Dissertations / Theses on the topic 'Nonlinear optical properties (NLO)'

Transition metal compounds, especially the oxides, containing dn (0 ≤ n ≤ 10) electronic configuration, constitute the backbone of solid state/materials chemistry aimed at realization of novel materials properties of technological importance. Some of the significant materials properties of current interest are spin-polarized metallic ferromagnetism, negative thermal expansion, second harmonic nonlinear optical (NLO) susceptibility, fast ionic and mixed electronic/ionic conductivity for application in solid state batteries, and last but not the least, high-temperature superconductivity. Typical examples for each one of these properties could be found among transition metal oxides. Thus, alkaline-earth metal (A) substituted rare-earth (Ln) manganites, Lnı.xAxMnΟ3, are currently important examples for spin-polarized magnetotransport, ZrV2O7 and ZrW2O8 for negative thermal expansion coefficient, KTiOPO4 and LiNbO3 for second harmonic NLO susceptibility, (Li, La) TiO3 and LiMn2O4 for fast-ionic and mixed electronic/ionic conductivity respectively, and the whole host of cuprates typified by YBa2Cu3O7 for high Tc superconductivity. Solid state chemists constantly endeavour to obtain structure-property relations of solids so as to be able to design better materials towards desired properties. Synthesis coupled with characterization of structure and measurement of relevant properties is a common strategy that chemists adopt for this task. The work described in this thesis is based on such a broad-based chemists' approach towards understanding and realization of novel materials properties among the family of metal oxides. A search for metallic ferro/ferrimagnetism among the transition metal perovskite oxides, metallicity and possibility of superconductivity among transition-metal substituted cuprates and second order NLO susceptibility among metal oxides containing d° cations such as Ti(IV), V(V) and Nb(V) - constitute the main focus of the present thesis. New synthetic strategies that combine the conventional ceramic approach with the chemistry-based 'soft1 methods have been employed wherever possible to prepare the materials. The structures and electronic properties of the new materials have been probed by state-of-the art techniques that include powder X-ray diffraction (XRD) together with Rietveld refinement, electron diffraction, thermogravimetry, measurement of magnetic susceptibility (including magnetoresistance), Mossbauer spectroscopy and SHG response (towards 1064 nm laser radiation), besides conventional analytical techniques for determination of chemical compositions. Some of the highlights of the present thesis are: (i) synthesis of new mixed valent [Mn(III)/Mn(IV)] perovskite-type manganites, ALaMn2O6-y (A = K, Rb) and ALaBMn3O9_y (A = Na, K; B = Ca, Sr) that exhibit ferromagnetism and magnetoresistance; (ii) investigation of a variety of ferrimagnetic double-perovskites that include ALaMnRuO6 (A = Ca, Sr, Ba) and ALaFeVO6 (A = Ca, Sr) and A2FeReO6 (A = Ca, Sr, Ba) providing new insights into the occurrence of metallic and nonmetallic ferrimagnetic behaviour among this family of oxides; (iii) synthesis of new K2NiF4-type oxides, La2-2xSr2XCui.xMxO4 (M = Ti, Mn, Fe, Ru) and investigation of Cu-O-M interaction in two dimension and (iv) identification of the structural rnotif(s) that gives rise to efficient second order NLO optical (SHG) response among d° oxides containing Ti(IV), V(V), Nb(V) etc., and synthesis of a new SHG material, Ba2-xVOSi2O7 having the fresnoite structure. The thesis consists of five chapters and an appendix, describing the results of the investigations carried out by the candidate. A brief introduction to transition metaloxides, perovskite oxides in particular, is presented in Chapter 1. Attention is focused on the structure and properties of these materials. Chapter 2 describes the synthesis and investigation of two series of anion-deficient perovskite oxides, ALaMn2O6-y (A = K, Rb, Cs) and ALaBMn3O9_y (A = Na, K; B = Ca, Sr). ALaMn2O6-y (A = K, Rb, Cs) series of oxides adopt 2 ap x 2 ap superstructure for K and Rb phases and √2 av x √2 ap x 2 ap superstructure (ap = perovskite subcell) for the Cs phase. Among ALaBMn3O9-y phases, the A = Na members adopt a new kind of perovskite superstructure, ap x 3 ap, while the A = K phases do not reveal an obvious superstructure of the perovskite. All these oxides are ferromagnetic (Tc ~ 260-325 K) and metallic exhibiting a giant magnetoresistance behaviour similar to alkaline earth metal substituted lanthanum manganites, Lai_xAxMnO3. However, unlike the latter, the resistivity peak temperature Tp for all the anion-deficient manganites is significantly lower than Tc. In Chapter 3, we have investigated structure and electronic properties of double-perovskite oxides, A2FeReO6 (A = Ca, Sr and Ba). The A = Sr, Ba phases are cubic (Fm3m) and metallic, while the A = Ca phase is monoclinic (P2yn) and nonmetallic. All the three oxides are ferrimagnetic with Tcs 315-385 K as reported earlier. A = Sr, Ba phases show a negative magnetoresistance (MR) (10-25 % at 5 T), while the Ca member does not show an MR effect. 57Fe Mossbauer spectroscopy shows that iron is present in the high-spin Fe3+ (S = 5/2) state in Ca compound, while it occurs in an intermediate state between high-spin Fe2+ and Fe3+ in the Ba compound. Monoclinic distortion and high covalency of Ca-O bonds appear to freeze the oxidation states at Fe+3/Re5+ in Ca2FeRe O6, while the symmetric structure and ionic Ba-O bonds render the FeReO6 array highly covalent and Ba2FeReO6 metallic. Mossbauer data for Sr2FeReO6 shows that the valence state of iron in this compound is intermediate between that in Ba and Ca compounds. It is likely that Sr2FeReO6 which lies at the boundary between metallic and insulating states is metastable, phase-seperating into a percolating mixture of different electronic states at the microscopic level. In an effort to understand the occurrence of metallicity and ferrimagnetism among double perovskites, we have synthesized several new members : ALaMnFeO6 (A = Ca, Sr, Ba), ALaMnRuO6 (A = Ca, Sr, Ba) and ALaVFeO6 (A = Ca, Sr) (Chapter 3). Electron diffraction reveals an ordering of Mn and Ru in ALaMnRuO6 showing a doubling of the primitive cubic perovskite cell, while ALaVFeO6 do not show an ordering. ALaMnRuOs are ferrimagnetic (Tcs ~ 200-250 K) semiconductors, but ALaVFeO6 oxides do not show a long range magnetic ordering . The present work together with the previous work on double perovskites shows that only a very few of them exhibit both metallicity and ferrimagnetism, although several of them are ferrimagnetic. For example, among the series Ba2MReO6 (M = Mn, Fe, Co, Ni), only the M = Fe oxide is both metallic and ferrimagnetic, while M = Mn and Ni oxides are ferrimagnetic semiconductors. Similarly, A2CrMoO6 (A = Ca, Sr), A2CrRe06 (A = Ca, Sr), and ALaMnRuO6 (A = Ca, Sr, Ba) are all ferrimagnetic but not metallic. While ferrimagnetism of double perovskites arise from an antiferromagnetic coupling of B and B' spins through the B-O-B' bridges, the occurrence of metallicity seems to require precise matching of the energies of d-states of B and B' cations and a high covalency in the BB'O6 array that allows a facile electron-transfer between B and B', Bn++B’m+↔B(n+1)++B’(m-1)+ without an energy cost, just as occurs in ReO3 and other metallic ABO3 perovskites. In an effort to understand the Cu-O-M (M = Ti, Mn, Fe, Ru) electronic interaction in two dimension, we have investigated K2N1F4 oxides of the general formula La2-2xSr2XCui.xMxO4 (M = Ti, Mn, Fe or Ru). These investigations are described in Chapter 4. For M = Ti, only the x = 0.5 member could be prepared, while for M = Mn and Fe, the composition range is 0 < x < 1.0, and for M = Ru, the composition range is 0 < x ≤ 0.5. There is no evidence for ordering of Cu(II) and M(IV) in the x = 0.5 members. While the members of the M = Ti, Mn and Ru series are semiconducting/insulating, the members of the M = Fe series are metallic, showing a broad metal-semiconductor transition around 100 K for 0 < x ≤ 0.15 that is possibly related to a Cu(II)-O-Fe(IV) < > Cu(III)-O-Fe(III) valence degeneracy. Increasing the strontium content at the expense of lanthanum in La2-2xSr2XCui.xFexO4 for x ≤ 0.20 renders the samples metallic but not superconducting. In a search for inorganic oxide materials showing second order nonlinear optical (NLO) susceptibility, we have investigated several borates, silicates and phosphates containing /ram-connected MO6 octahedral chains or MO5 square-pyramids, where M = d°: Ti(IV), Nb(V) or Ta(V). Our investigations, which are described in Chapter 5, have identified two new NLO structures: batisite, Na2Ba(TiO)2Si4O12, containing trans-connectd TiO6 octahedral chains, and fresnoite, Ba2TiOSi2O7, containing square-pyramidal T1O5. Investigation of two other materials containing square-pyramidal TiO5, viz., Cs2TiOP2O7 and Na4Ti2Si8O22. 4H2O, revealed that isolated TiO5 square-pyramids alone do not cause a second harmonic generation (SHG) response; rather, the orientation of T1O5 units to produce -Ti-O-Ti-O- chains with alternating long and short Ti-0 distances in the fresnoite structure is most likely the origin of a strong SHG response in fresnoite. Indeed, we have been able to prepare a new fresnoite type oxide, Ba2.xVOSi2O7 (x ~ 0.5) that shows a strong SHG response, confirming this hypothesis. In the Appendix, we have described three synthetic strategies that enabled us to prepare magnetic and NLO materials. We have shown that the reaction CrO3 + 2 NH4X > CrO2 + 2 NH3 + H2O + X2 (X = Br, I), which occurs quantitatively at 120-150 °C, provides a convenient method for the synthesis of CrO2. Unlike conventional methods, the method described here does not require the use of high pressure for the synthesis of this technologically important material. For the synthesis of magnetic double perovskites, we have developed a method that involves reaction of basic alkali metal carbonates with the acidic oxides (e.g. Re2O7) first, followed by reaction of this precursor oxide with the required transition metal/transition metal oxide (e.g. Fe/Fe2O3). By this method we have successfully prepared single-phase perovskite oxides, A2FeReO6, ACrMoO6 and ALaFeVO6. We have prepared the new NLO material Ba2_xV0Si207 from Ba2VOSi2O7 by a soft chemical redox reaction involving the oxidation of V(IV) to V(V) using Br2 in CH3CN/CHCI3. Ba2V0Si207 + 1/2 Br2 > Bai.5V0Si207 + 1/2 BaBr2. The work presented in this thesis was carried out by the candidate as part of the Ph.D. training programme. He hopes that the studies reported here will constitute a worthwhile contribution to the solid state chemistry of transition metal oxides and related materials.

