<p>Similar to thermodynamic phase transitions in matter, readily apparent changes in optical response arise in the transition from isotropic to anisotropic optical phases. Treating the anisotropy of the dielectric permittivity as a control parameter, which changes continuously from zero to a nonzero finite value at the transition, in this work we describe the resulting effect on light propagation. </p> <p><br></p> <p>We begin by investigating a simple case of the manifestations of such optical transition in lossy media. In the presence of loss, isotropic materials do not support Brewster phenomenon, however, if one changes the anisotropy continuously, the exact zero in the reflection at the Brewster incidence angle is recovered. Next, in the case of uniaxial anisotropy, we uncover dramatic changes in far-field thermal radiation induced by the transitions between metal, dielectric, and hyperbolic optical regimes that can be observed in the same material. We demonstrate that continuous evolution between different ''phases'' in the electromagnetic response imprints a characteristic signature in the far-field thermal emission. Finally, we show that the evolution of the optical anisotropy from uniaxial to biaxial symmetry brings qualitatively new optical modes which are different from the conventional propagating and evanescent fields. These emergent ''ghost'' waves offer a unique way to control mode interactions in optical systems. Our work uncovers the connection between the macroscopic properties of the optical materials and the transitions between different regimes of the electromagnetic response in these media. At last, we propose a range of potential applications of the resulting phenomena, from perfect absorption in lossy media to thermal radiation and optical sensing.</p>

Anderson localization occurs when an otherwise conductive solid becomes insulatingdue to a sufficiently large degree of disorder in the medium. The electron band energy(as a function of disorder) at which this transition between extended and localizedelectron states occur is called the mobility edge (ME) and is energy-dependent only in3-dimensional systems. In lower dimensional systems, energy-independent ME (allstates localized or all extended) has been demonstrated by replacing disorder withquasi-periodic potential. However, recent theoretical findings indicate that neitherdisorder nor quasi-periodic potential is necessary for a material to exhibit electronlocalization and existence of energy-dependent pseudo ME at finite system size.In this thesis work, we use light in coupled silicon nitride waveguides to simulatesingle-particle transport of a solid-state medium and investigate the coexistence ofdelocalized and localized states in disorder-free photonic lattices of finite systemsize. This was achieved by implementing a simulated linearly increasing electricpotential on even-numbered sites by varying the refractive index of the wave guide(ch. 3). Through our experimental setup, we successfully achieved a coexistence oflocalized and delocalized states, where the degree of localization varies depending onthe strength of the applied electric field.The findings have implications for the field of quantum technology, whereunderstanding and controlling quantum states is crucial. The ability to achievelocalization in the absence of disorder opens new possibilities for designing andengineering photonic devices for quantum information processing tasks. / Anderson-lokalisering uppstår när ett annars ledande fast material blir isolerande pågrund av en tillräckligt stor grad av oordning i mediet. Elektronbandsenergin (som enfunktion av oordning) vid vilken denna övergång mellan förlängda och lokaliseradeelektrontillstånd sker kallas mobilitetskanten (ME) och är energiberoende endasti 3-dimensionella system. I lägre dimensionella system har energioberoende ME(alla tillstånd lokaliserade eller alla förlängda) påvisats genom att ersätta oordningmed kvasi-periodisk spänning. Nya teoretiska fynd indikerar dock att varkenoordning eller kvasi-periodisk spänning är nödvändig för att ett material ska uppvisaelektronlokalisering och förekomsten av energiberoende pseudo-ME för system avfinita storlekar.I detta examensarbete använder vi ljus i kopplade vågledare av kiselnitrid föratt simulera transport av en partikel i ett fast tillståndsmedium och undersökersamexistensen av icke-lokaliserade och lokaliserade tillstånd i finita system utanoordning med fotoniska gitter. Detta uppnåddes genom att implementera ensimulerad linjärt ökande elektrisk potential på varje jämnt numrerat gitterläge platsgenom att öka vågledarbredderna och noll elektrisk spänning på varje udda. Genomvårt experimentella upplägg lyckades vi uppnå lokaliserade och förlängda tillstånd, därgraden av lokaliseringen varierade beroende på styrkan av det tillämpade elektriskafältet.Fynden har implikationer för kvantteknologi, där förståelse och kontroll avkvanttillstånd är avgörande. Förmågan att uppnå lokalisering i frånvaro avoordning öppnar nya möjligheter för att designa och konstruera fotoniska enheter förkvantinformationsprocesser.

