the atmosphere characteristics (ozone content, Rayleigh scattering and aerosols’ concentration), the solar UV irradiation flux registered by the dosimeter has been simulated throughout the day. This modelled hourly signal agrees with the measured TL signal at different hours. Moreover, these curves throughout the day are compared to the convoluted signal obtained from a gaussian curve centred at different UV-C wavelengths and the same solar spectrum. This comparison supports the idea that the KCl:Eu 2+ crystal behaves under the sun as a narrow band gaussian detector centred at about 265 nm, but with the main registered solar UV-B signal at 285 nm. On the other hand, a comparison of the KCl:Eu 2+ system with commercially available broad band UV-B biological sensors (biometers), shows that the europium doped crystals are more sensitive to minor changes of the solar UV-B flux and therefore, it is a good instrument for the study of small ozone layer depletions.

The results of high-pressure spectroscopic studies of near-stoichiometric ${\mathrm{LiNbO}}_{3}:\mathrm{C}\mathrm{r}(0.1%)$ and ${\mathrm{LiNbO}}_{3}:\mathrm{M}\mathrm{g}\mathrm{O},\mathrm{C}\mathrm{r}(0.1%)$ crystals are reported. Much lower inhomogeneous broadening of electronic transitions in near-stoichiometric crystals than in congruent ones allows us to obtain better resolution of the spectroscopic measurements. The results of measurements show that ${\mathrm{Cr}}^{3+}$ ions occupy mostly the same centers in near-stoichiometric crystals and in the congruent ones. In near-stoichiometric crystals doped only with chromium, the ${\mathrm{Cr}}^{3+}$ ions substitute the ${\mathrm{Li}}^{+}$ sites in the ${\mathrm{LiNbO}}_{3}$ host forming one major center (denoted as center \ensuremath{\gamma}) and two lower concentration centers, denoted by \ensuremath{\alpha} and \ensuremath{\beta}. In near-stoichiometric crystals doped additionally with a proper amount of MgO, the additional low-strength crystal-field center \ensuremath{\delta} dominates the spectrum. This center corresponds to the ${\mathrm{Cr}}^{3+}$ ions in ${\mathrm{Nb}}^{5+}$ sites. The hydrostatic pressure above 75 kbar transforms this center into the high-strength crystal-field center. Due to better resolution of measurements, it was possible to determine the splitting of the ${}^{2}E$ level for the \ensuremath{\delta} center, which is equal to about 17 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Estimated values of the energy difference between the position of the ${}^{4}{T}_{2}$ and the ${}^{2}E$ levels for centers \ensuremath{\beta}, \ensuremath{\gamma}, and \ensuremath{\delta} in these near-stoichiometric ${\mathrm{LiNbO}}_{3}$ crystals are equal to -20, -242, and more than -1000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively.

This work reports the energy levels of chromium ions for different sites in congruent LiNbO3 crystals doped with ZnO. Optical absorption and luminescence (fluorescence and excitation) experimental techniques were used. For crystals with Zn2+ concentration less than 4.7%, optical signals correspond to Cr3+ ions substituting for Li+ ions; for higher Zn2+ concentration, signals show two components coming from different sites for active Cr3+ ions. It is accepted that these two components can be assigned to Cr3+ ions substituting for Li+ ions and Cr3+ ions substituting for Nb+ ions. The energy levels of the former site are reported for the first time in this work; the 4T2 and 4T1 levels lie 13?421?cm-1 and 19?550?cm-1 above the 4A2 ground level, respectively. Analysis of the excitation spectra was the key result for achieving the resolution of the new site.

Sudden and transient decreases of ozone column values were detected over the midlatitudes of South America during the springs of 1996 and 1997 by a network of ground instruments. These low-ozone events were accompanied by air temperature decreases in the lower stratosphere. The analysis of the events suggests a connection with the springtime Antarctic ozone hole, Three-dimensional isentropic back trajectories showed that vertical motion also contributed to the decreases of total ozone observed at the midlatitudes of South America.

During 1996, 1997, and 1998, several total ozone depletion events were detected over Madrid (40°N, 3°W) in late spring and in winter. A broadband interferential filter instrument (UV-300) centered at 298 nm was used to monitor the solar UV-B irradiance on the ground, registering UV-B levels as high as 17.1 μW/cm 2 (May 2, 1997, solar zenith angle SZA = 35°, O 3 = 268 Dobson units). The data obtained have been compared to that measured with two commercial biometers (PMA2102 and Solar Light 501A), showing that the evolution of UV-B due to total ozone changes are better followed with broadband instruments with effective windows centered at lower wavelength than with instruments responding to the erythemal action spectra. The development of simple, portable, and reliable instruments for the analysis of solar UV-B phenomena at the high-energy side of that band is advocated.