Transition metal compounds, especially the oxides, containing dn (0 ≤ n ≤ 10) electronic configuration, constitute the backbone of solid state/materials chemistry aimed at realization of novel materials properties of technological importance. Some of the significant materials properties of current interest are spin-polarized metallic ferromagnetism, negative thermal expansion, second harmonic nonlinear optical (NLO) susceptibility, fast ionic and mixed electronic/ionic conductivity for application in solid state batteries, and last but not the least, high-temperature superconductivity. Typical examples for each one of these properties could be found among transition metal oxides. Thus, alkaline-earth metal (A) substituted rare-earth (Ln) manganites, Lnı.xAxMnΟ3, are currently important examples for spin-polarized magnetotransport, ZrV2O7 and ZrW2O8 for negative thermal expansion coefficient, KTiOPO4 and LiNbO3 for second harmonic NLO susceptibility, (Li, La) TiO3 and LiMn2O4 for fast-ionic and mixed electronic/ionic conductivity respectively, and the whole host of cuprates typified by YBa2Cu3O7 for high Tc superconductivity. Solid state chemists constantly endeavour to obtain structure-property relations of solids so as to be able to design better materials towards desired properties. Synthesis coupled with characterization of structure and measurement of relevant properties is a common strategy that chemists adopt for this task. The work described in this thesis is based on such a broad-based chemists' approach towards understanding and realization of novel materials properties among the family of metal oxides. A search for metallic ferro/ferrimagnetism among the transition metal perovskite oxides, metallicity and possibility of superconductivity among transition-metal substituted cuprates and second order NLO susceptibility among metal oxides containing d° cations such as Ti(IV), V(V) and Nb(V) - constitute the main focus of the present thesis. New synthetic strategies that combine the conventional ceramic approach with the chemistry-based 'soft1 methods have been employed wherever possible to prepare the materials. The structures and electronic properties of the new materials have been probed by state-of-the art techniques that include powder X-ray diffraction (XRD) together with Rietveld refinement, electron diffraction, thermogravimetry, measurement of magnetic susceptibility (including magnetoresistance), Mossbauer spectroscopy and SHG response (towards 1064 nm laser radiation), besides conventional analytical techniques for determination of chemical compositions. Some of the highlights of the present thesis are: (i) synthesis of new mixed valent [Mn(III)/Mn(IV)] perovskite-type manganites, ALaMn2O6-y (A = K, Rb) and ALaBMn3O9_y (A = Na, K; B = Ca, Sr) that exhibit ferromagnetism and magnetoresistance; (ii) investigation of a variety of ferrimagnetic double-perovskites that include ALaMnRuO6 (A = Ca, Sr, Ba) and ALaFeVO6 (A = Ca, Sr) and A2FeReO6 (A = Ca, Sr, Ba) providing new insights into the occurrence of metallic and nonmetallic ferrimagnetic behaviour among this family of oxides; (iii) synthesis of new K2NiF4-type oxides, La2-2xSr2XCui.xMxO4 (M = Ti, Mn, Fe, Ru) and investigation of Cu-O-M interaction in two dimension and (iv) identification of the structural rnotif(s) that gives rise to efficient second order NLO optical (SHG) response among d° oxides containing Ti(IV), V(V), Nb(V) etc., and synthesis of a new SHG material, Ba2-xVOSi2O7 having the fresnoite structure. The thesis consists of five chapters and an appendix, describing the results of the investigations carried out by the candidate. A brief introduction to transition metaloxides, perovskite oxides in particular, is presented in Chapter 1. Attention is focused on the structure and properties of these materials. Chapter 2 describes the synthesis and investigation of two series of anion-deficient perovskite oxides, ALaMn2O6-y (A = K, Rb, Cs) and ALaBMn3O9_y (A = Na, K; B = Ca, Sr). ALaMn2O6-y (A = K, Rb, Cs) series of oxides adopt 2 ap x 2 ap superstructure for K and Rb phases and √2 av x √2 ap x 2 ap superstructure (ap = perovskite subcell) for the Cs phase. Among ALaBMn3O9-y phases, the A = Na members adopt a new kind of perovskite superstructure, ap x 3 ap, while the A = K phases do not reveal an obvious superstructure of the perovskite. All these oxides are ferromagnetic (Tc ~ 260-325 K) and metallic exhibiting a giant magnetoresistance behaviour similar to alkaline earth metal substituted lanthanum manganites, Lai_xAxMnO3. However, unlike the latter, the resistivity peak temperature Tp for all the anion-deficient manganites is significantly lower than Tc. In Chapter 3, we have investigated structure and electronic properties of double-perovskite oxides, A2FeReO6 (A = Ca, Sr and Ba). The A = Sr, Ba phases are cubic (Fm3m) and metallic, while the A = Ca phase is monoclinic (P2yn) and nonmetallic. All the three oxides are ferrimagnetic with Tcs 315-385 K as reported earlier. A = Sr, Ba phases show a negative magnetoresistance (MR) (10-25 % at 5 T), while the Ca member does not show an MR effect. 57Fe Mossbauer spectroscopy shows that iron is present in the high-spin Fe3+ (S = 5/2) state in Ca compound, while it occurs in an intermediate state between high-spin Fe2+ and Fe3+ in the Ba compound. Monoclinic distortion and high covalency of Ca-O bonds appear to freeze the oxidation states at Fe+3/Re5+ in Ca2FeRe O6, while the symmetric structure and ionic Ba-O bonds render the FeReO6 array highly covalent and Ba2FeReO6 metallic. Mossbauer data for Sr2FeReO6 shows that the valence state of iron in this compound is intermediate between that in Ba and Ca compounds. It is likely that Sr2FeReO6 which lies at the boundary between metallic and insulating states is metastable, phase-seperating into a percolating mixture of different electronic states at the microscopic level. In an effort to understand the occurrence of metallicity and ferrimagnetism among double perovskites, we have synthesized several new members : ALaMnFeO6 (A = Ca, Sr, Ba), ALaMnRuO6 (A = Ca, Sr, Ba) and ALaVFeO6 (A = Ca, Sr) (Chapter 3). Electron diffraction reveals an ordering of Mn and Ru in ALaMnRuO6 showing a doubling of the primitive cubic perovskite cell, while ALaVFeO6 do not show an ordering. ALaMnRuOs are ferrimagnetic (Tcs ~ 200-250 K) semiconductors, but ALaVFeO6 oxides do not show a long range magnetic ordering . The present work together with the previous work on double perovskites shows that only a very few of them exhibit both metallicity and ferrimagnetism, although several of them are ferrimagnetic. For example, among the series Ba2MReO6 (M = Mn, Fe, Co, Ni), only the M = Fe oxide is both metallic and ferrimagnetic, while M = Mn and Ni oxides are ferrimagnetic semiconductors. Similarly, A2CrMoO6 (A = Ca, Sr), A2CrRe06 (A = Ca, Sr), and ALaMnRuO6 (A = Ca, Sr, Ba) are all ferrimagnetic but not metallic. While ferrimagnetism of double perovskites arise from an antiferromagnetic coupling of B and B' spins through the B-O-B' bridges, the occurrence of metallicity seems to require precise matching of the energies of d-states of B and B' cations and a high covalency in the BB'O6 array that allows a facile electron-transfer between B and B', Bn++B’m+↔B(n+1)++B’(m-1)+ without an energy cost, just as occurs in ReO3 and other metallic ABO3 perovskites. In an effort to understand the Cu-O-M (M = Ti, Mn, Fe, Ru) electronic interaction in two dimension, we have investigated K2N1F4 oxides of the general formula La2-2xSr2XCui.xMxO4 (M = Ti, Mn, Fe or Ru). These investigations are described in Chapter 4. For M = Ti, only the x = 0.5 member could be prepared, while for M = Mn and Fe, the composition range is 0 < x < 1.0, and for M = Ru, the composition range is 0 < x ≤ 0.5. There is no evidence for ordering of Cu(II) and M(IV) in the x = 0.5 members. While the members of the M = Ti, Mn and Ru series are semiconducting/insulating, the members of the M = Fe series are metallic, showing a broad metal-semiconductor transition around 100 K for 0 < x ≤ 0.15 that is possibly related to a Cu(II)-O-Fe(IV) < > Cu(III)-O-Fe(III) valence degeneracy. Increasing the strontium content at the expense of lanthanum in La2-2xSr2XCui.xFexO4 for x ≤ 0.20 renders the samples metallic but not superconducting. In a search for inorganic oxide materials showing second order nonlinear optical (NLO) susceptibility, we have investigated several borates, silicates and phosphates containing /ram-connected MO6 octahedral chains or MO5 square-pyramids, where M = d°: Ti(IV), Nb(V) or Ta(V). Our investigations, which are described in Chapter 5, have identified two new NLO structures: batisite, Na2Ba(TiO)2Si4O12, containing trans-connectd TiO6 octahedral chains, and fresnoite, Ba2TiOSi2O7, containing square-pyramidal T1O5. Investigation of two other materials containing square-pyramidal TiO5, viz., Cs2TiOP2O7 and Na4Ti2Si8O22. 4H2O, revealed that isolated TiO5 square-pyramids alone do not cause a second harmonic generation (SHG) response; rather, the orientation of T1O5 units to produce -Ti-O-Ti-O- chains with alternating long and short Ti-0 distances in the fresnoite structure is most likely the origin of a strong SHG response in fresnoite. Indeed, we have been able to prepare a new fresnoite type oxide, Ba2.xVOSi2O7 (x ~ 0.5) that shows a strong SHG response, confirming this hypothesis. In the Appendix, we have described three synthetic strategies that enabled us to prepare magnetic and NLO materials. We have shown that the reaction CrO3 + 2 NH4X > CrO2 + 2 NH3 + H2O + X2 (X = Br, I), which occurs quantitatively at 120-150 °C, provides a convenient method for the synthesis of CrO2. Unlike conventional methods, the method described here does not require the use of high pressure for the synthesis of this technologically important material. For the synthesis of magnetic double perovskites, we have developed a method that involves reaction of basic alkali metal carbonates with the acidic oxides (e.g. Re2O7) first, followed by reaction of this precursor oxide with the required transition metal/transition metal oxide (e.g. Fe/Fe2O3). By this method we have successfully prepared single-phase perovskite oxides, A2FeReO6, ACrMoO6 and ALaFeVO6. We have prepared the new NLO material Ba2_xV0Si207 from Ba2VOSi2O7 by a soft chemical redox reaction involving the oxidation of V(IV) to V(V) using Br2 in CH3CN/CHCI3. Ba2V0Si207 + 1/2 Br2 > Bai.5V0Si207 + 1/2 BaBr2. The work presented in this thesis was carried out by the candidate as part of the Ph.D. training programme. He hopes that the studies reported here will constitute a worthwhile contribution to the solid state chemistry of transition metal oxides and related materials.