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[pt] Embora a grande maioria dos materiais dilate quando aquecida e contraia quando resfriada, existe uma classe de materiais que se contrai, ou não muda de dimensões, ao aquecida, apresentando um coeficiente de expansão térmico negativo (ETN) ou próximo à zero (ETZ), respectivamente. A possibilidade de reduzir significativamente o coeficiente de expansão térmica, e ao mesmo tempo, incrementar suas propriedades físicas tem sido a principal força motriz na busca por fases cristalinas dentro da família A(2)M(3)O(12) e suas subfamílias. Tendo isso em vista, a proposta deste estudo foi sintetizar dois sistemas novos, ZrMg(1- x)Zn(x)Mo(3)O(12) (x=0,1; 0,3; 0,35; 0,4) e ZrMg(1-x)Ni(x)Mo(3)O(12) (x=0,05; 0,1; 0,15; 0,2), para tentar reduzir o coeficiente de expansão térmica da fase mãe, a ZrMgMo(3)O(12). O limite de solubilidade de Zn(2+) e Ni(2+) no sistema ZrMgMo(3)O(12) se encontra no intervalo de 0,35 menor ou igual à x menor ou igual à 0,4 e 0,1 menor ou igual à x menor ou igual à 0,2, respectivamente. O menor coeficiente de expansão térmica (alfa l =2,82x10(-7)K (-1)) foi obtido para a composição x=0,1 no sistema ZrMg(1-x)Zn(x)Mo(3)O(12) na faixa de temperatura de 213 K a 298 K. Neste sistema, a transição de fase de monoclínica para ortorrômbica foi observada, ocorrendo abaixo da temperatura ambiente para todas as composições de x=0,1 a x=0,4. Esta temperatura de transição aumenta conforme aumenta a composição de Zn(2+). As análises de termogravimetria indicaram que as fases dos dois sistemas não são higroscópicas. Aplicando a equação de Kubelka-Munk, e considerando uma transição indireta para o ZrMg(1-x)ZnxMo3O12, concluiu-se que não existem diferenças significativas na energia de banda proibida das fases analisadas. No entanto, para uma transição indireta para o ZrMg(1-x)Ni(x)Mo(3)O(12) existe um decréscimo da energia da banda de energia, conforme o conteúdo de Ni(2+) aumenta na composição, além do surgimento da absorção no espectro visível devido à transição d-d. Por fim, os resultados deste estudo mostraram que é possível obter um material cerâmico, dentro dos sistemas estudados, que apresente um comportamento de expansão térmica próxima à zero. / [en] Although the vast majority of materials dilates when heated and contract when cooled, there is a class of materials that contracts or does not change their dimensions when heated, presenting a negative thermal expansion coefficient (NTE) or close to zero (ZTE), respectively. The possibility of reducing the coefficient of thermal expansion while increasing its physical properties has been the main driving force in the search for crystalline phases within the A(2)M(3)O(12) family and its subfamilies. In the present study, we propose to synthesize two new systems, ZrMg(1-x)Zn(x)Mo(3)O(12) (x = 0.1, 0.3, 0.35, 0.4) and ZrMg(1-x)Ni(x)Mo(3)O(12) (x = 0.05; 0.1, 0.15, 0.2), to try to reduce the coefficient of thermal expansion of the ZrMgMo(3)O(12) phase. The solubility limit of Zn(2+) and Ni(2+) in the ZrMgMo(3)O(12) system is in the range of 0.35 less than or equal to x less than or equal to 0.4 and 0.1 less than or equal to x less than or equal to 0.2, respectively. The lowest coefficient of thermal expansion (alfa l=2.82x10(-7)K (-1)) was obtained for the composition x = 0.1 in the ZrMg(1-x)Zn(x)Mo(3)O(12)system in the temperature range of 213 K to 298 K. In this system, the phase transition from monoclinic to orthorhombic was observed, occurring below the room temperature for all compositions from x = 0.1 to x = 0.4. This transition temperature increases as the Zn(2+) composition increases. Analyzes of thermogravimetry indicated that the phases of the two systems are not hygroscopic. Applying the Kubelka-Munk equation, and considering an indirect transition to ZrMg(1-x)Zn(x)Mo(3)O(12), it was concluded that there are no significant differences in the band gap energy of the analyzed phases. However, for an indirect transition to ZrMg1-xNixMo3O12 there is a decrease in energy of the band energy, as Ni2+ content increases in composition, in addition to the appearance of absorption in the visible spectrum due to d-d transition. Finally, the results of this study showed that it is possible to obtain a ceramic material, within the systems studied, that presents a thermal expansion behavior close to zero.

