Liquid crystal (LC)-based adaptive optics are important for information processing, optical interconnections, photonics, integrated optics, and optical communications due to their tunable optical properties. In this dissertation, we describe novel liquid crystal photonic devices and their fabrication methods. The devices presented include inhomogeneous polymer-dispersed liquid crystal (PDLC), polymer network liquid crystals (PNLC) and phase-separated composite film (PSCOF). Liquid crystal/polymer composites could exist in different forms depending on the fabrication conditions. In Chap. 3, we demonstrate a novel nanoscale PDLC device that has inhomogeneous droplet size distribution. In such a PDLC, the inhomogeneous droplet size distribution is obtained by exposing the LC/monomer with a non-uniform ultraviolet (UV) light. An electrically tunable-efficiency Fresnel lens is devised for the first time using nanoscale PDLC. The tunable Fresnel lens is very desirable to eliminate the need of external spatial light modulator. Different gradient profiles are obtained by using different photomasks. The nanoscale LC droplets are randomly distributed within the polymer matrix, so that the devices are polarization independent and exhibit a fast response time. Because of the small droplet sizes, the operating voltage is higher than 100 Vrms. To lower the driving voltage, in Chap. 2 and Chap. 3, we have investigated a polymer-network liquid crystal (PNLC) using a rod-like monomer structure. Since the monomer concentration is only about 5%, the operating voltage is below 10 Vrms. The PNLC devices are polarization dependent. To overcome this shortcoming, stacking two cells with orthogonal alignment directions is a possibility. In Chap. 3, another approach to lower the operating voltage is to use phase-separated composite film (PSCOF) where the LC and polymer are separated completely to form two layers. Without multi-domain formed in the LC cell, PSCOF is free from light scattering. Using PNLC and PSCOF, we also demonstrated LC blazed grating and Fresnel lens. The diffraction efficiency of these devices is continuously controlled by the electric field. Besides Fresnel lens, another critical need for imaging and display is to develop a system with continuously tunable focal length. A conventional zooming system controls the lens distance by mechanical motion along the optical axis. This mechanical zooming system is bulky and power hungry. To overcome the bulkiness, in Chap. 4 we developed an electrically tunable-focus flat LC spherical lens which consists of a spherical electrode imbedded in the top flat substrates while a planar electrode on the bottom substrate. The electric field from the spherical and planar electrodes induces a centrosymmetric gradient refractive index distribution within the LC layer which, in turn, causes the focusing effect. The focal length is tunable by the applied voltage. A tunable range from 0.6 m to infinity is achieved. Microlens array is an attractive device for optical communications and projection displays. In Chap. 5, we describe a LC microlens array whose focal length can be switched from positive to negative or vise versa by the applied voltage. The top spherical electrode glass substrate is flattened with a polymer layer. The top convex substrate and LC layer work together like a zoom lens. By tuning the refractive index profile of the LC layer, the focal length of the microlens array can be switched from positive to negative or vise versa. The tunable LC microlens array would be a great replacement of a conventional microlens array which can be moved by mechanical elements. The fast response time feature of our LC microlens array will be very helpful in developing 3-D animated images. A special feature for LC/polymer composites is light scattering. The concept is analogous to the light scattering of clouds which consist of water droplets. In Chap. 6, we demonstrate polymer network liquid crystals for switchable polarizers and optical shutters. The PNLC can present anisotropic or isotropic light scattering behavior depending on the fabrication methods. The use of dual-frequency liquid crystal and special driving scheme leads to a sub-millisecond response time. The applications for display, light shutters, and switchable windows are emphasized. Although polymer networks help to reduce liquid crystal response time, they tend to scatter light. In Chap. 7, for the first time, we demonstrate a fast-response and scattering-free homogeneously-aligned PNLC light modulator. Light scattering in the near-infrared region is suppressed by optimizing the polymer concentration such that the network domain sizes are smaller than the wavelength. As a result, the PNLC response time is ~300X faster than that of a pure LC mixture except that the threshold voltage is increased by ~25X. The PNLC cell also holds promise for mid and long infrared applications where response time is a critical issue.

