Dissertations / Theses on the topic 'Optical thin films'

Evaluation of the wet etching properties of several different thin film oxidesgrown by physical vapour deposition was performed in this work. MgO, Al2O3,SiO2, TiO2, HfO2 ZrO2 and Y2O3 were coated on two types of substrates; Si andborosilicate glass and etching tests were performed in different etchingsolutions. MgF2 thin films have also been evaluated. Important aspects of the choice of the thin films was taken into account in orderto match to good optical properties such as refractive index (n), extinction coefficient (k) and optical thickness (TP) as well as good chemical properties in the wet etching process. A description is made of the physics of optical filters and how a combination of different oxides stacked onto each other can create interference filters. A description of the manufacturing process of the thin films where physical vapour deposition (PVD) was used is presented. Thermal shift of the optical spectra caused by porous coatings was investigated and analyses of the thin films by ellipsometry, surface profilometry and transmission spectrophotometry have been performed. The wet etching properties were evaluated by monitoring the transmission insituon transparent borosilicate glass substrates. A method of how to measure the wet etching rate for different thin films is described. A computer software was used to calculate the Pourbaix diagrams in order to understand the chemical behaviour of the etching solutions. The pH can have a significant impact on the etching behaviour. In case of TiO2, it can be dissolved in an alkaline solution of H2O2. The catalytically process behind this is evaluated. Etching rate for both Y2O3 andSiO2 were matched by adjusting the etchant concentration as a case example. The group IVB oxides are difficult to etch. The catalytic etching of TiO2 with peroxide is slow but detectable. Al2O3, Y2O3 and MgO are reasonably easy to etch but have too low refractive indices to be useful in multilayer optical filters. The In-situ etching instrument was found to be very useful for measuring etching rates.
Utvärdering av våtkemiska egenskaper för flera olika oxidtunnfilmer utfördes idetta arbete på tunnfilmer av MgO, Al2O3, SiO2, TiO2, HfO2 ZrO2 and Y2O3 vakuumdeponerade på både kiselwafers och borosilikatglas. Etstester gjordes med ett flertal etslösningar. Även MgF2-tunnfilmer utvärderades. Både optiska och kemiska egenskaper togs i beaktande vid utvärderingen av tunnfilmerna. De optiska lagar som gäller för tunnfilmer redovisas, bl a hur kombinationer av olika oxider kan skapa interferrensfilter. En beskrivning av tillverkningsprocessen varvid PVD användes presenteras. Termiskt skift av det optiska transmissionsspektrat orsakat av porositet undersöktes. Analyser av tunnfilmerna med ellipsometri, profilometri och transmissions spektroskopi utfördes. Våtetsningsegenskaperna utvärderades genom att mäta in-situ vid etsprocessen på transparenta borosilikatglassubstrat. Metoden för att mäta etshastigheten för olika oxider är beskriven. Datorberäkningar av pourbaixdiagram användes för att skapa en förståelse av de kemiska egenskaperna för etslösningarna. Etsegenskaperna påverkas till stordel av lösningens pH. TiO2 kan etsas i basisk lösning av peroxid. Denna process utvärderades, likaså utvärderades etshasigheten för Y2O3 och SiO2 för att erhålla matchande par avoxider som en fallstudie. Grupp IVB oxiderna är mycket svåra att etsa. Katalytisk etsning av TiO2 med peroxid är detekterbar men långsam. Al2O3, Y2O3 och MgO är förhållandevis enkla att etsa men har för låga brytningsindex för att var praktiskt använbara i optiska multilagerfilter. In-situ etsinstrumentet befanns vara ett utmärkt verktyg för att mäta etshastigheten för tunnfilmer.

Grâce à un long temps de vie de spin, le silicium est un matériau prometteur pour l'électronique de spin. Mais les approches classiques d'étude de la dynamique de spin basées sur la luminescence polarisée ne peuvent pas être utilisées dans ce matériau à cause du faible couplage spin-orbite et du gap indirect. Dans ce travail, nous avons étudié la polarisation de spin des électrons de conduction créée en condition de pompage optique par spectroscopie de photoémission. La surface du silicium est activée en affinité négative par dépôt de césium et d'oxygène de sorte que les électrons photoexcités avec une énergie proche du gap peuvent émis dans le vide. Nous utilisons un laser accordable qui permet de mesurer systématiquement le spectre de polarisation pour des énergies d'excitation allant du seuil d'absorption jusqu'à la bande Gamma2- au-dessus du gap direct. Nous avons obtenus les spectres de polarisation à partir de couches minces SOI d'épaisseurs différentes. A partir de ces résultats, nous déterminons la valeur de paramètres importants de la structure de bande comme le gap direct ou l'énergie du couplage spin-orbite. Cependant, contrairement aux prédictions, lorsque l'épaisseur de la couche de silicium diminue jusqu'à des valeurs inférieures à la longueur de diffusion de spin, la polarisation en spin des électrons émis reste proche de zéro (-0.4%), remettant en cause l'interprétation directe des valeurs théoriques de la polarisation initiale égale à -20%. Une approche théorique a donc été développée sur la base d'un modèle ab initio de structure de bande pour déterminer les spectres de polarisation en spin. Ces calculs sont encore en cours, mais les résultats déjà obtenus sur la structure électronique du silicium sous contrainte indiquent une piste intéressante pour les études futures.

V této práci je prezentována inovativní metoda zvaná \textit{Zobrazovací Reflektometrie}, která je založena na principu spektroskopické reflektometrie a je určena pro vyhodnocování optických vlastností tenkých vrstev .\ Spektrum odrazivosti je získáno z map intenzit zaznamenaných CCD kamerou. Každý záznam odpovídá předem nastavené vlnové délce a spektrum odrazivosti může být určeno ve zvoleném bodu nebo ve vybrané oblasti.\ Teoretický model odrazivosti se fituje na naměřená data pomocí Levenberg~-~Marquardtova algoritmu, jehož výsledky jsou optické vlastnosti vrstvy, jejich přesnost, a určení spolehlivosti dosažených výsledků pomocí analýzy citlivosti změn počátečních nastavení optimalizačního algoritmu.

The columnar microstructure of most thermally evaporated thin films detrimentally affects many of their properties through a reduction in packing density. In this work, we have investigated ion-assisted deposition as a means of disrupting this columnar growth for a number of coating materials. A Kaufman hot-cathode ion source bombarded thermally evaporated films with low-energy (< 1000 eV) positive ions during deposition in a cryopumped box coater. We have investigated MgF₂, Na₃AIF₆, AIF₃, LaF₃, CeF₃, NdF₃, Al₂O₃, and AIN. Argon ion bombardment of the fluoride coatings increased their packing densities dramatically. We achieved packing densities near unity without significant absorption for MgF₂, LaF₃, and NdF₃, while Na₃AIF₆, AIF₃, and CeF₃ began to absorb before unity packing density could be achieved. Fluorine was preferentially sputtered by the ion bombardment, creating anion vacancies. The films adsorbed water vapor and hydroxyl radicals from the residual chamber atmosphere. These filled the vacancy sites, eliminating absorption in the visible, but the oxygen complexes caused increased absorption in the ultraviolet. For LaF₃ and NdF₃, a sufficient amount of oxygen caused a phase transformation from the fluoride phase to an oxyfluoride phase. The refractive indices of Al₂O₃ films increased with ion bombardment. Values as high as 1.70 at 350 nm were achieved with bombardment by 500 eV oxygen ions. Since all of the Al₂O₃ films had packing densities near unity and were amorphous, we postulate that the increase in refractive index was due to a change in amorphous networking. Aluminum nitride was deposited by bombarding thermally evaporated aluminum with nitrogen ions. Films with N:Al ratios of 0.5-1.5 could be deposited by varying the deposition conditions. Films with low absorption for wavelengths longer than 1 μm could be deposited. Annealing the films at 500°C eliminated absorption at wavelengths longer than 500 nm.