[pt] O principal objetivo deste trabalho foi estudar o sistema Al2-xGaxW3O12 (x = 0,2; 0,4; 0,5; 0,6; 0,7; 0,8; 1; 2) visando compreender os efeitos da substituição parcial de Al3+ (r = 0,67 Angstrom) por Ga3+ (r = 0,76 Angstrom) em relação ao coeficiente de expansão térmica da fase Al2W3O12. Foi determinado que o limite de solubilidade de Ga3+ no sistema é x = 0,5, as composições x maior ou igual 0,6 evidenciaram, por difração de raios-X (DRX), a presença de WO3 como fase secundária. Os difratogramas das composições 0,2 menor ou igual x menor ou igual 0,5, a temperatura ambiente, apresentaram exclusivamente linhas características do sistema monoclínico (P21/a). A transição para a fase ortorrômbica (Pbcn), foi evidenciada por DRX in situ e dilatometria e ocorre abaixo de 100 C em todos os casos. A temperatura de transição de fase, determinada por dilatometria, aumentou conforme foi aumentada a incorporação de Ga3+ na estrutura cristalina. A análise termogravimétrica das composições monofásicas revelou que essas fases não são higroscópicas. Embora Al1,5Ga0,5W3O12, seja a composição monofásica com maior teor de Ga, a fase Al1.6Ga0.4W3O12 foi a que apresentou o menor coeficiente de expansão térmica linear, alfa L= 1.14 K -1, uma redução de 25 por cento quando comparado ao coeficiente linear de expansão da fase Al2W3O12. O refinamento pelo método de Rietveld do padrão de difração de raios-X obtido a 100 C da Al1.6Ga0.4W3O12 ortorrômbica, confirmou que o Ga3+ substituiu o Al3+ na proporção descrita pela fórmula química nominal e evidenciou que as distorções poliédricas, Al(Ga)O6 e WO4, foram maiores do que as observadas em fases desta família. A espectroscopia de Raman corroborou as análises de DRX quanto ao limite de solubilidade, porém, evidenciando que quantidades mínimas, indetectáveis por DRX, de Al2O3 e WO3 podem estar presentes nas composições x menor ou igual 0,5, quando a síntese é realizada pelo método de reação no estado sólido. Os gráficos de Kubelka-Munk do sistema Al2- xGaxW3O12 indicaram que a substituição parcial de parcial de Ga3+ por Al3+ aumenta o intervalo de banda em x menor ou igual 0,4, no entanto, foi observada uma saliência de absorção dentro da região do visível presente em todas as amostras, interpretada como uma conseqüência da presença de WO3 monoclínica, observada na espectroscopia Raman. A síntese da fase Ga2W3O12, não foi bem sucedida, embora a entalpia de formação deste composto, calculada por meio da equação generalizada de Kapustinskii e pelo ciclo de Born-Haber, seja fortemente exotérmica, ΔHF= −10149,15 Kj. mol -1. / [en] The aim of this work was to study the Al2-xGaxW3O12 system (x = 0.2, 0.4, 0.5, 0.6, 0.7, 0.8, 2) in order to investigate the relationship between the partial replacement of Al3+ (r = 67 Angstrom) by Ga3+ (r = 0.76 Angstrom) and the coefficient of thermal expansion on the Al2W3O12 phase. It was determined as limit of solubility of Ga3+ in Al2-xGaxW3O12 the sample 𝑥 = 0.5, once it was identified in the diffraction patter WO3 as a secondary phase in 𝑥 bigger or equal 0.6. Unlike Al2W3O12 which is orthorhombic (Pbcn) at room temperature, the phases 0.2 less or equal 𝑥 less or equal 0.5 in the Al2- xGaxW3O12 appeared, at room temperature, in the monoclinic system (P21/a). The transition to orthorhombic phase (Pbcn), determined by XRPD in situ and dilatometry, was observed below 100 C for all compositions. The phase transition temperature increases as the Ga3+ content was increased in the crystalline structure. The thermogravimetric analysis of the monophasic samples showed that they were not hygroscopic. Although the monophasic composition with the highest Ga3+ content was Al1.5Ga0.5W3O12, the phase Al1.6Ga0.4W3O12 presented the lowest linear coefficient of thermal expansion, alpha l = 1.14 K -1, a reduction of 25 percent comparing with the linear coefficient of thermal expansion of the phase Al2W3O12. The Rietveld fit to the orthorhombic Pbcn space group, of the Al1.6Ga0.4W3O12 diffraction pattern taken at 100 C, confirms that Ga3+ was replaced by Al3+ in the same proportion described in the nominal chemical formula, and showed that its polyhedral distortion , Al(Ga)O6 and WO4, is in a higher amount than generally noticed for other phases in this crystal family. The