<p>U poslednjih nekoliko decenija, feroelektrici su prepoznati kao dobri kandidati<br />za &scaron;irok spektar primene. Barijum titanat je jedan od najče&scaron;će istraživanih<br />perovskitnih materijala usled tipičnog feroelektričnog pona&scaron;anja na sobnoj<br />temperaturi, sa histerezisnom zavisnosti između polarizacije i električnog polja.<br />Specifična svojstva BaTiO₃ čine ovaj materijal veoma korisnim u proizvodnji<br />kondenzatora, memorija, senzora i dr. Pored toga, paraelektrična faza BaTiO₃<br />ima primenu u proizvodnji mikrotalasnih tunabilnih uređaja. BaTiO₃ pokazuje<br />paralaketrično pona&scaron;anje iznad Kirijeve temperature (120&deg;C) i veliki je izazov<br />modifikovati materijal da bude paraelektričan na sobnoj temperaturi. Delimičnom<br />supstitucijom Ba²⁺ ili Ti⁴⁺ jona, jonima Sr²⁺ ili Zr⁴⁺, respektivno, snižava<br />se Kirijeva temperatura i dolazi do pomeranja fazne transformacije feroelektri<br />čno/paraelektrično na niže temperature. Monolitna keramika na bazi bazi<br />BaTiO₃ je često istraživana kako bi se ispitale promene u strukturi i svojstvima<br />uzrokovane dodatkom dopanata, međutim ne postoji praktična primena ovih<br />materijala u mikrotalasnim tunabilnim tehnologijama. S druge strane, u trendu<br />sveop&scaron;te minijaturizacije elektronskih komponenti, tanki filmovi su prepoznati<br />kao dobri kandidati za proizvodnju tunabilnih uređaja. Jedan od ciljeva ovog<br />istraživanja bio je ispitivanje strukturnih i funkcinalnih svojstava tankih filmova na<br />bazi barijum titanata, BaTiO₃, Ba_1-xSr_xTiO₃ (x=0,1, 0,2, 0,3 i 0,4) i BaTi_1-xZr_xO₃<br />(x=0,1 i 0,2), pripremljenih hemijskom depozicijom iz tečne faze. Metalne<br />soli BaCO₃, C₄H₆O₄Sr i ZrOCl₂ H₂O i Ti(OCH₂CH₂CH₂CH₃)₄ rastvorene su<br />odvojeno u kiseloj sredini i pome&scaron;ane u transparentne solove. Inkdžet &scaron;tampa<br />i spin tehnika kor&scaron;ćene su za depoziciju funkcionalnih filmova na bazi BaTiO₃.<br />Nakon deponovanja, pripremljeni filmovi su termički tretirani na različitim<br />temperaturama do 1000&deg;C. U zavisnosti od koncentracije pripremljeni su filmovi<br />debljine od 100 do 700 nm,sa veličinom zrna od nekoliko desetina nanometara. Strukturna karakterizacija potvrdila je promene u strukturi BaTiO₃ tankih filmova<br />sa dodatkom Sr²⁺ i Zr⁴⁺. Tetragonalna (feroelektrična) faza BaTiO₃ je potvrđena<br />rentgenostrukturnom analizom i Ramanovom spektroskopijom. S druge strane,<br />smanjenje tetragonalnosti je primećeno kod dopiranih uzoraka. Promene u<br />funkcionalnosti dopiranih BaTiO₃ filmova analizirane su na osnovu dielektričnih<br />i feroelektričnih merenja. Izvr&scaron;ene analize su potvrdile feroelektrično pona&scaron;anje<br />kod BaTiO₃, dok se feroelektrični odgovor u tankim filmovima smanjuje sa<br />dopiranjem. Priprema elektroda specifičnih geometrija pogodnih za tunabilna<br />merenja različitim tehnikama depozicije bio je drugi cilj istraživanja. Tehnika spaterovanja u kombinaciji sa laserskim uklanjanjem, inkdžet &scaron;tampa i fotolitografija<br />su kori&scaron;ćene za pripremu kružnih i koplanarnih elektroda na povr&scaron;ini barijum<br />titanatnih tankih filmova. Uticaj procesnih parametara svake od pomenutih<br />metoda na dimenzionalnu preciznost pripremljenih elektroda je bio predmet istraživanja<br />u okviru teze. Odabrani laser se pokazao kao neprikladan za pripremu<br />elektroda na pripremljenim barijum titanatnim tankim filmovima. Inkdžet &scaron;tampa<br />se pokazala kao korisna u pripremi elektroda mikrometarskih fimenzija, dok je za<br />pripremu sofisticiranijih geometrija fotolitografija pokazala najbolje performanse.