In the past decades new interest in diamond and possible applications of diamond has emerged. In this master thesis, optical properties of thin films of diamond were investigated for the possible use as an Internal Reflection Element (IRE) in Attenuated Total Reflection Fourier Infra Red (ATR-FTIR) spectroscopy. First a theoretical study of optical properties was made using Fresnel’s equations. Secondly the ray-tracing software TracePro was tested as a possible tool for further investigations of optical properties in advanced geometries. Finally, experimental measurements were made to compare the simulated result with measured data. Results obtained with TracePro were consistent with the theoretically expected results as to dependence of layer thickness, polarization and wavelength, and TracePro was therefore a possible tool for the investigation of optical properties in thin films. Although no extra information was given from TracePro for this specific geometry, TracePro can be a valuable tool in the investigation of more advanced geometries. The experimental measurements showed the importance of a well-defined layer thickness and also showed a stronger dependence of the underlying coating materials than expected from simulations. Further investigations need to be made to fully understand the dependence of the underlying coating materials SiO2 and Si3N4, particularly regarding different polarization states.

Some basic concepts and principles of optical dielectric slab waveguiding and experimental methodology involving characterisation of waveguide films are introduced, Results from the characterisation of thin polymeric waveguide films and measurements of refractive indices of the films are presented. The birefringence of some polymer films is analysed and discussed. The photostabilities of several dopants (DEMI, Ultra-DEMI, Dicyclohexyl-DEMI, Mor2, Morpip and DCM) are investigated in a polymer matrix (PMMA), and their measured photostabilities are presented. These organic chromophores change their properties in the course of photodegradation. Degradation experiments are carried out by exposing the doped waveguide films to light in air, vacuum and nitrogen environments. The degradation mechanisms of these chromophores are discussed. It is found that the degradation of the DEMI, Ultra-DEMI, Dicyclohexyl-DEMI and DCM are due to photooxidation, their photostabilities are much higher in vacuum than in air. The Mor2 and Morpip degrade by direct photodecomposition, their photostabilities are in the same order when exposed to light in their main absorption bands. The oxygen free environment (e.g. vacuum) is essential to increase their photostabilities. A beam branching effect in DCM doped waveguide film is observed. Stacked multi-layer waveguides are investigated as possible humidity sensors. Symmetric structure (PMMA/P-4VP/PMMA/P-4VP/PMMA) (P4VP-I) and unsymmetric structure (Si02/P-4VP/Zeonex/P-4VP/Air) (P4VP-II) are studied. Special procedures and process have been developed to fabricate multi-layer waveguide structures in experiments. It is found that both structures have good reversibilities and show reasonable stabilities. 30 ppm concentrations of water vapour can be detected by the P4VP-II structure. The experimental results show that the overall response of P4VP-II structure exhibits good linearity with increase of the concentration of water vapour. The structures can not only measure the phase shift of interference, but also can measure the direction of fringe movement. The sensitivity of the structure can be further improved by using different combinations of polymers in the structure.

Silicon based hydrogenated amorphous semiconducting (intrinsic and n/p doped a-Si:H and a-Si1-xCx:H) thin films have been deposited by plasma enhanced chemical vapor deposition (PECVD) system. In order to analyze the optical response of these amorphous films, intrinsic optical absorption mechanisms have resumed and spectral variations of absorption coefficient &
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(E) are derived. The exponential variation of absorption coefficient for energies below the band edge is discussed in the frame of randomly distributed square well like potential fluctuations of localized states. Urbach constant EU and the slope B are deduced as disorder parameters. Both intensity sensitive transmittance and reflectance, and amplitude/phase sensitive ellipsometric techniques for multilayer thin films are theoretically and practically treated. Various methodologies are developed for the determination of thickness, refractive index and absorption coefficient of the films. A reflectance unit is adapted to the spectrometer and all the measuring instruments are computerized and relevant software packets have been developed. IR spectroscopy has been used for determination of mainly hydrogen concentrations and bonding properties. Establishing the production-characterization-improved growth conditions cycle successfully, the following results are obtained: (a) determination of lateral inhomogeneity of films along the radial direction of the plasma reactor, (b) determination of vertical inhomogeneity due to both substrate and air ambient, (c) perfect adjustment of refractive index and band gap of a-Si1-xCx:H films by changing carbon content of the films, (d) effect of plasma power density on both growth and carbon content.

A microcomputer software package was written to calculate the tranmission and reflection characteristics of multilayer thin films on a substrate. The program was written to be as "user-friendly", versatile and modular as possible. To test the program, a transparent oxide-type heat-mirror film capped with an antireflection coating was studied for greenhouse applications. Si0₂ and ZnO were considered as representative antireflection and heat-mirror materials respectively. The results of calculations of heat transfer coefficient and transmittance of the glazing construction polyethylene/air gap/Si0₂/ZnO/polyethylene are presented. The resulting structure is shown to give a visible light transmission spectrum which closely matches the plant sensitivity curve for plant growth and has higher thermal insulation compared to uncoated polyethylene.
Science, Faculty of
Physics and Astronomy, Department of
Graduate

Chloride, bromide, pyridinium and quinolinium homologues of 4-(N- hexadecylpyridinium-4-ylmethylidene-amino)-2,6-dichlorophenolate have been investigated in solution, Langmuir and Langmuir-Blodgett films. Techniques including spectroscopy, surface potential measurement, quartz crystal microbalance, surface plasmon resonance, atomic force microscopy, reectometry and X-ray diffraction have been used to characterise these molecular systems. In solution, solvatochroism was observed and Benisi-Hildebrand analysis revealed dimeric aggregation. Langmuir monolayers were compressed at the air/water interface and chromophore rotation was observed by surface potential measurement. Langmuir- Blodgett monolayers showed lm-thickness dependence on the deposition-pressure. Monolayer thicknesses between 6-24Ä were measured using SPR and molecular areas between 40-l25Ä2 were measured using a quartz crystal microbalance. Both the molecular/s/area)and monolayer thicknesses were deposition-pressure dependent. The high tilt phases were visually distinguishable from the low tilt phases using atomic force microscopy, The compounds showed phase behaviour that was predominantly alike for the bromide and chloride homologues but different for the pyridinium and quinolinum homologues. Multilayer Y-type films of the merocyanine dyes were analysed using reectometry and deposition-pressure dependent thicknesses were found. Alternate layer structures of NLO-active hemicyanine amphiphiles were used to achieve homogeneous. orientation ordering using active and inactive spacer layers. Ordering was achieved but the optical efficiency was reduced by high proportions of inactive material and interlayer dipole formation. Double chained hemicyanine molecules were used to form Z-type structures and subsequent layers were found to significantly interdigitate. Different chain lengths were found to interdigitate by the length of the shortest chain. Gas detection experiments were undertaken on the quinolinium, dichloro merocyanine using three optical geometries. The absorption method showed slow switching and poor sensitivity. The Kretschmann SPR geometry showed high sensitivity and rapid switching. The grating SPR geometry showed rapid switching but was less sensitive than the ATR method. Protonation of the monolayers was investigated using hydrochloric acid gas, acetic acid vapour and stearic acid immobilised within the lm.

This thesis reports on the development of photosensitive and piezoelectric thin films for optical devices. The thesis is in two parts, the first concentrating on research into novel photosensitive materials for optical Bragg gratings, while Part II studies the development of piezoelectric ZnO thin films for acousto-optic devices. Bragg gratings are an important element in the development of optical communications systems. Strongly photosensitive optical waveguiding materials are required to achieve more efficient Bragg gratings in both planar and fibre waveguides. In this thesis, metal oxides are studied for photosensitivity in their pure state as RF sputtered thin films. Photosensitivity is demonstrated for the first time in RF sputtered germanium oxide and tantalum oxide thin film waveguides via Bragg diffraction. A side-polished fibre coupler, capable of monitoring both the magnitude and sign of an index change, is then used to establish large, negative and permanent index changes in tantalum oxide thin films. UV-induced changes in absorption, measured in a similar tantalum oxide film, are contrasted with the observed index change via a Kramers-Kronig analysis, while a UV-induced reduction in film reflectivity consolidates the fibre coupler result. ZnO, with a kt of 0.28, is an efficient piezoelectric material, and thin ZnO transducers permit operating frequencies of up to 3.5 GHz. In addition, ZnO transducers can be configured on curved surfaces, whose focusing properties greatly enhance device efficiency. In contrast, the operating frequency of LiNbO3 transducers is limited to approximately 0.8 GHz and curved transducers are a problem. Part II of this thesis discusses the fabrication and optimisation of ZnO transducers. Acousto-optic beam deflection by ZnO transducers on TeO2 crystals is demonstrated for the first time, and efficient in-line fibre phase modulation is achieved using a annular ZnO transducer on an optical fibre.