The presented work describes new concepts of fast switching elements based on principles of photonics. The waveguides working in visible and infra-red ranges are put in a basis of these elements. And as materials for manufacturing of waveguides the transparent polymers, dopped by molecules of the dyes possessing second order nonlinear-optical properties are proposed. The work shows how nonlinear-optical processes in such structures can be implemented by electro-optical and opto-optical control circuit signals. In this paper we consider the complete cycle of fabrication of several types of integral photonic elements. The theoretical analysis of high-intensity beam propagation in media with second-order optical nonlinearity is performed. Quantitative estimations of necessary conditions of occurrence of the nonlinear-optical phenomena of the second order taking into account properties of used materials are made. The paper describes the various stages of manufacture of the basic structure of the integrated photonics: a planar waveguide. Using the finite element method the structure of the electromagnetic field inside the waveguide in different modes was analysed. A separate part of the work deals with the creation of composite organic materials with high optical nonlinearity. Using the methods of quantum chemistry, the dependence of nonlinear properties of dye molecules from its structure were investigated in details. In addition, the paper discusses various methods of inducing of an optical nonlinearity in dye-doping of polymer films. In the work, for the first time is proposed the use of spatial modulation of nonlinear properties of waveguide according Fibonacci law. This allows involving several different nonlinear optical processes simultaneously. The final part of the work describes various designs of integrated optical modulators and switches constructed of organic nonlinear optical waveguides. A practical design of the optical modulator based on Mach-Zehnder interferometer made by a photolithography on polymer film is presented.
Die vorliegende Arbeit beschreibt neue Konzepte für schnelle photonische Schaltelemente. Diese Elemente basieren auf optischer Wellenleitung im sichtbaren und nahen infraroten Spektralbereich. Die Arbeit ist auf organische Wellenleiter aus transparenten, farbstoffdotierten Polymeren fokussiert, welche Chromophore mit besonders großen optischen Nichtlinearitäten zweiter Ordnung enthalten. Insbesondere wird dargestellt, wie nichtlineare optische Prozesse in derartigen Bauelementen genutzt werden können, wenn man elektrische oder optische Steuersignale einsetzt. Es wird der gesamte Her-stellungszyklus verschiedener integrierter photonischer Bauelemente betrachtet. Die Arbeit umfasst weiterhin eine detaillierte theoretische Analyse der Wellenausbreitung in Medien mit großer nichtlinearer optischer Suszeptibilität zweiter Ordnung bei hoher Lichtintensität. Unter Annahme optischer Materialkonstanten, welche den experimentell ermittelten Werten entsprechen, erfolgte eine quantitative Abschätzung für das Auftreten von Szenarien, bei denen messbare Abwei-chungen der Strahlausbreitung vom linearen Regime auftreten, z:B. das Auftreten von zeitlichen, räumlichen und spektralen Satteliten zu den im Material propagierenden Laserpulsen. Es gelang, we-sentliche Aspekte der Strahlpropagationsprozesse durch Anwendung von Jacobi-Integralfunktionen in geschlossener mathematischer Form darzustellen. Darüber hinausgehende theoretische Untersuchungen nutzten die Finite-Elemente-Methode, um die Verteilung des elektromagnetischen Feldes im Inneren optischer Wellenleiter für verschiedene Pro-pagationsmoden zu analysieren. Ein weiterer Teil der Arbeit beschäftigt sich mit organischen Kompositmaterialen, welche große opti-sche Nichtlinearitäten aufweisen. Mittels quantenchemischer Verfahren erfolgte eine detaillierte Un-tersuchung der Zusammenhänge zwischen der molekularen Struktur und den linearen sowie den nichtlinearen optischen Eigenschaften der untersuchten Farbstoffmoleküle und Polymere. In Bezug auf die Probenpräparation sind unterschiedliche Verfahren zur Schichtbildung und zur molekularen Orientierung miteinander verglichen und bewertet worden, da letztere eine Voraussetzung für das Auftreten nichtlinearer optischer Prozesse zweiter Ordnung bildet. Die Untersuchungen erstreckten sich auf Vakuumaufdampfschichten aus niedermolekularen Chromophoren und auf Polymerschich-ten, welche durch Vakuumabscheidung sowie durch Spincoating oder andere Flüssigphasenabschei-dungsverfahren hergestellt worden sind. Auf orientierenden Substraten (z.B. geriebene Schichten aus Polytetrafluoräthylen) konnte eine spontane Orientierung der deponierten Chromophore nachgewie-sen werden. Die Chromophore in Polymerschichten wurden durch Coronapolung orientiert. In der Arbeit ist zum ersten Mal vorgeschlagen worden, eine räumliche Modulation der nichtlinearen optischen Eigenschaften durch gezielte lokale Coronapolung oder andere Orientierungstechniken derart zu generieren, dass die Abfolge gepolter Domänen dem Bildungsgesetz einer modifizierten Fibonacci-Reihe mit gebrochen rationalen Zahlen gehorcht. Der Vorteil von optischen Wellenleitern mit dieser Struktur darin, dass diese mehrere unterschiedli-che nichtlineare optische Prozesse gleichzeitig unterstützen und somit eine Vielzahl neuartiger Phä-nomene auftreten kann, welche man in neuartigen photonischen Bauelementen, z.B. für die optische Informations- und Kommunikationstechnik nutzen kann. Der letzte Teil der Arbeit beschreibt den Aufbau verschiedener integrierter optischer Modulatoren und Schalter, die sich mit Hilfe das neu entwickelten Verfahrens herstellen lassen. Zukünftige Entwicklungen in der Photonik, insbesondere der optischen Informations- und Kommuni-kationstechnik werden im abschließenden Teil der Arbeit diskutiert.