Raman spectroscopy corroborated the analyzes regarding the solubility limit, although it showed that the compositions 𝑥 less or equal 0,5 could have a minimum quantities, undetectable by XRPD, of Al2O3 and WO3, when synthesized by the solid state reaction method. Kubelka-Munk graphics of Al2-xGaxW3O12 suggest that the partial replacement of Al3+ by Ga3+ increases the band gap in x less or equal 0,4, however, the absorption of Al2-xGaxW3O12 in the visible region increase, this behavior is apparently caused by the presence of WO3, as deduced by Raman spectroscopy. Attempts to synthesize Ga2W3O12 was not successful, although the enthalpy of formation of this compound, calculated by Generalized Kapustinskii equation and the Born-Haber cycle, presented a high exothermic value, ΔHF = −10149,15 Kj. mol -1.

Tesis por compendio / [ES] Los sesquicalcogenuros con estequiometria B2X3, con A = Al, Ga e In y X = S, Se y Te, han recibido gran atención a lo largo de los últimos años, en particular en como modificarlos para obtener nuevas estructuras con propiedades inéditas, permitiendo su uso en una amplia variedad de aplicaciones. Si bien vías como el uso de altas/bajas temperaturas o modificar la composición química han sido bastante explotadas para modificar y obtener nuevas estructuras, las altas presiones están ganando auge como una tercera vía para obtener nuevos materiales. El uso de altas presiones implica emplear celdas de yunques de diamantes, preparadas para alcanzar altas presiones, además de altas temperaturas. A estos dispositivos se acoplan multitud de técnicas experimentales, como espectroscópicas (Raman e IR), difracción y absorción de rayos X, medidas de absorción óptica, de resistividad, etc., con el fin de estudiar como la materia evoluciona en dichas condiciones extremas. Adicionalmente, los cálculos teóricos son empleados como apoyo a los resultados experimentales. Dentro de los trabajos existentes a altas presiones de esta familia de compuestos, estos han llegado a conclusiones incluso contradictorias, arrojando más dudas acerca su comportamiento bajo presión. De todos los integrantes de estos sesquicalcogenuros, Ga2S3, In2S3 y In2Se3, han sido los más estudiados bajo presión. En esta tesis se han evaluado los efectos de la alta presión en estos tres sesquicalcogenuros, haciendo uso de espectroscopia Raman y difracción de rayos X, siempre con el soporte de los cálculos teóricos, con el fin de aclarar los resultados publicados anteriormente. Fruto de estos trabajos, la presente tesis recoge los cuatro artículos publicados en revistas indexadas. Dichos artículos han dado luz al comportamiento bajo presión de estos compuestos, como caracterización de propiedades vibracionales y estructurales bajo presión, mecanismos de transición, transiciones de fase inducidas bajo presión, así como caracterizar dichas fases de alta presión. Con todo ello, estos trabajos pretenden no solo conocer fehacientemente el comportamiento bajo presión de estos tres sesquicalcogenuros, sino impulsar futuros trabajos en el resto de los compuestos de esta familia y en otros similares, como en compuestos ternario AB2X4 con estructura tipo espinela y vacantes ordenadas. / [CA] Els sesquicalcogenurs amb estequiometria B2X3, amb B = Al, Ga, i In i X = S, Se, i Te, han rebut una gran atenció al llarg dels darrers anys, en particular sobre com modificar-los per tal d'obtindre noves estructures amb propietats inédites, permetent el seu ús en una àmplia varietat d'aplicacions. Si bé l'ùs d'altes/baixes temperatures o modificar la composició química han segut prou explotades per a modificar i obtindre noves estructures, les altes pressions estan guanyant importància com una tercera via per a obtindrer nous materials. L'ús d'altes presions implica emprar cel·les d'encluses de diamants, preparades per a assolir altes presions, a més a més d'altes temperatures. A aquestos dispositius s'acoblen multitud de tècniques experimentals, com ara espectroscòpiques (Raman i IR), difracció i absorció de raigs X, mesures òptiques, de resistivitat, etc, amb la finalitat d'estudiar com la matèria evoluciona en aquestes condicions extremes. Adicionalment, els càlculs teòrics son emprats com a recolçament dels resultats experimentals. Dins dels treballs existents a