Optical and structural characterisation of epitaxial nanoporous GaN grown by CVD

Producción CientíficaIn this paper we study the optical properties of nanoporous gallium nitride (GaN) epitaxial layers grown by chemical vapour deposition on non-porous GaN substrates, using photoluminescence, cathodoluminescence, and resonant Raman scattering, and correlate them with the structural characteristic of these films. We pay special attention to the analysis of the residual strain of the layers and the influence of the porosity in the light extraction. The nanoporous GaN epitaxial layers are under tensile strain, although the strain is progressively reduced as the deposition time and the thickness of the porous layer increases, becoming nearly strain free for a thickness of 1.7 μm. The analysis of the experimental data point to the existence of vacancy complexes as the main source of the tensile strain.Ministerio de Economía, Industria y Competitividad (Projects No. TEC2014-55948-R and MAT2016-75716-C2-1-R (AEI/FEDER, UE)Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. Projects VA293U13 and VA081U16)Comisión Interministerial de Ciencia y Tecnología (Proyect CICYT MAT2010-20441-C02)Ministerio de Economía, Industria y Competitividad (Projects No.ENE2014-56069-C4-4-R

New strategies invonving upconverting nanoparticles for determining moderate temperatures by luminescence thermometry

Producción CientíficaHere we analyze alternative luminescence thermometry techniques to FIR, such as intensity ratio luminescence thermometry between the emission arising from two electronic levels that are not necessarily thermally coupled, but that show different evolutions with temperature, and lifetime luminescence nanothermometry in (Ho,Tm,Yb):KLu(WO4)2 and (Er,Yb):NaY2F5O nanoparticles. (Ho,Tm,Yb):KLu(WO4)2 nanoparticles exhibited a maximum relative sensitivity of 0.61% K−1, similar to that achievable in Er-doped systems, which are the upconverting systems presenting the highest sensitivity. From another side, (Er,Yb):NaY2F5O nanocrystals show great potentiality as thermal sensors at the nanoscale for moderate temperatures due to the incorporation of additional non-radiative relaxation mechanisms that shorten the emission lifetime generated by the oxygen present in the structure when compared to (Er,Yb):NaYF4 nanoparticles exhibiting the highest upconversion efficiency. We used those nanoparticles for ex-vivo temperature determination by laser induced heating in chicken breast using lifetime-based thermometry. The results obtained indicate that these techniques might constitute alternatives to FIR with potential applications for the determination of moderate temperatures, with sensitivities comparable to those that can be achieved by FIR or even higher.Ministerio de Economía, Industria y Competitividad (Projects MAT2013-47395-C4-1-4-R, and TEC2014-55948-R)Catalan Government (Projects no. 2014SGR1358 and 2013FI_B 01032

Fully porous GaN p-n junction diodes fabricated by chemical vapor deposition

Producción CientíficaModern society is experiencing an ever-increasing demand for energy to power a vast array of electrical and mechanical devices. A significant amount of the energy consumed is used for lighting purposes. For instance, this demand is ~17% of the total energy consumed in the USA in 2011 [1]. Thus, any approach that can reduce energy consumption is important. In this context, the development of light emitting diodes (LEDs) incorporating at least one porous component, with improved light extraction efficiency, is being explored intensively [2]. However, up to now, only partially porous p-n junctions have been analyzed for this purpose.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA166A11-2 and VA293U1

Formation of polycrystalline TiO2 on the ablated surfaces of RbTiOPO4 single crystals by thermal annealing

We investigated the structural effects that ultrafast laser ablation has on RbTiOPO4 (RTP) single crystals. Raman scattering analysis revealed that a polycrystalline layer of RTP substituted the original material on the surface of the microstructured areas which is mainly responsible for the roughness observed in these structures. A later annealing treatment on the ablated samples resulted in the formation of a layer of sub-micrometric particles of TiO2 in a mixed form of anatase and rutile, due to the decomposition of the polycrystalline RTP layer. This allows the development of engineering strategies to fabricate RTP–TiO2 composites, with mixed properties, that might allow the combination of the non-linear optical properties of RTP and the light dispersive and electrical properties of TiO2.This work was supported by the EU Framework 7 under project FP7-SPA-2010-263044, by the Spanish Government under projects MAT2011-29255-C02-02, TEC2010-21574-C02-02, FIS2009-09522 and CSD2007-00013, by Generalitat de Catalunya under project 2009SGR235 and by Junta de Castilla y León under project SA086A12-2