Glass microfibre waveguides offer an intriguing platform for the investigation of nonlinear effects, due to their high effective nonlinearity which arises from the tight modal confinement down to dimensions comparable to the wavelength of guided light. This thesis presents theoretical and experimental work towards achieving efficient third and second harmonic generation in silica microfibres, as well as in microfibre loop resonators for enhancing the conversion. Since microfibre resonators themselves exhibit interesting nonlinear behaviour, the polarisation dependent properties of microcoil resonators were also studied. Efficient third harmonic generation is possible through intermodal phase matching, and experiments using short tapers have demonstrated significant efficiencies up to 10

The nonlinear optical properties of semiconductor heterostructures, such as GaAsAl/GaAs alloys, are studied with analytic and numerical methods on the basis of quantum mechanics. Particularly, second and third-order nonlinear optical properties of quantum wells described by the various types of confining potentials are considered within the density matrix formalism. We consider a Pö
schl-Teller type potential which has been rarely considered in this area. It has a tunable asymmetry parameter, making it a good candidate to investigate the effect of the asymmetry on the nonlinear optical properties. The calculated nonlinear quantities include nonlinear absorption coefficient, second-harmonic generation, optical rectification, third-harmonic generation and the intensity-dependent refractive index. The effects of the DC electric field on the corresponding nonlinearities are also studied. The results are in good agreement with the results obtained in other types of quantum wells, such as square and parabolic quantum wells. The effects of the Coulomb interaction among the electrons on the nonlinear intersubband absorption are considered within the rotating wave approximation. The result is applied to a Si-delta-doped, square quantum well in which the Coulomb interaction among the electrons are relatively important, since there has been no work on the nonlinear absorption spectrum of the Si-delta-doped quantum well. The results are found to be new and interesting, especially when a DC electric field is included in the calculations.

This thesis concerns nonlinear optical phenomena in liquids and at interfaces. Optical processes are considered in chiral liquids and at chiral surfaces to second order in the applied electric fields, and to third-order in fluids. Time-dependent perturbation theory is used to describe the response of molecules to electromagnetic fields in terms of molecular electric-dipole polarizabilities. The inclusion of phenomenological damping in the sum-overstates expressions for the nonlinear polarizabilities is discussed, both at optical frequencies and in the limit that one or more applied electromagnetic fields are static. The reality condition for the polarization and the applied field is shown to define the sign of the damping terms. Electro-optical effects, linear in the electrostatic field, are considered in chiral liquids. Symmetry arguments are presented to show that the Pockels effect is forbidden in any non-conducting isotropic medium, and it is shown that this also follows from the statistical average of its sum-over-states expression when damping is correctly incorporated. A linear effect of an electrostatic field on the intensity of sum- and difference-frequency generation in a chiral liquid is predicted. The effect changes sign with the enantiomer and on reversing the direction of the electrostatic field. The sum-frequency polarizability and the electric field-induced sum-frequency polarizability interfere and their contributions to the scattering power can be distinguished. Experiments are described where, symmetry permitting, the third-harmonic is generated simultaneously in a second and in a third-order nonlinear optical mixing process. The third-order nonlinear optical properties of some simple liquids are characterized by all-electric phase-matched four-wave mixing. However, no three-wave mixing was observed in chiral liquids. Finally, a theory of surface nonlinear optical activity, applicable to various surface symmetries, is developed for sum-frequency generation. The potential of the three-wave mixing processes as probes of molecular chirality is examined. An optically active conformation for water in a static electric field parallel to the surface is proposed.