Electro-optical properties of thin film are of great interest owing to the perpetual demand for miniaturization and higher speed devices for communication, electronic, and biomedical applications. The thickness of polymer films developed for these applications has decreased dramatically making interfacial effects significant. It is well documented that, in submicron thickness range, both film/substrate & film/air interface are critical. In this study, we probe the dynamics of electro-optical polymer thin films in the sub-micron thickness regime to understand interfacial effects. The polymer chain dynamics of Polypropylene oxide (PPO) under electric field are investigated in this study. The effects of electric field strength, frequency, and polymer molecular weight on the polymer chain dynamics under electric field are studied. Experimental results show that PPO exhibits both piezoelectric and electrorestrictive effects at significantly high frequencies (101kHz range). Conventional organic materials are responsive only at frequencies in <1kHz range. A high signal-to-noise ratio differential interferometry is designed to quantitatively study the effects of film thickness, electric field frequency and amplitude on the dynamic properties of PPO thin films ranging from 30 nm to 400 nm. The interferometer can concurrently monitor the index of refraction, thickness change of polymer films, and birefringence due to the applied electrical field.
Master of Science

Since the discovery of a class of superconducting materials with critical temperatures as high as 125 degrees kelvin, there has been a great deal of research interest in their possible application to optical radiation detection, particularly in the infrared spectrum. The motivation for this research is the promise of a fast detector operating at elevated temperatures that is sensitive to low level optical signals and that operates out to the far IR. It has been shown that thin films of these high temperature superconductors (HTS) exhibit a change in their electrical properties when exposed to optical radiation. However, in order to make a practical detector out of HTS materials, the mechanisms of this response must be fully understood. The purpose of this research is to investigate the spectral, temporal and thermal characteristics of this electrical response in an effort to better understand the mechanisms involved.