</p> / <p>In past few decades, ferroelectrics are recognized as good candidates for wide<br />range of applications. Barium titanate is one of the most investigated perovskite<br />materials due to typical ferroelectric behavior at room temperature, with hysteresis<br />dependence of the polarization and electric field. Specified properties<br />of BaTiO₃ make this material useful in production of capacitors, memories,<br />sensors, etc. Nevertheless, paraelectric phase of BaTiO₃ may have application<br />in production of microwave tunable devices. Barium titanate shows paraelectric<br />behavior at temperatures above the Currie temperature (120 &deg;C) and it is great<br />challenge to make material paraelectric at room temperature. Partial substitution<br />of Ba²⁺ or Ti⁴⁺ ions, by Sr²⁺ or Zr⁴⁺, respectively, decreases the Currie temperature<br />of barium titanate and moves phase transition ferroelectric/paraelectric<br />to lower temperatures. Bulk BaTiO₃was often investigated in order to improve<br />structure and functionality by addition of dopant, but do not have practical<br />application in microwave tunable technologies. On the other hand, with the<br />trend of overall miniaturization of electronic devices, thin films are recognized<br />as good candidates for production of tunable devices. One of the aims in this<br />research was investigation of structural and functional properties of barium<br />titanate based thin films, BaTiO₃, Ba_1-xSrxTiO₃ (x=0,1, 0,2, 0,3 i 0,4) and<br />BaTi_1-xZrxO₃ (x=0,1 i 0,2), prepared by chemical solution deposition. Metal<br />salts of BaCO₃, C₄H₆O₄Sr and ZrOCl₂ H₂O and Ti(OCH₂CH₂CH₂CH₃)₄were<br />separetly disolved in acetic environment and mixed in clear transparent sols.<br />Inkjet printing and spin coating were used for deposition of functional BaTiO3<br />based fims. After deposition prepared films were thermally treated at different<br />temperatures up to 1000 &deg;C. In dependence of sol concentration thickness of<br />obtained films is from 100 to 700 nm and grain size is few tens of nanometers. Structural characterization confirmed changes in structure of barium titanate thin<br />films by addition of Sr^₂₊ i Zr⁴⁺. Tetragonal (ferroelectric) phase of BaTiO₃ is<br />confirmed by X-ray diffraction and Raman spectroscopy. On the other hand, decreasing<br />of tetragonality was noticed in doped samples. Changes in functionality<br />of doped BaTiO₃ thin films were analyzed by dielectric and ferroelectric measurements.<br />Performed analysis confirmed ferroelectric behavior of barium titanate<br />thin films, and decrease in ferroelectric answer of doped films. Investigation of<br />possibility of complex shaped electrodes preparation,suitable for tunability measurements, by different deposition techniques was the second goal of this research.<br />Sputtering technique in combination with laser removal, inkjet printing and photolithography were used for preparation of complex circular and coplanar electrodes<br />on the surface of barium titanate based thin films. Influence of processing<br />parameters for the each of mentioned technique on dimensional precision of prepared<br />electrodes was investigated. Selected laser was not appropriate for production<br />of electrodes on prepared barium titanate based thin films. Inkjet printing<br />was useful for production of electrodes in micrometer range, but for more sophisticated<br />geometries photolithography shows the best performance.</p>