Silica optical fibres are renowned for the framework they have set in modern communications systems, sensors, and biotechnology. One particular trend in current research aims to investigate materials with enhanced optical functionality, high optical effciency, robustness, and a small device footprint. Amongst the many material choices, semiconductors are emerging as a promising route. In this work, optical fibres and semiconductors are elegantly unified to create a hybrid structure with the potential of seamless integration into current fibre infrastructures. Silica capillaries form the fibre templates in which amorphous semiconductor materials such as silicon and/or germanium are impregnated. This thesis will present the first comprehensive description of the fabrication, characterisation, and the implementation of silicon optical fibres for all-optical signal processing. The fibres are fabricated via a novel high pressure chemical deposition procedure. Each fibre is analysed to determine the exact material composition, uniformity, and more importantly the optical quality. Linear and nonlinear optical characterisations are performed experimentally and supported by intensive numerical studies to validate the results. The high nonlinearity of silicon is exploited for all-optical signal processing. Several investigations have been performed to determine key nonlinear coeffcients that were previously unknown in these fibres. Nonlinear absorption experiments allowed for the determination of the degenerate and non-degenerate two-photon absorption coeffcients, free carrier cross sections, and free carrier lifetimes of a number of silicon fibres. Nonlinear refraction investigations were then used to establish the Kerr nonlinearity. The strength of this parameter allowed for demonstration of strong self-phase and cross-phase modulation effects. With the insight gained in nonlinear absorption and refraction in silicon optical fibres, all-optical amplitude modulation and wavelength switching was demonstrated at ultrafast sub-picosecond speeds.

New mono-chelate complexes with ferrocenyl (Fc), octamethylferrocenyl (Me8Fc) and diaminophenyl (Dap) donor groups, connected through a conjugated bridge, to ZnCl2, Zn(OAc)2 or ReCl(CO)3 acceptor groups, are described. A thorough characterisation of the complexes is provided, including single crystal structures for one pro-ligand and three complexes. Visible d(FeII)→π* metal-to-ligand charge-transfer (MLCT) bands accompany π→π* intraligand charge-transfer absorptions in the UV region. TD-DFT calculations confirm the nature of these absorptions and indicate transitions at higher energies also contain some d(FeII)→π* character. Fc and Me8Fc containing chromophores display a fully reversible oxidation process when probed electrochemically. Molecular quadratic nonlinear optical (NLO) responses are determined using hyper-Rayleigh scattering (HRS) and Stark spectroscopy. Larger β values are determined for complexes with Dap donors and ReICl(CO)3 acceptors. A family of novel fac-ReI(Lax)(CO)3(N-N) complexes, where N-N is 4,4′-dicyano-2,2′-bipyridyl (dcnbpy) or 4,4′-bis-(p-cyanophenyl)-2,2′-bipyridine (bbnbpy), with various axial ligands (Lax) are reported. The ReI complexes are useful precursors for metalation with electron-rich RuII ammine donor groups, to prepare novel tri-metallic V-shaped molecules. Single crystal X-ray structures are reported for five ReI complexes, confirming the fac geometry of the carbonyl ligands; the ReI complexes also display stretching frequencies typical of fac-ReI(Lax)(CO)3(N-N) complexes. The UV-visible spectra contain a low intensity band due to a d(ReI)→π*(bpy) transition, along with a more intense π→π* band. 1H NMR studies reveal the formation of trimetallic complexes, upon treatment of ReI complexes with a molar excess of [RuII(NH3)5(H2O)][PF6]2.The synthesis of octupolar heptametallic complexes containing RuII ammine donor groups has been investigated. The ligand 4,4′-bis-[(E)-2-(4-cyanophenyl)ethenyl]-2,2′-bipyridine (bbnpe) was used to prepare tris-chelate complexes of various transition metals, in order to understand its complexation behaviour. The ZnII tris-chelate BPh4– salt was treated with a 10 molar excess of a RuII ammine aquo complex, to produce the heptametallic complex as a mixed anion salt. HRS and Stark spectroscopy have been used to determine the quadratic NLO response for the heptametallic mixed anion complex salts, the latter gives large β0 values, approximately 10–27 esu. Density functional theory (DFT) calculations have been carried out on twelve cationic, 2D NLO chromophores with pyrazinylbis(pyridinium) electron acceptors with either 4-(methoxy/dimethylamino)-phenyl or pyridyl coordinated {RuII(NH3)5}2+/trans-{RuII(NH3)4(py)}2+ electron donor groups and the results compared with data previously obtained experimentally. The B3LYP/6-311G(d) level of theory was used to model the absorption spectra and to calculate static hyperpolarisability (β0) values, whilst the B3LYP/LANL2DZ/6-311G(d) level was used for the complexes. The extent of prediction of trends in ICT bands and β0 is partial, with the main discrepancies relating to the progression from one to two electron donor groups. The quantitative accuracy of predictions diminishes as the systems depart from a relatively simple one-dimensional (1D) dipolar motif.

>Magister Scientiae - MSc
Recent research shows that the study of optical properties of organic material natural dyes has gained much consideration. The specific functional groups in several natural dyes remain essential for the large nonlinear absorption expressed in terms of nonlinear optical susceptibilities or other mechanism of absorption such as two photon absorption (TPA), reverse saturable absorption (RSA) or intensitydependent refractive index characteristic. In this thesis we highlight the optical limiting responses of selected natural dyes as nonlinear response in the femtosecond regime. This technique refers to the decrease of the transmittance of the material with the increased incident light intensity.Three dyes derived from beetroot, flame flower and mimosa flower dyes were investigated. The results showed a limiting behaviour around 795 mW for the beetroot and the flame dye while there is total transmission in the flame dye sample. The performance of the nonlinearity i.e. the optical limiting is related to the existence of alternating single and double bonds (i.e. C-C and C=C bonds) in the molecules that provides the material with the electron delocalization, but also it is related to the light intensity.Beside nonlinearity study, crystallographic investigation was carried out for more possible applicability of the selected dyes and this concerned only the mimosa and flame flower dye thin film samples since the beetroot thin film was very sensitive to strong irradiation (i.e. immediately destroyed when exposed to light with high intensity). For more stability,dye solutions were encapsulated in gels for further measurements.

This dissertation demonstrates the feasibility of using novel electrostatic self-assembly (ESA) methods to fabricate linear and nonlinear optical thin films and components. The ESA process involves the layer-by-layer alternate adsorption of anionic and cationic complexes from aqueous solutions. Selection of the molecules in each layer, their orientation at the molecular level, and the order in which the layers are assembled determine the film's bulk optical, electronic, magnetic, thermal, mechanical and other properties. In this work, the capability of nanoscale control over film optical properties allowed the fabrication of complicated refractive index profiles required for linear optical interference filters. The inherent ordered nature of ESA films yielded extremely stable noncentrosymmetric thin films for second-order nonlinear optical applications. The ESA technique offers numerous advantages over conventional thin film fabrication methods and offers great potential in commercial applications such as reflectance and AR filters, EO waveguides and modulators and other optoelectronic devices. The structure of each monolayer in ESA films is dependent on the processing parameters, producing subsequent variations in bulk film properties both intentionally and incidentally. As this method is still in its infancy, variations in ESA processing methods, including process automation, are considered first in this document. These results allowed carefully controlled refractive index experiments and the synthesis of both step and graded index structures, several microns thick. Dielectric stack, Rugate, and antireflection optical interference filters were designed, synthesized and demonstrated. c(2) films of both commercially available polymer dyes and novel polymers designed specifically for the ESA process were demonstrated using second harmonic generation. UV/vis spectroscopy, ellipsometry and atomic force microscopy analysis are presented.
Ph. D.