Optical microcavities can find broad application areas, like tunable and compact sources, dynamic filters for optical communications, biological and chemical sensors, etc. Optical microcavities are key component allowing one to obtain compact laser devices exhibiting small cavity volume and low threshold. Among the different resonators architectures for microcavities laser, photonic crystal (PC) structures are one of the most promising. These structures feature the periodicity in one or more dimensions, and are resonant for light waves of a specific wavelength. Two-dimensional PCs are planar dielectric waveguide where photons are vertically confined by the vertical profile of the optical index, while the crystal periodicity acts only in the slab plane. In photonic crystals the refractive index contrast of the periodic structure is high enough to open a full band gap, and thus to fully confine light at very small scale. Strong coupling is theoretically feasible, and quality factor of more than 106 have been experimentally achieved for small modal volumes. Substantial additional gains are possible with confinement improvement in microfabrication techniques and with implementation of low loss design. Photonic crystal whose index contrast is lower can be used as DFB gratings. In a DFB device, the laser modes receive feedback at one specific wavelength, determined by the grating period of the structure. The feedback strength is related to the coupling constant, ?, which in turn depends on the grating index contrast, and to the grating length, L. The ?L product must be high enough to ensure the feedback required for lasing. In an optically pumped laser, an external source supplies the excitation energy necessary to get the population inversion. To do that, it must be on resonance with one of the absorption transitions of the active medium. When the external source provides enough energy, the active material exhibits gain: more photons are generated than lost. For intense incoming beams, i.e. intense laser sources, multiple-photon absorption processes become appreciable. It is thus possible to have absorption also by pumping with sources having photon energies lower than the resonance energy of the active medium. The two-photon absorption (TPA), described from the 3rd order susceptibility, involves the simultaneous absorption of two photons with energy: E_exc-E_ground=2?? The absorption of the first photon causes the promotion of the electron to the virtual level. Here the simultaneous absorption of the second photon promotes the electron to the real excited state. The system can then decay to the ground state emitting an up-converted photon, i.e., a photon having energy higher than that of the exciting ones. A large part of this work is devoted to the realization and characterization of active microresonators behaving as laser sources. Two main research subjects will be pursued: An integrated InP semiconductors photonic crystal microcavity laser, operating in the telecommunication wavelength. A distributed feedback laser for two-photon induced IR-to-visible up-conversion lasing. Within the second subject, particular attention will be devoted to the characterization of the two-photon induced emission properties of organic push-pull dyes and II-VI semiconductors quantum dots (QDs) to evaluate their potentiality as candidates for all-solid-state up-converted laser devices. The third reported research subject is the exploitation of hybrid silica-titania sol-gel films for UV lithography application, finalized to the production of surface relief gratings. MICROLASER BASED ON EFFECTIVE INDEX CONFINED, SLOW-LIGHT MODES IN PC WAVEGUIDES This research regards the study of photonic crystal microcavity having small mode volume V and high quality factor Q, for the production of low threshold integrated laser devices. The light propagation inside the PC is modified because of its periodicity. In this study we exploit the low-light guided modes at the high symmetry point of the dispersion curve of a PC-W1 waveguide. The PC-W1 waveguide is a PC having triangular symmetry with a missing row of hole along the ?K direction. The linear defect entails the appearance of defect modes with frequencies localized inside the unperturbed PC band gap, and thus modes that exponentially decay inside the PC. The band associated with the defect mode becomes flat at the K point of the band diagram, leading to slow-modes whose group velocity goes to zero. The lateral confinement of low-group velocity modes is controlled by locally changing the refractive index of the two dimensional photonic crystal waveguides. The index modulation is induced by post-processing a dielectric strip on top of the two-dimensional PC waveguide. This results in a photonic heterostructure whose confinement properties are the result of the effective index shift and the local curvature of the band associated with the waveguide mode. In this thesis the results of the device simulation, experimental realization and characterization will be reported. Computational tools, such as MPB and 3D-FDTD software have been used for the device design and for the study the electromagnetic field behavior inside the cavity. The realization of the PC structure has been accomplished through lithography techniques like e-beam lithography and reactive ion etching. Intense clean-room activities have been necessary to reach optimized structure quality. The characterization of the microcavity laser has been pursued with a proper optical set-up, in such a way to determine its performance. UP-CONVERTED LASING The up-converted lasing is an alternative method to convert the emission of a cheap, easily available IR laser into that of a more technological valuable visible laser. It involves the two photon pumping (TPP) mechanism, i.e., the NLO system is excited through the simultaneously absorption of two photons in the near-IR range. In this work we report our effort towards the realization of a solid-state visible laser device based on a TPP induced emission. The starting point of this technology is to find a system able to efficiently convert the IR incoming radiation to a visible one. We have studied the up-conversion process both in push-pull organic dyes embedded in sol-gel hybrid films and in semiconductor core-shell CdSe-CdS-ZnS quantum dots embedded in zirconia films. The excitation source is an amplified Ti:Sapphire laser at 800 nm. Concerning the organic compounds, it has been possible to characterize the emission properties only in solution, because of their poor photostability when they are embedded in sol-gel matrices. On the opposite, quantum dots embedded in zirconia films show promising amplified emission properties, with interesting gain value and extremely long time stability. We have investigated the possibility to implement this material with a distributed feedback optical resonator for obtaining compact and integrated laser devices. The grating parameters have been determined with MPB software, and first attempts of e-beam lithography of the pattern have been done. We have also prepared a devoted optical set-up for the optical characterization of the laser devices. SILICA-TITANIA SOL-GEL FILM FOR DIRECT PHOTOPATTERNIG APLICATIONS The possibility to exploit the photocatalytic action of hybrid silica-titania sol-gel film towards the decomposition of their organic component, for the direct patterning of surface structure has been investigated. These films have been characterized to study their microstructural properties, and to confirm the presence of crystalline titanium oxo-clusters. Their photocatalytic efficiency has been measured using stearic acid as reference material. To test the potentiality of this system for UV-lithography, it has been exposed to a UV-lamp. The organic component decomposition leads to a film shrinkage of about 60%, accompanied by a refractive index increases of about 0.1 By irradiating the spin-coated films through an UV-mask, structures of different shapes and micrometer dimension have been achieved.
Lo studio delle microcavità ottiche riveste un grande interesse per applicazioni in svariati campi, quali la ricerca di sorgenti laser tunabili e compatte, filtri per le telecomunicazioni, sensori chimici e biologici, etc. Le microcavità ottiche sono fondamentali per l’ottenimento di dispositivi laser compatti, aventi bassa soglia di emissione laser, ove il campo elettromagnetico è confinato in volumi estremamente ridotti, con conseguente aumento dell’interazione radiazione-materia,. Tra le possibili architetture della cavità risonante, per dispositivi pompati otticamente, i cristalli fotonici rappresentano una delle soluzioni più promettenti. Questi ultimi sfruttano la periodicità in una o più direzioni e sono risonanti con determinate lunghezze d’oda della radiazione elettromagnetica. In un cristallo fotonico bidimensionale il confinamento verticale è garantito dal profilo verticale dell’indice di rifrazione, mentre il confinamento nel piano del cristallo è opera della strutture periodica. Nei cristalli fotonici il contrasto di indice di rifrazione della struttura periodica è tale da aprire un intervallo completo di energie proibite per la propagazione della radiazione nel mezzo. Essa può quindi essere confinata in volumi molto piccoli, dell’ordine del cubo della lunghezza d’onda, con fattori di qualità sperimentali superiori a 106. Inoltre i valori ottenuti sperimentalmente sono inferiori a quelli previsti teoricamente, e ulteriori passi in avanti saranno possibili con lo sviluppo delle tecniche litografiche e di produzione del materiale attivo. I cristalli fotonici nei quali il contrasto di indice di rifrazione è insufficiente per aprire un band-gap completo si comportano come reticoli distributed feedback, DFB. In un dispositivo DFB, i modi risonanti ricevono il feedback a lunghezze d’onda specifiche, determinate dal periodo del reticolo. La forza dell’accoppiamento è legata alla costante di accoppiamento ?, la quale, a sua volta, dipende dal contrasto di indice nel reticolo e all’estensione totale del reticolo. Il prodotto ?L deve essere sufficiente per garantire il feedback richiesto per l’emissione laser. In un laser a pompaggio ottico, una sorgente esterna fornisce al mezzo attivo l’energia di eccitazione richiesta per raggiungere l’inversione di popolazione, requisito necessario per ottenere il guadagno all’interno del mezzo e quindi l’amplificazione. Affinché si verifichi assorbimento, l’energia del fascio di pompa deve essere in risonanza con una delle transizioni del mezzo attivo. Per campi incidenti molto intensi, come possono essere quelli legati a fasci laser focalizzati, diventano tuttavia apprezzabili anche fenomeni di assorbimento multi fotonici. Si può quindi avere assorbimento anche utilizzando sorgenti di pompa aventi energie inferiori all’energia di risonanza del mezzo attivo. L’assorbimento a due fotoni (TPA), legato alla suscettibilità non lineare al terzo ordine, comporta l’assorbimento simultaneo di due fotoni, con energia: E_exc-E_ground=2?? L’assorbimento del primo fotone promuove l’elettrone dallo stato fondamentale a uno stato virtuale, dal quale esso passa immediatamente allo stato eccitato attraverso l’assorbimento simultaneo di un secondo fotone incidente. Infine il sistema può tornare allo stato fondamentale, attraverso l’emissione di un fotone a energia superiore rispetto ala pompa. Gran parte del lavoro di dottorato è incentrato sulla realizzazione e caratterizzazione di microcavità attive per l’ottenimento di sorgenti laser. All’interno di tale attività sono stati studiati due sistemi differenti: Una microcavità laser a semiconduttore, realizzata sfruttando le proprietà dei cristalli fotonici bi-dimensionali, che emette alla lunghezza d’onda delle telecomunicazioni. Un dispositivo laser DFB, pompato oticamente a due fotoni, per la conversione di emissione laser dall’infrarosso al visibile. All’interno della seconda tematica, particolare attenzione è stata rivolta alla caratterizzazione delle proprietà di emissione indotta a due fotoni di un cromoforo organico e di quantum dots di un semiconduttore II-VI, il CdSe, entrambi inglobati in matrice sol-gel. Un terzo soggetto è costituito dallo studio delle proprietà foto catalitiche di film sol-gel ibridi a base di silica e titania, in vista di possibili applicazioni per il patterning diretto tramite radiazione UV. CONFINAMENTO DI MODI LENTI IN GUIDA D’ONDA A CRISTALLO FOTONICO PER L’OTTENIMENTO DI MICROCAVITA’ LASER Questa ricerca riguarda lo studio di cavità, ottenute sfruttando cristalli fotonici bidimensionali, a basso volume modale e alto fattore di qualità Q, finalizzate all’ottenimento di dispositivi laser integrati a bassa soglia. Questo lavoro si basa sull’utilizzo dei modi guidati lenti corrispondenti al punto ad elevata simmetria K della curva di dispersione di una guida d’onda W1-PC. Una guida d’onda W1-PC si ottiene da un cristallo fotonico a simmetria triangolare, attraverso la rimozione di una fila di buche lungo la direzione ?K. In questo modo si introduce un difetto lineare, il quale si riflette nella comparse di modi del difetto, aventi frequenze localizzate all’interno del band-gap del cristallo fotonico, che pertanto decadono esponenzialmente all’interno del cristallo. Le bande associate ai modi del difetto hanno curvatura nulla in corrispondenza dei punti a elevata simmetria, e ciò implica una velocità di gruppo del modo nulla in corrispondenza di tali punti. L’estensione laterale dei modi lenti viene controllata agendo sull’indice di rifrazione del cristallo fotonico, in modo da creare una etero struttura in grado di confinarli efficacemente. L’indice effettivo della guida viene modificato localmente depositando un film di polimero all’interfaccia superiore della guida. La forza del confinamento dipende dall’entità della variazione dell’indice e dalla curvatura della banda associata al modo lento. L’attività svolta all’interno di questo progetto consiste nel design della struttura, nella sua realizzazione sperimentale e infine nella caratterizzazione ottica del dispositivo. Per ottimizzare i parametri del dispositivo e comprendere il comportamento della radiazione elettromagnetica all’interno della cavità, sono stati impiegati strumenti di calcolo computazionale, quali i software MPB e TESSA 3D-FDTD. I parametri delle simulazioni sono stati poi utilizzati per la realizzazione del cristallo fotonico, effettuata tramite tecniche litografiche, quali la litografia con fascio elettronico e l’etching ionico. La caratterizzazione ottica del dispositivo è stata effettuata con un apposito set-up, al fine di determinarne le prestazioni. EMISSIONE LASER CON CONVERSIONE DI FREQUENZA La conversione di frequenza laser fornisce l’interessante possibilità di convertire una sorgente laser economica e di facile reperibilità nell’infrarosso, in una sorgente laser nel visibile di enorme interesse tecnologico. Essa si basa sull’emissione indotta a seguito di processi di assorbimento a due fotoni nel vicino IR. In questo lavoro verranno presentati gli sforzi profusi e i risultati preliminari ottenuti nella ricerca di un dispositivo laser allo stato solido per la conversione di frequenza. A tal fine sono state investigate le proprietà di conversione di un cromoforo push-pull organico disperso in matrici sol-gel ibride, e di quantum dots di semiconduttore II-VI, CdSe-CdS-ZnS, dispersi in una matrice inorganica a base di zirconia. Il composto organico presenta interessanti proprietà di emissione indotta a due fotoni in soluzione. Tuttavia la sua scarsa resistenza al pompaggio ottico in matrice solida preclude un suo possibile impiego e rende estremamente problematica la stessa caratterizzazione ottica. Al contrario i film di QDs-ZrO2 mostrano una buona efficienza di conversione di frequenza, con valori di guadagno per l’emissione spontanea amplificata interessanti, e elevata stabilità del segnale emesso nel tempo. E’ stata pertanto studiata la possibilità di implementare i film di QDs-ZrO2 all’interno di una cavità risonante di tipo distributed feedback per ottenere un dispositivo laser compatto e integrabile. I parametri del reticolo sono stati determinati con il software MPB e sono stati fissati in modo da avere amplificazione in corrispondenza del massimo di emissione dei QDs. Sono tutt’ora in corso delle prove di realizzazione del reticolo DFB tramite litografia elettronica su film sol-gel appositamente sviluppati per il patterning diretto. Infine è stato messo appunto un set-up dedicato per la caratterizzazione ottica dei dispositivi prodotti. FILM SOL-GEL IBRIDI A BASE DI SILICA-TITANIA PER IL PATTERNING DIRETTO CON LUCE UV E’ stata studiata l’attività fotocatalitica di film sol-gel ibridi a base di silica-titania, promossa dalla radiazione UV. I film sono stati caratterizzati a livello micro strutturale tramite spettroscopia infrarossa, e sono stati osservati al microscopio elettronico per confermare la presenza di cluster di titanio cristallino al loro interno. L’efficienza del processo di fotocatalisi è stata determinata mediante test standard che si avvalgono dell’acido stearico come materiale di riferimento. Quest’ultimo infatti è in grado di simulare efficacemente i comuni inquinanti organici, è può essere depositato facilmente per spin-coating. Successivamente è stata valutata la possibilità di sfruttare l’attività foto catalitica per il patterning diretto dei film. Tale studio parte dall’osservazione che la fotocatalisi si manifesta anche nei confronti della componente organica dei film sol gel ibridi.. Questo fenomeno è accompagnato da una diminuzione dello spessore del film, fino al 60% sullo spessore iniziale, e può pertanto essere sfruttato per la realizzazione di strutture a rilievo. Test di patterning diretto sono stati effettuati irradiando il film con una lampada UV attraverso una maschera in quarzo, ottenendo strutture di dimensione micrometrica ben definite.