Fully porous GaN p-n junctions fabricated by Chemical Vapor Deposition: a green technology towards more efficient LEDs

Producción CientíficaPorous GaN based LEDs produced by corrosion etching techniques demonstrated enhanced light extraction efficiency in the past. However, these fabrication techniques require further postgrown processing steps, which increase the price of the final system. In this paper, we review the process followed towards the fabrication of fully porous GaN p-n junctions directly during the growth step, using a sequential chemical vapor deposition (CVD) process to produce the different layers that form the p-n junction.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA302U13

Surface ablation of RbTiOPO4 by femtosecond laser

We report here the results obtained in surface ablation of RbTiOPO4 single crystals by femtosecond laser. We fabricated and characterized one-dimensional (1D) diffraction gratings with different lattice spacings of 15 and 20 μm, and with a sub-modulation of the period introduced in the later. The optical and electronic microscopy characterization and filling factor analysis of these diffraction gratings are reported. We also show that the roughness generated on the grooves by the ablation process can be improved by chemical etching.This work was partially funded by the European Commission under the Seventh Framework Program under Project Cleanspace FP7-SPACE-2010-1-GA-263044, supported by the Spanish Government under Projects PI09/90527, TEC2009-09551, AECID A/024560/09, FIS2009-09522, HOPE CSD2007-00007 and SAUUL CSD2007-00013 (Consolider-Ingenio 2010), by Catalan Government under Projects 2009SGR235 and 2009SGR549, by Junta de Castilla y León under Project GR27, and by the Research Center on Engineering of Materials and Systems (EMaS) of the URV. J.J.C. is supported by the Education and Science Ministry of Spain and European Social Fund under the Ramón y Cajal program, RYC2006-858. We also acknowledge support from the Centro de Laseres Pulsados, CLPU, Salamanca, Spain

Thermal and Optical Characterization of Undoped and Neodymium-Doped Y3ScAl4O12 Ceramics

Y3–3xNd3xSc1Al4O12 (x = 0, 0.01, and 0.02) ceramics were fabricated by sintering at high temperature under vacuum. Unit cell parameter refinement and chemical analysis have been performed. The morphological characterization shows micrograins with no visible defects. The thermal analysis of these ceramics is presented, by measuring the specific heat in the temperature range from 300 to 500 K. Their values at room temperature are in the range 0.81–0.90 J g1–K–1. The thermal conductivity has been determined by two methods: by the experimental measurement of the thermal diffusivity by the photopyroelectric method, and by spectroscopy, evaluating the thermal load. The thermal conductivities are in the range 9.7–6.5 W K–1 m–1 in the temperature interval from 300 to 500 K. The thermooptic coefficients were measured at 632 nm by the dark mode method using a prism coupler, and the obtained values are in the range 12.8–13.3 × 10–6 K–1. The nonlinear refractive index values at 795 nm have been evaluated to calibrate the nonlinear optical response of these materials.This work is supported by the Spanish Government under projects MAT2011-29255-C02-01-02, MAT2013-47395-C4-4-R, and the Catalan Government under project 2014SGR1358. It was also funded by the European Commission under the Seventh Framework Programme, project Cleanspace, FP7-SPACE-2010-1-GA No. 263044

Case-control study for colorectal cancer genetic susceptibility in EPICOLON: previously identified variants and mucins