La croissance rapide du marché des télécommunications a conduit à une augmentation significative du nombre de bandes de fréquences allouées et à un besoin toujours plus grand en terminaux offrant un accès à un maximum de standards tout en proposant un maximum de services. La miniaturisation de ces appareils, combinée à la mise en place de fonctions supplémentaires, devient un vrai challenge pour les industriels. Une solution consiste à utiliser des fonctions hyperfréquences accordables (filtres, commutateurs, amplificateurs,…). A ce jour, trois technologies d'accord sont principalement utilisées : capacités variables, matériaux agiles ou encore MEMS RF. Dans le cadre de cette thèse, nous avons travaillé sur l’optimisation de dispositifs hyperfréquences reconfigurables en utilisant des couches minces ferroélectriques KTN et des diodes varactor. Nos premiers travaux étaient relatifs à l’optimisation des dispositifs hyperfréquences accordables à base de couche minces KTN. Dans ce sens, nous avons tout d’abord caractérisé le matériau KTN en basse et haute fréquence afin de déterminer ses caractéristiques diélectriques et ses caractéristiques en température. Nous avons ensuite réalisé des dispositifs hyperfréquence élémentaires tels des capacités interdigitées et des déphaseurs à base de KTN. Leurs performances ont alors été comparées aux mêmes dispositifs réalisés cette fois à base de la solution la plus utilisée BST. Bien qu’un léger avantage soit acquis à la solution BST, il n’en reste pas moins vrai que les résultats avec le matériau KTN sont très proches indiquant que cette voie peut également, après optimisation, apporter une alternative au BST. La seconde partie de nos travaux concerne la réalisation de filtres planaires accordables en fréquence à base de matériaux KTN et de diodes varactor. Nous avons ainsi réalisé deux filtres passe-bande accordables. Un premier filtre passe-bande de type « open loop » possédant deux pôles agiles en fréquence centrale et un second filtre passe-bande de type SIR rendant possible l’accord de sa fréquence centrale ainsi que de sa bande passante à partir de diodes varactor. Lors de la conclusion sur nos travaux, nous évoquons les suites à donner à ce travail et les perspectives. / The rapid growth of the telecommunications industry has led to a significant increase in the number of allocated frequency bands and a growing need for terminals providing access to an increasing number of standards while offering maximum services. The miniaturization of these devices combined with the implementation of additional functions has become a real challenge for the industry. The use of tunable microwave functions (filters, switches, amplifiers ...) appears as a solution to this issue. In this way, three main technologies are mainly used: variable capacitors, tunable materials and RF MEMS. Within the scope of this thesis work, our investigations focused on tunable microwave devices optimization through the use of KTN ferroelectric thin films and varactor diodes. The first part of our study deals with the optimization of tunable microwave devices based on KTN ferroelectric thin films. In this way, we initially characterized KTN material in low and high frequency to determine its dielectric properties and characteristics according to the temperature. Then, we designed basic microwave devices such as interdigitated capacitors and phase shifters based on KTN thin films. Their performances were then compared with BST solution. Despite results highlighting a slight advantage to BST solution, KTN material, after optimization process, could be a BST alternative solution. In a second part, our work focused on the realization of tunable planar filters based on KTN materials and varactor diodes. We made two tunable bandpass filters. The first one is a center frequency tunable bandpass two pole open loop filter and the second one is a center frequency and bandwidth tunable SIR bandpass filter using varactor diodes. Finally, we discussed follow-up to give to this work and outlooks.

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