We established measurement and analysis techniques necessary to investigate the optical properties of some transition metal compounds: specifically the carbides and nitrides of Ti, Zr, and Hf. Two distinct techniques determined the optical constants of these films: a Nestell-Christy method to invert measurements of thickness, reflection, and transmission and a Kramers-Kronig analysis of reflection. The compositions of the samples were evaluated by X-ray diffraction, Auger spectroscopy, scanning electron microprobe analysis, and nuclear analysis. We found it possible to correlate these materials' optical and electronic properties and relate these to compositional changes. The ability to engineer a specific optical response of materials is discussed. Additions of carbon and nitrogen change the optical properties in a specific manner; the roles of these elements as electron donors and their effect on the location and population of the d electron bands with respect to the Fermi level are postulated.

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 development of materials for optical thin film application is essential to progress in fields such as optical data storage and signal processing. Samarium sulfide (SmS) thin films were prepared by reactive evaporation of samarium in hydrogen sulfide (H₂S). These displayed optical switching properties despite the presence of large amounts of carbon and oxygen. They are therefore potentially useful for data storage. The semiconductor to metal phase transition was characterized by x-ray diffraction and spectrophotometry. The observed optical response was modelled by a Bruggeman effective medium calculation. Success with this analysis suggests it as a means for predicting performance in subsequent applications. Zinc sulfide (ZnS) thin films were prepared by molecular beam epitaxy (MBE). Implimentation of an H₂S treated silicon surface provided good chemical bond match in addition to a good lattice match. Atomic layer epitaxy was unsuccessfully explored as a means to grow ZnS from zinc and H₂S reactants, therefore other reactants are proposed. Both the MBE and ALE work is directed at the long term goals of producing p-type ZnS, which is suitable for semiconductor lasing at short wavelengths, and high quality SmS thin films.

The goal of the thesis is to make contributions to the development of two new technologies for data storage: perpendicular recording and magneto-optic recording. CoCr and rare earth-transition metal multilayers are the most suitable media for perpendicular recording and magneto-optic recording technologies, respectively. In part A of the thesis, magnetic properties of CoCr thin films produced by dc magnetron sputtering are studied for various deposition conditions. Dielectric constants and extraordinary Hall effect are also studied to provide information complementary to magnetic properties. In part B, new methods are developed for theoretical analysis of the magneto-optics of rare earth-transition metal multilayers, which can be used to optimize the readout of the recording system. Part A For dc magnetron sputtered CoCr films the perpendicular and parallel magnetic coercivities are found to be mainly controlled by the substrate temperature during film growth. Substrate temperatures between 180 and 300 C are necessary to fabricate CoCr thin films for recording media. Films produced in this manner have magnetic anisotropy constants ranging from —1.0 to +0.5 10⁶erg/cc. The magnetic anisotropy has a complicated dependence on a large number of deposition parameters and can be best controlled by the dc sputtering power and the target-to-substrate distance. Based on microstructural analysis film properties are interpreted in terms of the adatom diffusion during film growth. It is found that high adatom mobility and low deposition rate promote positive magnetic anisotropy. The dielectric constants measured by ellipsometry are found to depend on the film thickness because of the change in film morphology during film growth. The effects of asymmetric sputtering are analyzed, and the relationship between the extraordinary Hall effect and the magnetic properties is investigated. Part B The 4x4 matrix method proposed by Lin-Chung and Teitler[P. J. Lin-Chung and S. Teitler, J. Opt. Soc. Am. A 1 703(1984)] is applied to the magneto-optics of the rare earth-transition metal multilayer system. Based on a plane wave model, the above method enables one to calculate the sensitivity of the readout to the layer thicknesses as well as effects of oblique angle of incidence, anisotropy in the nonmagnetic part of the dielectric constants and misalignment of the magnetization. Finally, an improved model is presented to take into account the fact that the reading laser is a strongly focused beam instead of a plane wave. This new model is used to optimize the magneto-optic multilayer system. When the focal spot size of the reading laser beam is less than about three wavelengths, significantly different results are obtained from the focused beam and the plane wave models.
Science, Faculty of
Physics and Astronomy, Department of
Graduate

The effect of the configuration of the confining magnetic field on the operating characteristics of a planar magnetron has been observed. In particular, the heat load and charged particle bombardment of the substrate were measured. A circular planar magnetron was constructed with a magnetic field which could be controlled by the adjustment of the current flowing through coils placed around the circumference and with a separate supply feeding a coil around the centre pole. This allowed the magnetic field configuration to be unbalanced and caused the negative glow plasma to be incident on the substrate. It was found that the "unbalanced" electrically-controlled model could give six times the heat load to a substrate in comparison with a balanced permanent magnet structure. Such a load resulted from the bombardment of the substrate with the plasma (revealed by the measurement of the I-V characteristics), and gave a 30 volt negative bias as well as saturated electron and ion currents of around 90 and 1 mA respectively (a magnetron operating current: 0.8A). The heat load was lkWm⁻². This compared to a 1 volt positive bias with current of 1.4 and 0.5 mA for the balanced case. This bombardment was used as a neutralised ion beam to enhance film growth during sputtering. The effect of the magnetic field configuration has been observed on the operating characteristics of a planar magnetron. In particular, the deposition rate, the currents to a probe and heat load, as a function of the outer pole solenoid current, were measured. The deposition rate/input power was constant at 19 mÅ/s/W, independent of the outer pole solenoid current. In the region from 20 to 40 A of the outer pole solenoid current, the configuration of the magnetic field seemed to be concentric and the electrons were confined by it. But from 40 to 80 A, the magnetic field became dispersed outwards along the direction to the magnetron and the electrons followed it. Consequently, following bombardment of the surface by electrons and neutralising ions, the currents to a probe and the heat load, gave a behaviour. The effect of the pressure has been observed on probe current, floating potential and heat load. With the pressure increasing, they decreased. It seems that the mean free path of particles becomes shorter and then scattering of them increases. The influence of the magnetic field strength on the plasma, created at the rear of a substrate, has been observed. In particular, the floating potential and the charged particle bombardment of the substrate were measured. Permanent magnets at the rear of the substrate, outside of the chamber, could make the configuration of the magnetic field change and control the floating potential from -3 to -59 V. At the same time, they influenced the current-voltage characteristics of a probe and made it change following the direction of the field. Ti0₂ is a very important thin film material in optics because the refractive index is high and it can also satisfy the requirements of a hard, dense and chemically stable coating. In our study, the effect of the floating potential, and the resulting ion beam bombardment on the properties of Ti0₂ thin films deposited by reactive planar magnetron sputtering, has been observed. In reactive magnetron sputtering of Ti0₂, as the partial pressure of oxygen is increased, reaction products form on the target (it is poisoned) which lead to an unstable situation with the cathode switching from metal to oxide uncontrollably. More sophisticated control techniques are required in order to get a stoichiometric Ti0₂, film. The oxygen flow was controlled by observation of the light emission of the Ti spectral line. This was measured using a band pass filter and a photomultiplier. It maintained the oxygen flow by regulating a piezovalve. The optical emission controller gave a complete range of Ti to oxygen ratios. As a result, we obtained samples with a high refractive index of 2.52 at 633 nm and a high deposition rate of 4 Å/s. The properties measured were the refractive index, the surface composition and morphology, and the crystallinity. The relationship between the refractive index and the proportions of 'anatase' and 'rutile' crystal structures of the Ti0₂ films was especially considered and the dependence on the floating potential was assessed. Also, Al₂0₃, Cuo, ZnO have been sputtered and the effect of pressure instability assessed. Their properties were measured.