<p>Abstract</p> <p>Background</p> <p>Colorectal cancer (CRC) is the second leading cause of cancer death in developed countries. Familial aggregation in CRC is also important outside syndromic forms and, in this case, a polygenic model with several common low-penetrance alleles contributing to CRC genetic predisposition could be hypothesized. Mucins and GALNTs (N-acetylgalactosaminyltransferase) are interesting candidates for CRC genetic susceptibility and have not been previously evaluated. We present results for ten genetic variants linked to CRC risk in previous studies (previously identified category) and 18 selected variants from the mucin gene family in a case-control association study from the Spanish EPICOLON consortium.</p> <p>Methods</p> <p>CRC cases and matched controls were from EPICOLON, a prospective, multicenter, nationwide Spanish initiative, comprised of two independent stages. Stage 1 corresponded to 515 CRC cases and 515 controls, whereas stage 2 consisted of 901 CRC cases and 909 controls. Also, an independent cohort of 549 CRC cases and 599 controls outside EPICOLON was available for additional replication. Genotyping was performed for ten previously identified SNPs in <it>ADH1C</it>, <it>APC</it>, <it>CCDN1</it>, <it>IL6</it>, <it>IL8</it>, <it>IRS1</it>, <it>MTHFR</it>, <it>PPARG</it>, <it>VDR </it>and <it>ARL11</it>, and 18 selected variants in the mucin gene family.</p> <p>Results</p> <p>None of the 28 SNPs analyzed in our study was found to be associated with CRC risk. Although four SNPs were significant with a <it>P</it>-value < 0.05 in EPICOLON stage 1 [rs698 in <it>ADH1C </it>(OR = 1.63, 95% CI = 1.06-2.50, <it>P</it>-value = 0.02, recessive), rs1800795 in <it>IL6 </it>(OR = 1.62, 95% CI = 1.10-2.37, <it>P</it>-value = 0.01, recessive), rs3803185 in <it>ARL11 </it>(OR = 1.58, 95% CI = 1.17-2.15, <it>P</it>-value = 0.007, codominant), and rs2102302 in <it>GALNTL2 </it>(OR = 1.20, 95% CI = 1.00-1.44, <it>P</it>-value = 0.04, log-additive 0, 1, 2 alleles], only rs3803185 achieved statistical significance in EPICOLON stage 2 (OR = 1.34, 95% CI = 1.06-1.69, <it>P</it>-value = 0.01, recessive). In the joint analysis for both stages, results were only significant for rs3803185 (OR = 1.12, 95% CI = 1.00-1.25, <it>P</it>-value = 0.04, log-additive 0, 1, 2 alleles) and borderline significant for rs698 and rs2102302. The rs3803185 variant was not significantly associated with CRC risk in an external cohort (MCC-Spain), but it still showed some borderline significance in the pooled analysis of both cohorts (OR = 1.08, 95% CI = 0.98-1.18, <it>P</it>-value = 0.09, log-additive 0, 1, 2 alleles).</p> <p>Conclusions</p> <p><it>ARL11</it>, <it>ADH1C</it>, <it>GALNTL2 </it>and <it>IL6 </it>genetic variants may have an effect on CRC risk. Further validation and meta-analyses should be undertaken in larger CRC studies.</p

Growth and characterisation of RbTiOPO4. A new self-frequency doubling crystal.

