Diffusion de la lumière dans les milieux complexes à base de cristaux liquides

Cette thèse présente les étapes de fabrication et les résultats de la caractérisation des modèles de tissu biologique fabriqués en utilisant des cristaux liquides et des polymères qui permettent de contrôler la diffusion et la transmission de la lumière par un champ électrique. Ce champ électrique entraîne un changement d’orientation des molécules de cristaux liquides, ce qui induit des variations locales de l’indice de réfraction dans ces matériaux. Nous avons utilisé ce contrôle de l’indice de réfraction pour contrôler le niveau de diffusion et de transmission dans différents types d’échantillons. Les échantillons utilisés sont faits a) de cristaux liquides purs, b) de sphères de polymère dans un environnement de cristaux liquides, c) de cristaux liquides ou de gouttelettes de cristaux liquides dans un environnement de polymère. Les travaux réalisés nous ont permis de proposer une méthode électro-optique pour simuler, à l’aide de cristaux liquides, des milieux diffusants tels que les tissus biologiques. Dans la recherche de modèles, nous avons montré qu’il est possible de contrôler, par la fréquence du champ électrique, des gouttelettes de cristaux liquides dispersées dans une matrice de polymère contenant des ions lithium. Une sensibilité sélective à la fréquence de la tension électrique appliquée en fonction de la taille des gouttelettes est observée. Par la suite, nous avons démontré l’effet de « quenching » interférentiel de la diffusion pour une sonde optique cohérente en étudiant des modèles non alignés (clustérisés) de cristaux liquides. Nous avons aussi démontré des applications potentielles de ce phénomène tel le filtrage en longueur d’onde par l’ajustement d’un champ électrique. Nous avons ensuite démontré que l’utilisation des cristaux liquides - dits « dual-frequency » - permet une modulation rapide des états ou des structures du matériau ainsi que de la transmission et de la diffusion de lumière de ce dernier. Enfin, nous avons aussi découvert que ces modèles peuvent être contrôlés pour retrouver le degré de polarisation de la lumière diffusée.

Highly Efficient and Thermally Stable Second-Order Nonlinear Optical Chromophores and Electrooptic Polymers

Organic polymeric electro-optic (E-O) materials have attracted significant attention because of their potential use as fast and efficient components of integrated photonic devices (1,2). However, the practical application of these materials in optical devices is somewhat limited by the stringent material requirements imposed by the device design, fabrication processes and operating environments. Among the various material requirements, the most notable ones are large electro-optic coefficients (r(33)) and high thermal stability (3). The design of poled polymeric materials with high electro-optic activity (r(33)) involves the optimization of the percent incorporation of efficient (large beta mu) second order nonlinear optical (NLO) chromophores into the polymer matrices and the effective creation of poling-induced non-centrosymmetric structures. The factors that affect the material stability are a) the inherent thermal stability of the NLO chromophores, b) the chemical stability of the NLO chromophores during the polymer processing conditions, and c) the long-term dipolar alignment stability at high temperatures. Although considerable progress has been made in achieving these properties (4), organic polymeric materials suitable for practical E-O device applications are yet to be developed. This chapter highlights some of our approaches in the optimization of molecular and material nonlinear optical and thermal properties.

Dielectric Studies of laser ablated Ca doped BaTiO3 thin films

Recently, there has been growing interest in Ca modified BaTiO3 structures due to their larger electro-optic coefficients for their use in optical storage of information over conventional BaTiO3 crystals. Barium Calcium Titanate (BCT) shows promising applications in advanced laser systems, optical interconnects and optical storage devices. BaTiO3 thin films of varied Ca (3 at. % - 15 at. %) doping were deposited using pulsed laser ablation (KrF excimer laser) technique over Pt/Si substrates. The stoichiometric and the compositional analysis were carried out using EDAX and SIMS. The dielectric studies were done at the frequency regime of 40 Hz to 100 kHz at different ambient temperatures from 200 K to 600 K. The BCT thin films exhibited diffuse phase transition, which was of a typical non lead relaxor behavior and had high dielectric constant and low dielectric loss. The phase transition for the different compositions of BCT thin films was near the room temperature, showing a marked departure from the bulk phase transition. The C - V and the hysteresis behavior confirmed the ferroelectric nature below the phase transition and paraelectric at the room temperature.

Measurement of Electron Spin Lifetime and Optical Orientation Efficiency in Germanium Using Electrical Detection of Radio Frequency Modulated Spin Polarization

We propose and demonstrate a technique for electrical detection of polarized spins in semiconductors in zero applied magnetic fields. Spin polarization is generated by optical injection using circularly polarized light which is modulated rapidly using an electro-optic cell. The modulated spin polarization generates a weak time-varying magnetic field which is detected by a sensitive radio-frequency coil. Using a calibrated pickup coil and amplification electronics, clear signals were obtained for bulk GaAs and Ge samples from which an optical spin orientation efficiency of 4.8% could be determined for Ge at 1342 nm excitation wavelength. In the presence of a small external magnetic field, the signal decayed according to the Hanle effect, from which a spin lifetime of 4.6 +/- 1.0 ns for electrons in bulk Ge at 127 K was extracted.