Passive intrinsic bistability is of great interest. Our Fabry-Perot interference filters exhibit bistability due to a temperature dependent refractive index. Operating across the entire viable spectrum permits wide application to the demonstration and development of concepts for optical computing. Thermal evaporation of zinc sulfide and chiolite is used to produce Fabry-Perot interference filters. The effect of the thickness of the spacer, and reflectivities of the mirrors, on the nonlinear characteristics of these devices is investigated. The mathematic description of these etalon's linear, as well as their nonlinear, behavior is presented. The experimental apparatus is described. Measured values of the minimum critical detuning and minimum observed power are presented.

This work concerns the vacuum deposition of novel thin films that exhibit nonlinear optical effects due to their unusual microstructure. We discuss four different materials: 1) Tilted columns of aluminum-oxide 2) Gold particles in aluminum-oxide 3) Cadmium sulpho-selenide particles in aluminum-oxide 4) Silver particles in zinc-sulphide. We begin with a description of the vacuum system and some the techniques used to characterize the optical and structural properties of the films. This leads to our study of second-harmonic generation (SHG) in aluminum-oxide thin films deposited at an angle to the evaporant source. We show that SHG is very sensitive to the non-isotropic microstructure that results from such a deposition. and the behavior of the SHG signal with sample orientation provides insight to the symmetry properties of the microstructure. In a related study we show that AU/Al₂O₃ composite films produce a large SHG signal. We investigate the dependence of the strength of the SHG signal with fill-fraction of gold and show that it increases quadratically. in agreement with theory. The third material we discuss is cadmium sulpho-selenide doped aluminumoxide. We describe attempts at nucleating semiconductor crystallites in a variety of hosts through a process of co-deposition and subsequent annealing. We also deposit alternate layers of CdS-Se and Al₂O₃ with the semiconductor layer thin enough that interspersed crystallites form. This results in suspended. isolated crystallites similar to the doped-glass materials of interest to nonlinear optics. A waveguide of a CdS/Al₂O₃ "sandwich" demonstrates optical nonlinearity through a power-dependent prism coupling experiment, and the degree of nonlinearity is much greater than undoped glass, though less than doped glass. The final section of the dissertation is a theoretical description of nonlinear optical behavior in a novel composite material consisting of metal particles in a nonlinear dielectric host. We assume the enhanced field around the resonating particles drives the host locally nonlinear through either a Kerr-type or thermal nonlinearity. We calculate the change in optical properties of the medium due to this effect and show that for a system of silver in zinc-sulphide the nonlinearity can be significant.

The objective of the present dissertation is to advance the science and engineering of metal-dielectric thin-film structures for ultrafast all-optical applications. The research presented consists of three parts: first, the linear and nonlinear optical (NLO) properties of Au and Ag/Au bilayer metallic thin films are comprehensively studied; then the design and properties of a novel nonlinear device structure are presented and finally an ultrafast all-optical shutter is developed and applications are discussed. In the first part, this study describes the linear and NLO properties of bilayer metallic films and shows that they can be tuned by controlling the mass-thickness ratio between Au and Ag. The combined properties of these bilayers are attractive for active plasmonic applications and nonlinear optical filters. Detailed physical models describing the linear and NLO response of Au and Ag/Au bilayers are presented and compared with experiments. In the second part, these models are used to optimize the NLO response of a novel Au-based NLO device. With only four layers, this novel device strongly amplifies the NLO response of the component Au thin film. NLO devices with broad spectral and angular bandwidths in the visible spectral region are demonstrated. The narrow band dependent NLO response of the NLO device is shown to lead to all-optical controls of high peak-power optical signal pulses. Finally, the NLO device technology is integrated into a novel ultrafast all-optical shutter, which allows temporal opening windows (the time shutter remains open) as short as a few ps. Ultrafast all-optical shutter potentially can temporally shape high peak-power nanosecond optical pulses, which could benefit biomedical and micromachining applications. Other possible optical applications such as short electron, X-ray pulse generations, ultrafast photography, and biomedical imaging will also be discussed.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.
Includes bibliographical references (leaves 41-43).
Pulsed laser deposition was used to create thin films of Ce-Fe-O and Y-Fe-O systems. Deposition temperature and ambient oxygen pressure were varied systematically between samples to determine which deposition conditions were most favorable to the formation of cerium/yttrium orthoferrite. The structure and composition of each film were then determined using X-ray diffraction and wavelength dispersive spectroscopy respectively. In addition, the magnetic and optical properties of the yttrium films were characterized to determine the suitability of these materials as Faraday isolators at A=1550 nm. Results show that orthoferrite crystal structures in these systems are not stable in the temperature and oxygen ranges tested. It was also found that increasing oxygen pressure caused exponential decay in the deposition rate. Most films were amorphous, exhibiting a paramagnetic M-H plot and a Verdet coefficient between 0.37 and 0.89 deg cm-1 Gauss-1
by William Wagner Supplee, Jr.
S.B.

Nearly all the deficiencies of conventional vacuum evaporated coatings trace to a single physical property of condensed films: low packing density. One way to increase packing density is to bombard the growing film with ions during deposition, called ion-assisted deposition (IAD). The beginning chapters of this dissertation analyze IAD as a perturbation of the conventional vacuum evaporation process. The experimental chapters begin with an examination of the effect on moisture penetration behavior of oxygen-ion bombarding completed optical filters. Moisture adsorption and desorption is retarded after bombardment in filters composed of titania and silica, but not in those of zirconia and silica. Bombardment evidently induces a crystalline-to-amorphous transition in titania, causing the surface to swell and occluding the pores. The transition in zirconia is the reverse, and no impediment to moisture appears. Argon-ion-assisted magnesium fluoride (MgF₂) can show ultraviolet (UV) absorption. The primary mechanism is probably the formation of F-centers (single fluorine-ion vacancies), although an unsaturated oxygen bond may also be responsible. Absorption can be removed by baking and often by irradiation with UV. After baking, fluorine is lost and replaced by oxygen. Absorption-free MgF₂ films can be deposited by minimizing the substrate temperature and bombardment flux. Ion-assisted films contain up to 2% argon and up to 170 parts-per-million of tungsten from the ion gun filaments. They show a slightly higher refractive index, are much less porous, and are much more resistant to damage by abrasion and exposure to fluorine gas. Ion-assisted aluminum oxide (alumina, Al₂O₃) films show a small increase in UV absorption after argon-ion bombardment; however, a mixture of argon and oxygen ions avoids the problem. Excess oxygen is often incorporated into alumina films, and depresses both the mass density and the refractive index. IAD increases refractive index and decreases porosity. Ion-assisted alumina films are somewhat more stable in humid environments. Ion-assisted deposition has been shown by this study to cause substantial improvements in many of the physical and some of the optical and chemical properties of evaporated magnesium fluoride and aluminum oxide films.