altes presions a aquesta familia de compostos s'ha arribat a determinades conclusions algunes de les quals son contradictòries, el que ha sembrat moltes dubtes al voltant del seu comportament sota pressió. De tots els integrants d'aquestos sesquicalcogenurs, Ga2S3, In2S3 i In2Se3 han sigut els més estudiats sota pressió. En aquesta tesi doctoral s'han evaluat els efectes de les altes pressions a aquestos tres sesquicalcogenurs, fent ús de l'espectroscopia Raman i la difracció de raigs X, sempre amb el suport dels càlculs teòrics, amb el fi d'aclarir els resultats previament publicats. Fruit d'aquestos treballs, la present tesi doctoral recull els quatre articles publicats a revistes indexades. Aquestos articles han vessat llum sobre el comportament sota pressió d'aquestos compostos, com ara la caracterització de les seues propietats vibracionals i estructurals sota pressió, les transicions de fase induides sota pressió i els mecanismes d'eixes transicions, així com la caracterització de les seues fases d'alta pressió. Amb tot, aquestos treballs pretenen no només conèixer el comportament sota pressió d'aquestos tres sesquicalcogenurs, sino també impulsar futurs treballs a la resta de compostos d'aquesta familia i altres compostos rel·lacionats, com ara els compostos ternaris AB2X4 de tipus espinela i de vacants ordenades. / [EN] B2X3 sesquichalcogenides (A = Al, Ga and In, X = S, Se y Te) have received special attention along last years, with great emphasis in tailor them to attain new structures to novel properties, driving them in a huge number of applications. Although high/low temperature or varying chemical composition have been extensively used to modify and obtain new structures, high pressure is gaining relevance as an alternative way to synthetised new materials. To reach such pressures and additionally high/low temperatures, diamond anvil cells are used. Many experimental techniques can be coupled to these tools to study matter under extreme conditions (Raman and IR spectroscopy, X-ray diffraction and absorption, optical absorption, and resistivity measurements, among others). Additionally, computational simulations are used to give further support to the experimental results. Despite the several existing works devoted to the behaviour under pressure of this family, controversial results have been reported. The most studied of these sesquichalcogenides have been Ga2S3, In2S3 and In2Se3. The aim of this thesis is to revisit the pressure effects by means of Raman spectroscopy and X-ray diffraction, with the help of computational simulations, for the purpose of clarify the results published earlier. The current thesis contains the four articles published in indexed journals, resulting from the study of these three sesquichalcogenides. Such articles shed light to the pressure behavior of these compounds, their vibrational and structural properties under pressure, pressure-induced phase transitions and the mechanisms behind them and characterize such high-pressure phases. With these works, we pursue not only a depth understanding of the pressure behavior of these sesquichalcogenides, but boost future high-pressure works on the rest of the family and other similar compounds, as AB2X4 with spinel structure and ordered vacancies. / The authors thank the financial support from Spanish Research Agency (AEI) under projects MALTA Consolider Team network (RED2018-102612-T) and projects MAT2015- 710, MAT2016-75586-C4-2/3-P, FIS2017-83295-P, PID2019-106383GB-41/42/43, and PGC2018-097520-A-100, as well as from Generalitat Valenciana under Project PROMETEO/2018/123 (EFIMAT). / Gallego Parra, S. (2022). Experimental and Theoretical Study of B2X3 Sesquichalcogenides under Extreme Conditions [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/191502 / Compendio