The shape and size of nanostructures formed by the precipitation of europium ions in KBr single crystal has been analyzed using optical absorption (OA), luminescence (LM) and atomic force microscopy (AFM). This work reports a relationship between the emission band peak (EBP) and the nanostructure size. The shift of the t2g absorption band towards longer wavelengths and a larger 10 Dq value reveals the growth of the nanostructures (NS). The AFM images show that the precipitates have circular shape and ∼ 20 to 400 nm diameter according to the position of EBP at around 428–433 nm. The AFM technique allowed us to relate the shape and size of the nanostructures with our previous research by OA and LM. To our knowledge, the topography of europium ions forming NS of different diameter is observed for first time.

The EuBr 2 nanostructures self-assembled (SA) in ring form are presented in this letter. The atomic force microscopy (AFM) images show rings of ~ 190 to 2500 nm diameter formed on [100] surface. The sample optical response with rings displays an emission band peak (EBP) at 435 nm. The europium absorption bands reveal structural changes and shift toward infrared wavelength. The emission bandwidth of EuBr 2 nanostructures in ring form is narrower than dispersed nanostructures of similar size. To our knowledge, is the first time that there is clear evidence of the arrangement in ring form in the KBr:Eu 2+ crystal samples.

This work reports on the use of laser as an efficient tool in environmental studies. Application of LIDAR (LIght Detection And Ranging) and LIDAR-DIAL (DIfferential Absorption Lidar) technologies in the ultraviolet range related with the tropospheric ozone and sulphur dioxide profiles in the surroundings of Madrid are given. Sulphur dioxide measurements performed in a coal-burning power plant are also presented. Finally, the LIDAR technique is applied to study the cloud's density and its temporal evolution.

The results of high-pressure studies of ${\mathrm{LiNbO}}_{3}:\mathrm{C}\mathrm{r}(0.2%),$ Mg crystals doped with two concentrations of magnesium (2% and 5.5%) are reported. The results reveal information about the electronic structure of different ${\mathrm{Cr}}^{3+}$ centers in lithium niobate crystals. There are three major ${\mathrm{Cr}}^{3+}$ centers (denoted by \ensuremath{\alpha}, \ensuremath{\beta}, and \ensuremath{\gamma}) in the crystal with 2% of magnesium. These centers correspond to ${\mathrm{Cr}}^{3+}$ ions in ${\mathrm{Li}}^{+}$ sites with different crystal field. At ambient pressure the \ensuremath{\alpha} center experiences strong crystal field and the \ensuremath{\beta} and \ensuremath{\gamma} centers are the intermediate crystal-field centers. The energy differences between the ${}^{4}{T}_{2}$ and ${}^{2}E$ levels are positive and negative for the \ensuremath{\beta} and the \ensuremath{\gamma} centers, respectively. Additional broadband luminescence observed in the sample with 5.5% of magnesium even at pressure of almost 100 kbar testifies that another very weak crystal-field center exists in this crystal. This center (denoted by \ensuremath{\delta}) correponds to ${\mathrm{Cr}}^{3+}$ ions located in ${\mathrm{Nb}}^{5+}$ sites. The R lines of the ${\mathrm{Cr}}^{3+}$ centers exhibit very large redshift with pressure of about 3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$/kbar.

The time evolution of infrared stimulated luminescence has been systematically investigated in potassium feldspar and albite minerals subjected to different doses of gamma irradiation (ranging from 0 to 8 kGy). Experimental results reveal a 300-600 nm signal which is greatly intensified for irradiated samples, following the same irradiation dose dependence observed in thermoluminescence. The luminescence intensity disappears after few seconds of illumination, following a time evolution which is proportional to (1 + Bt) -P . The influence of both B and P factors upon the irradiation dose has been systematically analysed.

The α-Al 2 O 3 :C system has been studied as an outdoor ultraviolet B solar dosemeter with no previous ionising radiation pre-treatment. The thermoluminescence signal fades when illuminated with solar ultraviolet A and visible light. An ultraviolet bandpass filter and an α-Al 2 O 3 :C sample were used to measure solar ultraviolet B dose. The hourly distribution of the solar ultraviolet dose has been used to characterise independently the system response function to solar ultraviolet radiation.