Comparison of relative flexoelectric coefficients measured by different techniques

While it is well known that it is possible to determine the effective flexoelectric coefficient of nematic liquid crystals using hybrid cells [1], this technique can be difficult due to the necessity of using a D.C. field. We have used a second method[2], requiring an A.C. field, to determine this parameter and here we compare the two techniques. The A.C. method employs the linear flexoelectrically induced linear electro-optic switching mechanism observed in chiral nematics. In order to use this second technique a chiral nematic phase is induced in an achiral nematic by the addition of a small amount of chiral additive (∼3% concentration w/w) to give helix pitch lengths of typically 0.5-1.0 μm. We note that the two methods can be used interchangeably, since they produce similar results, and we conclude with a discussion of their relative merits.

Terahertz time domain spectroscopy of amino acids and sugars

A time-domain spectrometer for use in the terahertz (THz) spectral range was designed and constructed. Due to there being few existing methods of generating and detecting THz radiation, the spectrometer is expected to have vast applications to solid, liquid, and gas phase samples. In particular, knowledge of complex organic chemistry and chemical abundances in the interstellar medium (ISM) can be obtained when compared to astronomical data. The THz spectral region is of particular interest due to reduced line density when compared to the millimeter wave spectrum, the existence of high resolution observatories, and potentially strong transitions resulting from the lowest-lying vibrational modes of large molecules.

The heart of the THz time-domain spectrometer (THz-TDS) is the ultrafast laser. Due to the femtosecond duration of ultrafast laser pulses and an energy-time uncertainty relationship, the pulses typically have a several-THz bandwidth. By various means of optical rectification, the optical pulse carrier envelope shape, i.e. intensity-time profile, can be transferred to the phase of the resulting THz pulse. As a consequence, optical pump-THz probe spectroscopy is readily achieved, as was demonstrated in studies of dye-sensitized TiO₂, as discussed in chapter 4. Detection of the terahertz radiation is commonly based on electro-optic sampling and provides full phase information. This allows for accurate determination of both the real and imaginary index of refraction, the so-called optical constants, without additional analysis. A suite of amino acids and sugars, all of which have been found in meteorites, were studied in crystalline form embedded in a polyethylene matrix. As the temperature was varied between 10 and 310 K, various strong vibrational modes were found to shift in spectral intensity and frequency. Such modes can be attributed to intramolecular, intermolecular, or phonon modes, or to some combination of the three.

Fabrication of microelectrodes deeply embedded in LiNbO3 using a femtosecond laser

We present a novel technique to fabricate deeply embedded microelectrodes in LiNbO3 using femtosecond pulsed laser ablation and selective electroless plating. The fabrication process mainly consists of four steps, which are (1) micromachining of microgrooves on the surface of LiNbO3 by femtosecond laser ablation; (2) formation of AgNO3 films on substrates; (3) scanning the femtosecond laser beam in the fabricated microgrooves for modi. cation of the inner surfaces; and (4) electroless copper plating. The void-free electroless copper plating is obtained with appropriate cross section of microgrooves and uniform initiation of the autocatalytic deposition on the inner surface of grooves. The dimension and shape of the microelectrodes could be accurately controlled by changing the conditions of femtosecond laser ablation, which in turn can control the distribution of electric field inside LiNbO3 crystal for various applications, opening up a new approach to fabricate three-dimensional integrated electro-optic devices. (C) 2008 Elsevier B. V. All rights reserved.

Compact parallel optical modified-signed-digit arithmetic-logic array processor with electron-trapping device

A compact two-step modified-signed-digit arithmetic-logic array processor is proposed. When the reference digits are programmed, both addition and subtraction can be performed by the same binary logic operations regardless of the sign of the input digits. The optical implementation and experimental demonstration with an electron-trapping device are shown. Each digit is encoded by a single pixel, and no polarization is included. Any combinational logic can be easily performed without optoelectronic and electro-optic conversions of the intermediate results. The system is compact, general purpose, simple to align, and has a high signal-to-noise ratio. (C) 1999 Optical Society of America.

I. A study of the dynamics of triplet excitons in molecular crystals. II. An investigation of delayed light emission from Chlorella pyrenoidosa

I. PREAMBLE AND SCOPE

Brief introductory remarks, together with a definition of the scope of the material discussed in the thesis, are given.