The linear and nonlinear optical properties of a spin-coated polydiacetylene, [5,7-dodecadiyn-1,12-diol-bis(n-butoxy-carbonyl-methyl-urethane)], or poly(4BCMU), were measured to predict its performance in all-optical devices at 1.319 μm. Material requirements for all-optical devices were identified and figures-of-merit noted. A two-photon absorption figure of merit was verified by numerical simulation of a waveguide device. The refractive index and waveguide loss in spin-coated poly(4BCMU) films were measured. A photo-induced bleaching was observed, and its effect on linear and nonlinear optical properties was quantified. Fabrication of integrated-optical structures using this photobleaching process was demonstrated. The nonlinear refractive index and absorption were measured at 1.319 μm with 60 picosecond laser pulses, using poly(4BCMU) strip-loaded channel waveguides. A novel pulse-modulated interferometer was developed for measuring the intensity-dependent refractive index. The fast electronic contribution was found to be n₂ = (4.8 ± 2.7) x 10⁻⁸ cm²/MW, an a slower thermal contribution of n₂(T) = -(7.9 ± 4.5) x 10⁻¹¹ cm²/MW was measured. The thermal index change was shown to limit the duty cycle of operation for a poly(4BCMU) device. The two-photon absorption coefficient was also measured, yielding γ < 0.25 cm/GW. These values were used to estimate performance of a poly(4BCMU) all-optical device using standard figures-of-merit. For this specific waveguide, the figures-of-merit indicated poor performance. If waveguide losses were neglected, (by assuming improved fabrication for example), and assuming the nonlinearity does not saturate at intensities below the damage threshold, the figures-of-merit improve to useful levels. The limit on duty cycle imposed by thermal effects appears to restrict operation to GHz frequencies of slower.

Le but de cette thèse et de la recherche associée est une démonstration des avantages d’une combinaison de propriétés inhabituelles de nanostructures plasmoniques avec des aspects parmi les plus intéressants de l’optique non-linéaire. Pour cet effet, la modélisation analytique et numérique a été combiné avec le travail expérimental, qui comprenait la production de nanostructures et les mesures effectuées au moyen de la microscopie confocale non-linéaire résolue en polarisations et de la technique Z-scan modifiée (nommée “f-scan”).Il a été montré que l’anisotropie efficace de génération de seconde-harmonique dans les cristaux plasmoniques (formés par des réseaux rectangulaires de cavités tétraédriques sur une surface d’argent) peut être contrôlée par un choix approprié des paramètres de maille. Il a aussi été montré que cette anisotropie provient principalement d’une structure de bande photonique elle-même anisotrope, présentant une bande interdite plasmonique avec des états plasmoniques en bord de bande, permettant de renforcer le champ électrique local. Les arrangements chiraux bidimensionnels de nanoparticules triangulaires d’or, forment des “meta-molécules” plasmoniques énantiomériques, ont été analysés par microscopie non-linéaire à la lumière polarisée circulairement et par modélisation numérique, révélant un fort effet chiroptique par génération de seconde harmonique en rétro-réflexion. La petite taille des énantiomères uniques permet de créer “des filigranes” (“watermarks”) codés par la chiralité des meta-molécules, qui peuvent être lu par imagerie de la génération de seconde harmonique excitée par un rayon polarisé circulairement. Les caractéristiques quantitatives de la non-linéarité optique du troisième ordre et de l’efficacité d’absorption saturable des solutions aqueuses de fragments de graphène et de graphène dopé par des nanoparticules d’or a été effectuée par une nouvelle technique “f-scan”, qui a été créée et développée par incorporation d’une lentille à distance focale accordable dans une technique de Z-scan traditionnelle. Ces études ont montrées que le graphène présente une absorption saturable ultra-rapide très efficace, qui est parfois convertie en absorption saturable inverse. Il apparaît alors qu’une décoration du graphène par des nanoparticules d’or peut causer une légère amélioration du paramètre d’efficacité d’absorption saturable dans la plage spectrale de leurs résonances plasmoniques. En résumé, cette thèse présente une variété de propriétés optiques non-linéaires apparaissant dans les nanostructures plasmoniques. Différentes possibilités de contrôle de ces propriétés au moyen d’une démarche de nano-ingénierie, soutenue par des modélisations à la fois analytique et numérique ont été démontrées et analysées. Ces travaux ouvrent la voie à la fabrication et à l‘optimisation sur mesure de nouveaux nano-matériaux et nano-dispositifs photoniques reposant sur des effets de nano-plasmonique non-linéaire
The aim of this thesis and the underlying research work is to demonstrate the benefits emerging from combination of the peculiar properties of plasmonic nanostructures with the most interesting aspects of nonlinear optics. For this purpose, analytical and numerical modeling was combined with experimental work, which included nanofabrication and measurements performed by means of polarization-resolved nonlinear confocal microscopy and by modified Z-scan technique (called "f-scan").It has been shown that the effective anisotropy of the second-harmonic generation in plasmonic crystals (formed by rectangular arrays of tetrahedral recesses in silver surface) can be controlled by proper choice of lattice constants. It also has been shown that this anisotropy arises mainly from the anisotropic photonic band structure, exhibiting plasmonic band gap with plasmonic band edge states, enabling enhancement of the local electric field.Two-dimensional chiral arrangements of triangular gold nanoparticles, forming plasmonic enantiomeric "meta-molecules", have been studied by nonlinear microscopy operating with circularly polarized light and by numerical modeling, revealing strong chiroptical effect in backscattered second-harmonic radiation. Small size of individual enantiomers allows to create "watermarks", encoded by the chirality of meta-molecules, which can be readout by imaging of second-harmonic generation excited by circularly polarized laser beam.Quantitative characterization of the third-order optical nonlinearity and saturable absorption efficiency of aqueous solutions of graphene and gold-nanoparticle decorated graphene has been performed by novel "f-scan" technique, which has been created and developed by incorporation of a focus-tunable lens into traditional Z-scan. These studies have shown that the graphene exhibits very efficient ultrafast saturable absorption, which is occasionally suppressed by reverse saturable absorption. Moreover, it turns out that decoration of graphene by gold nanoparticles may cause a slight improvement of the saturable absorption efficiency parameter within spectral range of their plasmon resonances.In summary, the following thesis presents various nonlinear optical properties of plasmonic nanostructures. Different possibilities of controlling these properties by means of nano-engineering, supported by analytical and numerical modeling, is also analyzed and demonstrated. This work opens up new perspectives for fabrication and rational design of novel photonic nano-materials and nano-devices based on nonlinear nanoplasmonic phenomena

Organometallic chromophores with both luminescent and second-order nonlinear optical properties are of growing interest as new molecular multifunctional materials, since they offer additional flexibility compared to organic chromophores by introducing active electronic charge-transfer transitions between the metal and the ligand, which are tunable by virtue of the nature, oxidation state and coordination sphere of the metal centre. This is the main reason why our interest was brought in this specific research area. My PhD thesis is divided in two main parts: 1) Luminescent properties of cyclometallated metal complexes. 2) Second-order NLO properties of organometallic complexes and third-order NLO activity of organic π-conjugated and coordination compounds. The first section is dedicated to the preparation and study of the luminescence properties of some novel platinum(dipyridylbenzene) complexes. These organometallic compounds, known for being extremely bright emissive and having long lifetimes, are of particular interest for applications in electroluminescent devices. My aim was to achieve mono- and dinuclear complexes possessing emission properties that can be tuned with external factors. The two dinuclear platinum(II) complexes that I synthesized alternate monomer-like and eximeric emission, depending on the temperature, the solvent and the presence of metal cations in solution. The mononuclear platinum(II) complexes that I studied, instead, bear a photochromic unit linked with different tethers to the metal moiety: this allows the modulation of their switching ability and the study of the effect of the linker. These complexes were prepared in Rennes, France, during my stay in the group of Dr. V. Guerchais. The second section is dedicated to the second-order nonlinear optical properties of mononuclear platinum(dipyridylbenzene) complexes. In the first place, we will discuss how the second-order NLO properties of such compounds can be modified by virtue of the presence of different π-conjugated pendants linked to the central metal unit with different bridges: this will allow a fine tuning of the NLO properties. Secondly, we will study related photochromic platinum(II) complexes and see that their second-order NLO properties are switchable, too. The two-photon absorption (TPA) ability of different organic π-conjugated and organometallic complexes will be discussed. At first, the outstanding TPA properties of some 3,4,9,10-perylenetetracarboxylic diimide derivatives will be presented. These purely organic compounds will be used as springboard to achieve new multinuclear ruthenium(II) complexes with a highly π-conjugated ligand. Afterwards, the TPA activity of some ruthenium(II) σ-acetylide and iridium(III) tris-(phenylpyridine) complexes will be discussed. The two-photon absorption measurements were performed in the laboratory of Dr. K. Kamada, in Ikeda, Japan, during my stay there. Throughout this dissertation, the luminescent, second- and third-order nonlinear optical properties of a new multifunctional cationic iridium(III) complex will also be presented.