Growth and Characterisation of RbTiOPO4: (Nb,Ln). A New frequency Doubling Crystal.Joan Josep Carvajal MartíFísica I Cristal·lografia de MaterialsDepartament de Química Física i InorgànicaUniversitat Rovira i VirgiliResumDes que Schawlow i Townes varen publicar que el principi del M.A.S.E.R. es podia estendre a la part òptica de l'espectre electromagnètic, i un any i mig més tard Mainman demostrava el primer làser, la importància d'aquest, tant per la societat, s'ha anat incrementant. Avui, els làser formen part de la nostra vida quotidiana.Actualment, l'interès en desenvolupar nous làser per l'emmagatzematge de dades i altres aplicacions s'ha incrementat gràcies a la recerca en làsers de semiconductors de banda ampla, generació d'harmònics per phase-matching en cristalls d'òptica no-lineal, quasi-phase-matching en cristalls, fibres òptiques i altres guies d'ona, i làsers d'up-conversion en cristalls i fibres. Els làsers d'estat sòlid compactes són útils en una amplia gamma d'aplicacions, com l'emmagatzematge de dades en alta densitat, projecció d'imatges en color, impressió làser, medicina, substitució dels làsers d'Ar, biofluorescència, comunicacions submarines, litografia estereogràfica, teràpia fotodinàmica.... Aquests nous làsers es poden aconseguir per processos d'up-conversion, que emeten a una longitud d'ona més curta que la de bombeig. Una alternativa interessant és la conversió de freqüència per processos d'òptica no-lineal. Els cristalls autodobladors de freqüència són cristalls d'òptica no-lineal amb posicions estructurals ocupades per ions actius làser, preferiblement Yb3+, que poden combinar l'emissió làser al voltant d'1 um amb la generació de segon harmònic de la matriu per produir emissió verda per autodoblatge de freqüència. L'objectiu principal d'aquesta Tesi Doctoral ha estat l'obtenció de cristalls de la família del KTiOPO4, coneguts per les seves excel·lents propietats d'òptica no-lineal, dopats amb ions lantànids (Ln). Hem mostrat que és poden obtenir cristalls de RbTiOPO4: (Nb,Ln) de bona qualitat òptica per la tècnica top-seeded solution growth amb un refredament lent de la solució, de mida útil per posteriors caracteritzacions i aplicacions, i amb una concentració de Ln similar a l'obtinguda en altres matrius làser i un codopant, que afavoreix l'entrada d'ions lantànids en la matriu a l'hora que fa de compensador de càrrega, com és el niobi (Nb5+). El refinament de l'estructura ha demostrat que tant el Nb com els lantànids substitueixen únicament al Ti en la xarxa cristal·lina. Mentre que el Nb5+ només substitueix al Ti en la posició Ti (1), degut a un efecte electrostàtic més que no pas estèric, els ions lantànids substitueixen als àtoms de Ti en els octàedres Ti(1)O6 i Ti(2)O6 amb la mateixa probabilitat. Un altre aspecte a destacar és l'habilitat del material per tal d'autocompensar-se elèctricament de la substitució pentavalent del Nb5+ a través de la creació de vacants de rubidi. L'evolució d'aquesta estructura amb la temperatura mostra com el canvi de fase de l'estructura ferroelèctrica depèn del contingut en Nb i que la substitució de Ti4+ pel parell (Nb5+-Rb+) estabilitza la fase d'alta temperatura d'aquests materials. La caracterització òptica de la matriu cristal·lina ha mostrat que l'increment de la birefringència de cristall possibilita l'increment de l'interval de doblatge de longituds d'ona que es pot efectuar amb aquests cristalls. L'àmplia finestra òptica juntament amb l'eficiència de conversió a segon harmònic mostren que aquesta és una bona matriu per hostatjar ions lantànids. Aquestes excel·lents propietats es mantenen fins i tot quan els ions lantànids es troben al cristall. Finalment, la caracterització espectrocòpia del ions actius ha mostrat que es pot obtenir una emissió eficient per part dels dos ions. Er3+b3+ la banda d'emissió ampla i l'elevada vida mitja obtingudes obren la possibilitat d'utilitzar aquests cristalls en aplicacions sintonitzables i de subpicosegons. Totes aquestes observacions ens duen a concloure que amb aquests cristalls es pot obtenir un nou material autodoblador de freqüència.Paraules clau: Òptica No-Lineal, Generació de Segon Harmònic, Autodoblatge de freqüència, Top-Seeded Solution Growth, KTiOPO4, RbTiOPO4, Difacció de raig X, Difracció de neutrons de pols.Growth and characterisation of RbTiOPO4: (Nb, Ln). A New Self-frequencyDoubling Crystal.Joan Josep Carvajal MartíFísica I Cristal·lografia de MaterialsDepartament de Química Física i InorgànicaUniversitat Rovira i VirgiliAbstract Since Schawlow and tornes reported that it should be possible to extend the maser principle to the optical frequency part of the spectrum and only one and a half years later. Mainman demonstrated the first laser, the importance of lasers to both science and society has increased dramatically. Today lasers are part of everyday life. Actually, the interest in developing compact blue and green laser sources for data storage and display applications has increased due to promising investigations into wide-gap semiconductor diode lasers, harmonic generation by phase matching in non-linear crystals, quasi-phase-matching in bulk, optical fibres and other waveguides, and up conversion lasers in crystals and fibers. Compact all-solid-state laser sources are needed for a variety of applications such as colour projection, high-density optical data storage, laser printing, medicine, Argon-laser replacement, biofluorescence, underwater communications, stereo lithography, photodynamic therapy. A class of these sources is achieved with up-conversion lasers emitting at a shorter wavelength than the pump source. An alternative to up-conversion lasing is frequency conversion by non-linear optical processes such as frequency doubling and sum-frequency mixing. Self-frequency doubling crystals are non-linear optical crystals with suitable sites for laser-active ions, preferably Yb3+, which can combine the 1 um laser emission of the active ions with the second harmonic generation properties of the host to produce green-laser radiation by self-frequency doubling. The aim of this Thesis was to investigate how to obtain crystals of the KTiOPO4 family, which are known for their excellent non-linear optical properties, doped with lanthanide (Ln) ions. This Thesis shows that high-optical quality RbTiOPO4(Nb,Ln) single crystals can be obtained by the top-seeded solution growth technique together with a slow-cooling of the solution, with large volumes useable for later characterisations and technical applications, and with a similar Ln concentration to that of other laser matrices. We introduced a codopant that enhances the concentration of Ln in the crystals at the same time that compensate the electrical charge of the framework. The refinement of the structure demonstrates that Nb and Ln ions only go to the Ti positions. Then, while Nb only substitutes Ti in the Ti(1) crystallographic position due to an electrostatic effect that an steric one, the lanthanide ion substitute Ti in both Ti(1)O6 octahedra with the same probability. Another interesting aspect to notice is the ability of the material to self-compensate the excess of electrical charge introduced by the pentavalent substitution of Nb5+ by the creation of Rubidium vacancies. The evolution of this structure with temperature showed that the transition from the ferroelectric phase to the paraelectric phase depended on the Nb content in the crystals, and that substituting Ti4+ by the (Nb5+-Rb+) pair stabilises the cubic high-temperature phase of these crystals. The optical characterisation of the host showed that the increase of birefringence in the crystals containing Nb causes the limit fundamental wavelength, at which second harmonic generation can be obtained, to shift to lower wavelengths. The large transparency window of the material and the conversion efficiency to second harmonic generation showed that this crystal is a good matrix to host lanthanide ions. These excellent properties are maintained even when the lanthanide ions is hosted in the crystalline matrix. Finally, the spectroscopic characterisation of the active ions showed that efficient emission can be obtained for Er3+ at 1.5 and 1.0 um, respectively. In the case of Yb3+ the broad emission band and the large lifetime obtained open up the possibility to use these crystals in tunable and subpicosecond applications. All these observations seems to converge in the conclusion that with these crystals a new self-frequency doubling material can be obtained.Keywords: Non-linear Optics, Second Harmonic Generation, Self-frequency Doubling, Top-Seeded Solution Growth, KTiOPO4, RbTiOPO4, X-ray Diffraction, Powder Neutron Diffraction