Ce mémoire se penche sur les propriétés photophysiques de différents complexes métalliques, particulièrement leur changement de structure moléculaire, leur état fondamental ainsi que leur état émissif par spectroscopie Raman et de luminescence. Ces techniques spectroscopiques peuvent être utilisées pour détecter et suivre des changements, comme des transitions de phase structurales. Pour ce mémoire, quatre composés seront étudiés par spectroscopie. Le premier est un composé de nickel(II) ayant un ligand azoture pontant, dont l’agencement spatial change avec une diminution de la température, cette dernière causant un changement dans la susceptibilité magnétique du composé. Les spectres obtenus montrent bel et bien un changement abrupt dans le pic du cisaillement du ligand azoture aux environs de 210 K. Les mesures à pression variable montrent également un changement soudain, entre 1 bar et 1,1 kbar. Le deuxième composé contient deux centres de manganèse(II) ayant chacun une coordination inhabituelle de sept, ce qui n’est pas souvent observé. Il s’avère que la spectroscopie Raman n’est pas assez précise pour mesurer un changement dans ce composé. Tous les déplacements de maxima des bandes mesurés se retrouvaient dans un intervalle de 1 cm-1 ; la résolution de l’appareil limite la confirmation s’il y a bel et bien une transition de phase. Les spectres de luminescence ne montrent pas de changement évident non plus. Le troisième composé est un complexe de platine plan carré qui possède des interactions platine(II)-platine(II), et qui montre un spectre de luminescence. Des complexes similaires ont déjà été étudiés par d’autres membres du groupe de recherche; il s’agit donc de compléter la banque de résultats et de comparer avec les données connues. Les résultats ne montrent aucun déplacement dans la transition, mais un amincissement du pic, ce qui est comparable aux données antérieures. Le quatrième composé est également un complexe de platine(II) plan carré, qui montre un spectre de luminescence. Un composé similaire a déjà été étudié par un étudiant de notre groupe de recherche, en rapportant un changement soudain dans le pic de luminescence. Les spectres mesurés montrent un changement de structure, qui se fait de manière graduelle plutôt que soudainement. Ce mémoire est donc axé sur la spectroscopie Raman et de luminescence à températures et pression variables. Les spectres mesurés, notamment les déplacements des maxima, donnent de l’information sur des changements de structure, de l’état fondamental et de l’état émissif. / This master’s thesis is focused on the vibrational spectroscopy of coordination compounds displaying or undergoes spatial or structural changes as well as variations in their ground state and their emitting state, studied via Raman and luminescence spectroscopy. These spectroscopic techniques can be used to detect and observe various changes, such as phase transitions. In this thesis, four compounds will be studied. The first one is a nickel(II) compound featuring azide bridging ligand, whose spatial arrangement changes by lowering the temperature, causing also a change in the magnetic susceptibility of the compound. The spectra we measured show clearly a sudden change in the scissoring vibration for the azide, at temperatures around 210 K. The variable pressure data we collected also show a sudden change between 1 bar and 1,1 kbar. The second compound contains two manganese(II) centers, both having a coordination number of seven, which is rarely seen for this metal center. Raman spectroscopy isn’t accurate enough to detect any structural changes. Any changes of band maxima were under 1 cm-1, at the limits of the instrument’s resolution. Luminescence graphs do not show anything in particular. The third compound is a square planar platinum(II) complex showing platinum(II)-platinum(II) interactions, influencing its luminescence spectrum. Similar complexes have already been studied by other members of this research group; this research will therefore add results to those we already have, and will be compared to these results. The results don’t show any transition, but the peaks are narrowing at low temperature, a behaviour similar to other systems. The fourth compound is also a square planar platinum(II) complex showing a luminescence spectrum. A similar complex has already been studied by a student in this research group, reporting a sudden shift in the luminescence peak. The spectrum shows indeed a structural change, but this change is gradual instead of sudden. This thesis is focusing on variable temperature and pressure Raman and luminescence spectroscopy. The experimental spectra show information about structural change, fundamental state and emissive state.