This dissertation reports on the development of a nonlinear surface spectroscopy and the observation of nonlinear optical phenomena using sputtered zinc oxide waveguides. The first is known as Surface Coherent Raman Spectroscopy, or SCRS, and is capable of monolayer sensitivity. The second, discovered during the development of SCRS, is optical limiting and a previously unobserved form of optical switching based on an absorptive nonlinear coupling mechanism. Overviews of the theories of waveguiding, linear coupling, and SCRS are given. Experiments showing that the spectrum of a monolayer coverage of molecules on the surface of a metal oxide waveguide can be obtained using SCRS are reported. For this purpose ZnO waveguides were fabricated using rf magnetron sputtering; the details of which are presented. The results of the characterization of these films, using an optical loss technique, Rutherford backscattering, and X-ray diffraction, are also presented. Experiments are described and data are presented to show the existence of optical limiting and optical switching phenomena in ZnO waveguides. The experimental dependence of these phenomena on input pulse energy, wavelength, temporal pulse width, and type of distributed coupling mechanism is described. Existing nonlinear distributed coupler theory is extended to include the effect of an absorptive nonlinearity and the results of this theory are used to explain some of the characteristic features of the experimental results. A value of n₂ ≅ 2 x 10⁻¹⁶ m²/W for the nonlinear coefficient of sputtered ZnO films is obtained.

In this thesis the nonlinear and nonequilibrium properties of graphene are experimentally investigated using degenerate four--wave mixing and time--resolved pump--probe spectroscopy. High quality exfoliated natural graphite and large area epitaxial graphene on silicon carbide are investigated with femtosecond and picosecond ultrafast pulses in the near--infrared. A bespoke technique for suspending exfoliated graphene is also presented. In Chapter 3, the third--order nonlinear susceptibility of graphene is measured for the first time and shows a remarkably large response. Degenerate four--wave mixing at near--infrared wavelengths demonstrates an almost dispersionless emission over a broad spectral range. Quantum kinetic theory is employed to estimate the magnitude of the response and is in good agreement with the experimental data. The large susceptibility enables high contrast imaging, with a monolayer flake contrast of the order 10^{7} times higher than for standard reflection imaging. The degenerate four--wave mixing technique is utilised in Chapter 4 to measure the interface carbon signal of epitaxially grown graphene on silicon carbide. Comparable third--order signal from the silicon carbide bulk prevents true interface imaging. Excluding the third--order emission from detection by elongating the emission to outside a band--pass filter range allows for pure interfacial luminescence imaging. Features within the two growth faces are investigated with Raman spectroscopy. Nonlinear measurements are an increasingly popular tool for investigating fundamental properties of graphene. Chapter 5 investigates the influence of ultrafast pulses on the nonlinear response of graphene. High instantaneous intensities at the sample are shown to reduce the nonlinear emission by a factor or two. Comparing the Raman peak positions, widths and intensities before and after irradiation points to a huge doping of the samples, of the order 500 meV. In Chapter 6 the relaxation of photoexcited carriers is measured via time--resolved pump--probe spectroscopy, where a layer dependence of hot phonon decay is observed. Single layer flakes are observed to relax faster than bilayers and trilayers, with an asymptote reached at approximately four layers. Removing the substrate and measuring fully suspended samples reveals the same trend, suggesting that substrate interactions are not the cause of the enhanced decay. The decay mechanism is therefore intrinsic to graphene, perhaps due to coupling to out--of--plane, flexural phonons. The thickness dependence of epitaxial graphene on silicon carbide is compared to that of exfoliated flakes where the layer dependence is not observed. Phonon relaxation times, however, are in good agreement. Predictions for future investigations into this novel material based on the works here are suggested in Chapter 7. Preliminary pump--probe measurements at high carrier concentrations are an example of such progress, which will offer an insight into further decay mechanisms in graphene.

The principles of nanoengineering of metamaterials which support optical electromagnetic and elastic waves with negative group velocity are described. Extraordinary properties of nonlinear-optical energy transfer between contra-propagating short pulses of electromagnetic and elastic waves are investigated and prospective unique photonic devices are discussed. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35381

Thesis (Ph.D)--Chemistry and Biochemistry, Georgia Institute of Technology, 2010.
Committee Chair: Perry, Joseph; Committee Member: Bunz, Uwe; Committee Member: Lyon, Andrew; Committee Member: Marder, Seth; Committee Member: Trebino, Rick. Part of the SMARTech Electronic Thesis and Dissertation Collection.

The object of the present research is to design and fabricate metal-dielectric thin film multilayer structures that make use of the nonlinear optical (NLO) response of Ag for efficient nonlinear absorption for sensor protection. These structures employ structural resonances to overcome the challenges of reflection and absorption that limit access to this large NLO response. The research consists of three parts: first, we present a comprehensive analysis of the contributions to the nonlinear optical response of Ag. Second, we present a systematic investigation of the linear optical properties of Metal-Dielectric Photonic Band-Gap (MDPBG) structures, including optimization of the structure for a particular transmittance spectrum. Third, we study the linear and nonlinear optical properties of Induced Transmission Filters (ITFs). Each of these parts includes experimental results backed by modeling and simulation.

In this thesis the electrical poling and second order non-linear optical, NLO properties of guest-host, side-chain and main-chain polymers are studied. In bisphenol-A polycarbonate, BPA-PC guest-host systems an enhancement in both the thermal stability and linear electro-optic coefficient can be observed. This can occur in guest- host systems where hydrogen bonding can arise. One example is DAN/BPA-PC guest- host polymer. In side-chain materials a commercially available polymer P-4VP can be poled. This material shows moderate electro-optic response following poling. A novel main-chain type material, where the NLO group is rigidly bonded to the main-chain in a 'V shape is studied for its poling and second-order NLO properties. The rigidity of the polymer is so high that only chain-end groups contribute to the response.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.
Cataloged from PDF version of thesis.
Includes bibliographical references.
Nonlinear optical properties and ultrafast carrier dynamics of slab-coupled optical waveguide amplifiers, silicon nanowaveguides, and III-V semiconductor saturable Bragg reflectors are studied. The limits imposed by two photon absorption and free-carrier absorption on the gain and output powers of an InGaAsP/InP slab-coupled optical waveguide amplifier with a confinement factor of [gamma] = 0.5% are determined. The two-photon absorption coefficient and the induced freecarrier absorption cross-section were measured to be 65cm/GW and 7x10-4 cm2, respectively. The effects of two-photon absorption begin to limit the gain significantly for pulses shorter than 40ps. The carrier recovery times were observed to vary between 390 to 160ps for 1A to 4A bias currents, and the short-pulse saturation fluence of the gain was determined to be 1.4mJ/cm2. Furthermore, nonlinear optical processes in high-index-contrast waveguide circuits consisting of 106nm x 497nm silicon waveguides with Si0 2 and HSQ cladding layers were studied using a heterodyne pump probe experimental setup. The linear loss of the waveguides was determined to be 6.5dB/cm. The two-photon absorption coefficient and free-carrier absorption effective crosssection were determined to be 0.68cm/GW, and 1.9x10-17 cm 2, respectively. Coefficients for the index changes due to optical Kerr effect, and free-carrier density were determined to be 3.2x10- 4 cm 2/W, and -5.5x10-21 cm3. Effects of the proton bombardment on linear loss and carrier lifetimes in the devices were also studied. Carrier lifetime reduction to 33ps with a linear loss of only 14.8dB/cm was achieved using a proton bombardment level of 105 /cm 2. Ultrafast dynamics of semiconductor saturable absorber mirrors were also investigated. The addition of resonant layers to the absorbers resulted in lower saturation fluence and increased non-saturable loss. Proton bombardment was utilized on these devices as well, to decrease the carrier recovery times. With proton bombardment of single-absorber layer devices with 40KeV proton energies at a dose of 1015/cm2, a 1.5ps carrier recovery time was achieved in single-absorber structures.
by Ali Reza Motamedi.
Ph.D.