This thesis presents optical characterizations of interfaces in ferromagnetic heterostructures and thin films used for spin polarized electronic devices. In these experiments, femtosecond laser spectroscopies are exploited to investigate the interface magnetization reversal, spin precession, and band offset, which are crucial in determining the performances of spintronic devices.;First, magnetization-induced second-harmonic-generation (MSHG) is applied to study interface magnetism in a hybrid structure containing a noncentrosymmetric semiconductor---Fe/AlGaAs. The reversal process of Fe interface layer magnetization is compared with the bulk magnetization reversal. In Fe/AlGaAs (001), the interface magnetization is found to be decoupled from the bulk magnetization based on the different switching characteristics---single step switching occurs at the interface layer, whereas two-jump switching occurs in the bulk. In contrast, the interface layer in Fe/AlGaAs (110) is rigidly coupled with the bulk Fe, indicating a strong impact of electronic structure on the magnetic interaction despite the same chemical composition. Furthermore, a time-resolved MSHG study demonstrates a coherent interface magnetization precession in Fe/AlGaAs (001), implying the feasibility of fast precessional control of interfacial spin. The interface magnetization precession exhibits a higher frequency and opposite phase for a given applied field compared to the bulk magnetization precession.;Second, uniform magnetization precession in the Lac0.67Ca 0.33MnO3 (LCMO) and La0.67Sr0.33MnO 3 (LSMO) films grown on different substrates are investigated by time-resolved magneto-optic Kerr effect. The parameters of magnetic anisotropy are determined from the field dependence of the precession frequency. The strain-free LCMO films grown on NdGaO3 exhibit a uniaxial in-plane anisotropy induced by the tilting of the oxygen octahedra in NdGaO3 An easy-plane magnetic anisotropy is found in the tensile-strained films grown on SrTiO 3, whereas the compressive-strained film grown on LaAlO3 exhibits an easy normal-to-plane axis.;Third, a table-top internal photoemission system is developed to measure the band offsets across semiconductor heterointerfaces by utilizing an optical parametric amplifier as the bright light source. The conduction band offsets DeltaE c = 660 meV and 530 meV at the CdCr2Se4-GaAs and CdCrZSe4-ZnSe interfaces are determined from the threshold energies of the photocurrent spectrum. The band offset is shown to be reduced by engineering the interface bonding and stoichiometry.

>Magister Scientiae - MSc
Amorphous silicon carbide (a-SiC) is a versatile material due to its interesting mechanical, chemical and optical properties that make it a candidate for application in solar cell technology. As a-SiC stoichiometry can be tuned over a large range, consequently is its bandgap. In this thesis, amorphous silicon carbide thin films for solar cells application have been deposited by means of the electron-beam physical vapour deposition (e-beam PVD) technique and have been isochronally annealed at varying temperatures. The structural and optical properties of the films have been investigated by Fourier transform Infrared and Raman spectroscopies, X-ray diffraction, Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy and UV-VIS-NIR spectroscopy. The effect of annealing is a gradual crystallization of the amorphous network of as-deposited silicon carbide films and consequently the microstructural and optical properties are altered. We showed that the microstructural changes of the as-deposited films depend on the annealing temperature. High temperature enhances the growth of Si and SiC nanocrystals in amorphous SiC matrix. Improved stoichiometry of SiC comes with high band gap of the material up to 2.53 eV which makes the films transparent to the visible radiation and thus they can be applied as window layer in solar cells.

The Electrostatic Self-Assembly (ESA) technique possesses great advantages over traditional thin film fabrication methods, making it an excellent choice for a number of applications in the fields of linear and nonlinear optics, electronics, sensing and surface coatings. The feasibility of fabricating linear optical interference filters by ESA methods is demonstrated in this thesis work. Basic single-anion/single-cation ESA films are synthesized and their optical parameters -- refractive index and average thickness for individual bilayer -- are investigated to provide a basis for the in-depth design of optical filters. High performance dielectric stack filters and narrowband and wideband antireflection coatings are designed using TFCalc simulation software and are fabricated by ESA. Both bulk film sensitivity and layer sensitivity to manufacturing errors are provided. The significant agreement between simulation and experiment demonstrates the strong capability of ESA to precisely control the refractive index and produce excellent thin film filters. The performance of optical thin film filters is largely enhanced compared to the results of previous methods. The experiment results indicate that the ESA process may be used to fabricate optical filters and other optical structures that require precise index profile control.
Master of Science

Chloride, bromide, pyridinium and quinolinium homologues of 4-(N- hexadecylpyridinium-4-ylmethylidene-amino)-2,6-dichlorophenolate have been investigated in solution, Langmuir and Langmuir-Blodgett films. Techniques including spectroscopy, surface potential measurement, quartz crystal microbalance, surface plasmon resonance, atomic force microscopy, reectometry and X-ray diffraction have been used to characterise these molecular systems. In solution, solvatochroism was observed and Benisi-Hildebrand analysis revealed dimeric aggregation. Langmuir monolayers were compressed at the air/water interface and chromophore rotation was observed by surface potential measurement. Langmuir- Blodgett monolayers showed lm-thickness dependence on the deposition-pressure. Monolayer thicknesses between 6-24Ä were measured using SPR and molecular areas between 40-l25Ä2 were measured using a quartz crystal microbalance. Both the molecular/s/area)and monolayer thicknesses were deposition-pressure dependent. The high tilt phases were visually distinguishable from the low tilt phases using atomic force microscopy, The compounds showed phase behaviour that was predominantly alike for the bromide and chloride homologues but different for the pyridinium and quinolinum homologues. Multilayer Y-type films of the merocyanine dyes were analysed using reectometry and deposition-pressure dependent thicknesses were found. Alternate layer structures of NLO-active hemicyanine amphiphiles were used to achieve homogeneous. orientation ordering using active and inactive spacer layers. Ordering was achieved but the optical efficiency was reduced by high proportions of inactive material and interlayer dipole formation. Double chained hemicyanine molecules were used to form Z-type structures and subsequent layers were found to significantly interdigitate. Different chain lengths were found to interdigitate by the length of the shortest chain. Gas detection experiments were undertaken on the quinolinium, dichloro merocyanine using three optical geometries. The absorption method showed slow switching and poor sensitivity. The Kretschmann SPR geometry showed high sensitivity and rapid switching. The grating SPR geometry showed rapid switching but was less sensitive than the ATR method. Protonation of the monolayers was investigated using hydrochloric acid gas, acetic acid vapour and stearic acid immobilised within the lm.

This study characterizes thin films of eleven lanthanide trifluorides which are potentially useful for optical applications. The natural and controlled transparency and real refractive index of single layers of these trifluorides are examined and accompanied by studies of composition and crystal structure of the films which are made by conventional, high temperature, and ion-assisted thermal evaporation. Additionally, since these particular fluoride compounds provide an excellent opportunity for doing so, the mechanisms active in film modification through high temperature and ion-assisted thermal evaporation are explored.