Novel low-cost, compact and fast signal processing sensor for ratiometric luminescent nanothermometry

DOI: 10.1016/j.sna.2016.08.031 URL: http://www.sciencedirect.com/science/article/pii/S0924424716304162 Filiació URV: SIWe developed a new compact, low-cost and non-invasive temperature sensor based on a ratiometric luminescence technique. The setup included a commercial digital color sensor, which collects simultaneously signals in the blue, green and red regions of the electromagnetic spectrum, usually used to assess the quality of computer screens and used for the first time here as a sensor for luminescent thermometry, coupled to an optical system that focuses an excitation laser beam onto luminescent nanoparticles emitting at least in two of these electromagnetic regions, which simplifies considerably the design, alignment and measurement procedures of setups used up to now for the same purpose. The same optical system collects the emission arising from the luminescent nanoparticles and directs it towards the digital color sensor through a dichroic mirror. We probed the potentiality of this setup for luminescence thermometry in the biological range of temperatures using Er,Yb:NaYF4, and up to 673 K for microelectronic applications using Tm,Yb:GdVO4 up-converting nanoparticles. The thermal sensitivity obtained in both cases is similar to that previously reported for the same kinds of nanoparticles using conventional systems. This validates our setup for temperature measurements. Also, we developed new flexible and transparent polymer composites, in which we embedded upconversion luminescent nanoparticles of Er,Yb:NaYF4 in PDMS, a standard polymer for microfluidic devices used for biomedical studies, which allow fabricating thermometric microfluidic chips in which temperature can be determined using our setup. The thermal sensitivity for these composites is slightly smaller than that of the bare nanoparticles, bur still allowing for precise and fast temperature measurements