Kyoto University (京都大学)
0048
新制・論文博士
博士(工学)
乙第12249号
論工博第3998号
新制||工||1440(附属図書館)
26406
UT51-2008-L174
京都大学大学院工学研究科材料化学専攻
(主査)教授 平尾 一之, 教授 横尾 俊信, 教授 田中 勝久
学位規則第4条第2項該当

The successful implementation of nonlinear devices, for example for all-optical switching, depends critically on the availability of appropriate nonlinear optical materials. Most of the currently used methods to measure optical nonlinearities of materials are either indirect or inadequate for separating the fast electronic effects from slow thermo-optic processes. The motivation of this Ph.D. research was to develop a direct and accurate measurement method to evaluate the nonlinear optical properties of various, recently available waveguide materials for all-optical switching applications. A pulse modulated Mach-Zehnder scanning interferometer was built and revised to obtain a resolution of π/100 for nonlinear phase measurements. The evolution of this instrument included the development of single pulse extraction from a mode-locked pulse train, intensity modulation of single pulses, numerical Hilbert transformation of fringe data set, mode profile calculation inside waveguides with a numerical Fourier method, and a careful study of pulse breakup effect associated with instantaneous nonlinear phase shift. Electronic and thermal nonlinear refractive indices of various newly developed materials, especially DANS channel waveguides, DAN single crystal fibers, LiNbO₃ channel waveguide were examined with this method at the 1.32 μm wavelength. For the DAN single crystal cored fibers, the physical origin of the exceptionally large nonlinear phase changes in single crystal fibers was identified to be the cascading of two second order nonlinear processes. In the LiNbO₃ waveguide, cascaded nonlinear phase changes near the second harmonic phase matching temperature were demonstrated for the first time. Based on the results above, single crystal organic fibers appear very promising for ultrafast all optical switching applications. This demonstrates that the interferometric measurement method based on a scanning pulse modulated Mach-Zehnder Interferometer has proven to be one of the best methods for identifying nonlinear materials for all-optical switching applications at the 1.32 μm communications wavelength.

This work presents the nonlinear properties of six Sn(IV) Phthalocyanines. Three of the phthalocyanines are linked by an alkylthiol substituent and the rest are linked with a phenoxy substituent. For all six compounds non-linear optic analysis was carried out in four solvents: chloroform, toluene, dichloromethane, and tetrahydrofuran, and their differences were recorded. Calculation of the linear, singlet excited, triplet excited and two photon absorption cross-sections were also carried out and the results compared. To form a comparison the first order hyperpolarizabilities, DFT calculations were also performed and the results compared to see if the behaviour between the two properties can be predicted using DFT.

This dissertation studies the physics of room-temperature optical nonlinearities in GaAs and their application to the optical logic gates. The microscopic origins of the room-temperature optical nonlinearities in GaAs are investigated experimentally and theoretically. The data of nonlinear absorption measurement are analyzed in the framework of a semiconductor plasma theory in combination with excitation-dependent line broadening. The importance of the plasma screening of the continuum-state Coulomb enhancement and band filling are emphasized for GaAs at room temperature. Optical bistability and optical logic gating are direct consequences of the nonlinear refractive index changes in etalons. The nonlinear index changes are directly measured by a new technique of observing the Fabry-Perot transmission peak shift using the self-photoluminescence as a broad-band source. The validity of a Kramers-Kronig technique under quasi-steady state conditions is crosschecked by an independent measurement of Δn under identical pumping conditions. Thermal index changes are also directly measured to establish the criteria on the temperature stability condition that is needed for reliable operation of devices based on dispersive nonlinearities. Optical logic gates based on dispersive optical nonlinearities may be the critical components of an all-optical computer in the future. Five optical logic functions are demonstrated in a nonlinear GaAs/AlGaAs MQW etalon. Specially designed dielectric mirrors are used to observe low-energy (3-pJ) operation of optical logic gates. Parallel operation using as many as eight optical logic devices is achieved with Wollaston prisms. Toward practical devices, optical logic gating using diode lasers is demonstrated in a setup much smaller than the usual argon-laser pumped dye laser setup. The cycle time of optical logic devices is limited, not by the switch-on time, but by the switch-off time which depends on the carrier relaxation rather than the switch-on time. To reduce the carrier relaxation time windowless GaAs is employed to take advantage of the faster surface recombination of carriers at the GaAs/dielectric mirror interface compared to that at the GaAs/GaAlAs interface. The speed and effectiveness of the windowless GaAs are compared with those of the proton-bombarded GaAs as optical logic gates.

Nonlinear optics is well known to be a highly powerful tool to investigate the properties of the materials. In this thesis we used two important nonlinear optical techniques known as Electric Field Induced Second Harmonic Generation (EFISH) and Harmonic Light Scattering (HLS) to study the first hyperpolarizability of various molecular objects. Firstly, we evidenced the pi donor conjugation in cyclometallated Ir complexes. We have also explored the series of trinuclear organometallic triaryl-1, 3, 5-triazinane-2, 4, 6-triones functionalized by d6-transition metal acetylides complexes at their periphery- large hyperpolarizabilities, far higher than those of related purely organic derivatives are reported. Secondly, a series of dipolar and octupolar dithienylethene (DTE)-containing 2, 2-bipyridine ligands with different metal ions featuring two, four and six photochromic dithienylethene units have been synthesized and fully characterized. The study reveals a large increase in the hyperpolarizability after UV irradiation and subsequent formation of ring-closed isomers. This efficient enhancement clearly reflects the delocalization of the pi-electron system and the formation of strong push&pull chromophores in the closed forms. Thirdly, we have investigated NLO properties of bis (phthalocyaninato) lanthanide-(III) double-decker complexes with crosswise ABAB (phthalocyanine bearing alternating electron-donor and electron-acceptor groups), AB3 (3 donor groups), A4 (4 donor groups) and B4 (no donor groups) ligands. First-order hyperpolarizability is measured and displays the highest quadratic hyperpolarizability ever reported for an octupolar molecule. The direct contribution of f-electrons in coordinated lanthanides ions is also observed on second order nonlinear activity. Finally, gold nanospheres (AuNSs) and gold nanorods (AuNRs) with different aspect ratios (AR) ranging from 1.7 to 3.2 nm have been synthesized by Radiolysis method. Second harmonic intensity collected from AuNRs is clearly higher than that of the nanospheres and reveals their dependence on the AR. We have also mixed the chromophore 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) derivative with AuNRs and observed the enhancement of DAST NLO properties in the presence of AuNRs. A clear increase in the hyperpolarizability (by a factor of 8) of DAST derivatives has been evidence.