In the thesis CuPc thin films were investigated by (in situ) SE and RAS, which are employed to determine the out-of-plane and in-plane optical anisotropy and molecular orientation, respectively. CuPc is a promising candidate of organic semiconductors used in organic field effect transistors, organic light emitting diodes and organic solar cells. Vicinal Si(111) substrates are interesting due to the in-plane anisotropy caused by the steps and terraces on the surface. The strength of in-plane anisotropy of vicinal Si(111) is dependent on the offcut angle. The influence of offcut angle on out-of-plane and in-plane molecular orientation in CuPc thin films is explored. The in situ investigation of CuPc films suggests that structural changes occur during film growth. In addition, two different surface modification layers were utilized to examine the effect on CuPc molecular orientation: OTS monolayer with upright standing molecules and PTCDA layers with flat lying molecules. Metal-organic interface plays an important role in organic electronic devices. In-CuPc is chosen to be an example system investigated employing in situ SE and RAS. When In was thermally evaporated onto CuPc film, In atoms firstly diffuse into the CuPc film underneath and then aggregate to form clusters on top. Hafnium dioxide (HfO2) is currently a hot topic to replace the conventionally used SiO2 as gate dielectrics in order to minimize leakage current when further scaling down microelectronic devices. Since HfO2 films are often crystalline, in order to obtain amorphous films which are beneficial to minimize leakage current, aluminum oxide (Al2O3) (k value: 9) which stays amorphous at much higher temperatures are combined to overcome this difficulty. Two series of ultra-thin samples were deposited by atomic layer deposition: mixed layers HfxAl1-xOz and bilayers HfO2 on Al2O3. Optical constants and bandgap are determined using SE in the energy range of 0.7-10 eV. It is found that the (effective) optical bandgap of both mixed layer and bilayer structures can be tuned by the film composition. Aging effect of high-k films was observed after storage of samples in air for two months, which is attributed to further oxidation of the dielectric films caused by the oxygen diffusion from ambient air to high-k films
In dieser Arbeit werden dünne Schichten aus Kupferphthalozyanin (CuPc) mittels spektroskopischer (in-situ) Ellipsometrie (SE) und (in-situ) Reflektions-Anisotropie-Spektroskopie (RAS) untersucht, um die optische Anisotropie in einer Ebene parallel und senkrecht zur Schichtoberfläche und die molekulare Orientierung zu bestimmen. CuPc ist ein aussichtsreicher Kandidat als organischer Halbleiter in organischen Feldeffekt-Transistoren, organischen Leuchtdioden und organischen Solarzellen. Vizinale Si(111)-Substrate sind wegen der Anisotropie in der Substratebene interessant, die durch die Treppen und Terrassen auf der Oberfläche verursacht wird. Die Stärke der Anisotropie der vizinalen Si(111)-Oberfläche ist vom Schnittwinkel (Offcut) abhängig. Es wird der Einfluss des Offcut-Winkels auf die molekulare Orientierung in dünnen CuPc-Schichten parallel und senkrecht zur Substratoberfläche untersucht. Die in-situ Untersuchungen von CuPc-Schichten weisen darauf hin, dass strukturelle Veränderungen beim Wachstum auftreten. Darüber hinaus wurden zwei unterschiedliche Oberflächenmodifizierungsschichten, um deren Wirkung auf die molekulare Orientierung von CuPc zu untersuchen, verwendet: eine OTS-Monoschicht mit aufrecht stehenden Molekülen und PTCDA-Schichten mit flach liegenden Molekülen. Metall-organische Grenzflächen spielen eine wichtige Rolle in organischen elektronischen Bauelementen. In-CuPc wird als Beispiel für ein Metall-organisches System durch in-situ SE und RAS untersucht. Wenn In thermisch auf eine CuPc-Schicht aufgedampft wird, diffundieren In-Atome zunächst in die darunterliegende CuPc-Schicht und bilden dann Cluster auf der Schicht. Hafniumdioxid (HfO2) ist ein heißer Kandidat für das Ersetzen des herkömmlich als Gate-Dielektrikum verwendeten SiO2 mit dem Ziel, die Leckströme bei der weiteren Verkleinerung mikroelektronischer Bauelemente zu minimieren. Um amorphe Schichten, die vorteilhaft zur Minimierung der Leckströme sind, zu erhalten, werden die HfO2-Schichten, die oft kristallin sind, mit Aluminiumoxid (Al2O3) (k-Wert: 9) kombiniert, das bei wesentlich höheren Temperaturen amorph bleibt. Zwei Serien von ultra-dünnen Proben wurden durch Atomlagenabscheidung hergestellt: Mischschichten HfxAl1-xOz und Doppelschichten HfO2 auf Al2O3. Die optischen Konstanten und Bandlücken wurden mittels SE im Energiebereich von 0,7 bis 10 eV bestimmt. Es hat sich gezeigt, dass die (effektive) Bandlücke der Misch- und Doppelschichten durch die Komposition abgestimmt werden kann. Nach Lagerung der High-k-Schichten für zwei Monate an Luft konnte ein Alterungseffekt beobachtet werden. Dieser wird auf die weitere Oxidation der dielektrischen Schichten, die durch Sauerstoffdiffusion aus der Umgebungsluft in die High-k-Schichten ermöglicht wird, zurückgeführt

In recent years there has been considerable interest in media which display significant non-linear optical properties; the telecommunications industry may exploit chin films of such materials for signal processing applications. The Langmuir-Blodgett (LB) technique provides a means of depositing organic layers of a precisely defined thickness. Moreover, by alternating layers of different materials, supermolecular arrays may be fabricated in which there is no centrosymmetry, and therefore the second-order non-linearity of the constituent molecules may be exploited. An investigation of the properties of water-surface monolayers of a number of novel materials with potentially large non-linearities is described. Several of these compounds are shown to form high quality homogeneous or heterogeneous LB films. The optical and electrical properties of the layers are characterized by optical absorption spectroscopy, surface plasmon resonance, and measurements of capacitance, whilst their structure is examined by electron diffraction. Monolayers of a nitrostilbene dye are shown to exhibit: on exceptionally high degree of crystalline order. Data are also given for theoretical calculations of non-linear coefficients and for the relative efficiency of second harmonic generation from bulk samples of various materials. Studies of second harmonic generation from monolayer and alternate multilayer films are reported. Optical non-linearity in an alternating donor-acceptor: inverted donor-acceptor dye system is demonstrated for the first time; the results are analysed in terms of second harmonic surface susceptibilities, and the value of the second-order hyperpolarizability determined for the first bilayer is found to be much superior to that expected by the simple addition of the hyperpolarizabilities of the separate layers. Monolayers containing a mixture of hemicyanine and cadmium arachldate are found to give rise to second harmonic generation which is enhanced relative to that obtained from a pure monolayer of the dye. Corresponding changes in the absorption spectra of the layers can be observed. These findings may have important implications for improving the efficiencies of any non-linear optical device which utilises IB films.

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 we investigate theoretically various types of sculptured thin film (STF) envisioned as platforms for optical sensing. A STF consists of an array of parallel nanowires which can be grown on a substrate using vapour deposition techniques. Typically, each nanowire has a diameter in the range from ~ 10-300 nmwhile the film thickness is ~<1μm. Through careful control of the fabrication process, both the optical properties and the porosity of the STF can be tailored to order. These abilities make STFs promising for optical sensing applications, wherein it is envisaged that the material to be sensed infiltrates the void region in between the parallel nanowires and hence changes the optical properties of the STF. Various homogenization formalisms can be used to estimate the constitutive parameters of the infiltrated STFs. In this thesis two different homogenization formalisms were used: the Bruggeman formalism (extended and non–extended versions) and the strong-permittivityfluctuation theory (SPFT). These were used in investigations of the following optical–sensing scenarios: (i) Electromagnetic radiation emitted by a dipole source inside an infiltrated chiral STF. The effects of using the extended Bruggeman homogenization formalism, which takes into account the nonzero size of the component particles,were studied. (ii) Surface–plasmon– polariton waves on ametal–coated, infiltrated columnar thin film. The influences of using the extended SPFT formalism, which takes into account the nonzero size of the component particles and their statistical distributions, were explored. (iii) A metal-coated infiltrated chiral STF which supports both surface-plasmon-polariton waves and the circular Bragg phenomenon. The possibility of using in parallel both surface-plasmon-polariton waves and the circular Bragg phenomenon was investigated using the non–extended Bruggeman formalism. Our numerical studies revealed that the design performance parameters of the infiltrated STF are bode well for these optical–sensing scenarios. The use of inverse Bruggeman formalism was also investigated: this was found to be problematic in certain constitutive parameter regimes, but not those for optical–sensing scenarios considered in this thesis.