II. A STUDY OF THE DYNAMICS OF TRIPLET EXCITONS IN MOLECULAR CRYSTALS

Phosphorescence spectra of pure crystalline naphthalene at room temperature and at 77˚ K are presented. The lifetime of the lowest triplet ³B_1u state of the crystal is determined from measurements of the time-dependence of the phosphorescence decay after termination of the excitation light. The fact that this lifetime is considerably shorter in the pure crystal at room temperature than in isotopic mixed crystals at 4.2˚ K is discussed, with special importance being attached to the mobility of triplet excitons in the pure crystal.

Excitation spectra of the delayed fluorescence and phosphorescence from crystalline naphthalene and anthracene are also presented. The equation governing the time- and spatial-dependence of the triplet exciton concentration in the crystal is discussed, along with several approximate equations obtained from the general equation under certain simplifying assumptions. The influence of triplet exciton diffusion on the observed excitation spectra and the possibility of using the latter to investigate the former is also considered. Calculations of the delayed fluorescence and phosphorescence excitation spectra of crystalline naphthalene are described.

A search for absorption of additional light quanta by triplet excitons in naphthalene and anthracene crystals failed to produce any evidence for the phenomenon. This apparent absence of triplet-triplet absorption in pure crystals is attributed to a low steady-state triplet concentration, due to processes like triplet-triplet annihilation, resulting in an absorption too weak to be detected with the apparatus used in the experiments. A comparison of triplet-triplet absorption by naphthalene in a glass at 77˚ K with that by naphthalene-h₈ in naphthalene-d₈ at 4.2˚ K is given. A broad absorption in the isotopic mixed crystal triplet-triplet spectrum has been tentatively interpreted in terms of coupling between the guest ³B_1u state and the conduction band and charge-transfer states of the host crystal.

III. AN INVESTIGATION OF DELAYED LIGHT EMISSION FROM Chlorella Pyrenoidosa

An apparatus capable of measuring emission lifetimes in the range 5 X 10^-9 sec to 6 X 10^-3 sec is described in detail. A cw argon ion laser beam, interrupted periodically by means of an electro-optic shutter, serves as the excitation source. Rapid sampling techniques coupled with signal averaging and digital data acquisition comprise the sensitive detection and readout portion of the apparatus. The capabilities of the equipment are adequately demonstrated by the results of a determination of the fluorescence lifetime of 5, 6, 11, 12-tetraphenyl-naphthacene in benzene solution at room temperature. Details of numerical methods used in the final data reduction are also described.

The results of preliminary measurements of delayed light emission from Chlorella Pyrenoidosa in the range 10^-3 sec to 1 sec are presented. Effects on the emission of an inhibitor and of variations in the excitation light intensity have been investigated. Kinetic analysis of the emission decay curves obtained under these various experimental conditions indicate that in the millisecond-to-second time interval the decay is adequately described by the sum of two first-order decay processes. The values of the time constants of these processes appear to be sensitive both to added inhibitor and to excitation light intensity.

Quantitative measurement of domain inversion in RuO2 : LiNbO3 crystal by digital holographic interferometry

We quantitatively study the domain inversion in a RuO2:LiNbO3 crystal wafer by the digital holographic interferometry. The crystal wafer is placed into one arm of a Mach-Zehnder-type interferometer to record a series of holograms. Making use of the angular spectrum backward propagation algorithm, we reconstruct the optical wave field in the crystal plane. The extracted phase difference from the reconstructed optical wave field is a well linear function of the applied external voltage. We deduce that the linear electro-optic coefficient of the detected RuO2:LiNbO3 crystal sample is 9.1x10(-12) m/V. An unexpected phase contrast at the antiparallel domain wall is observed and the influence of the applied external voltage on it is studied in detail. Also the built-in internal field is quantitatively measured as 0.72 kV/mm. (c) 2006 American Institute of Physics.

基于PLZT的电光偏转器的特性研究

采用具有高二次电光效应的掺镧锆钛酸铅陶瓷材料（PLZT），设计和制备了一种基于折射率随外加电压发生变化的电光偏转器。构建了测试系统，测得PLZT的电光系数是R33=2.1

单晶集成的1×N电光开关

为了使光学系统更加紧凑化和稳定化，本文提出了一种单晶集成的1×N电光开关：它有2个折射面，4个互相垂直的全内反射面，和N-1个电光调制器．还得到了该种1×N电光开关的构成条件：发生两相邻反射的反射面必须互相垂直．并对晶体的结构进行了优化，通过调整光束输出面与光轴的夹角来改变光束出射角，使得接收端易于接收输出的光信号．给出1×3电光开关的实例．

双中心全息记录的矢量分析与记录方向优化

对描述双掺杂晶体非挥发性全息记录动力学过程的Kukhtarev方程进行了矢量分析, 分析中考虑了体光生伏特效应和外加电场的作用。在小信号近似的基础上给出了双中心全息记录中记录与固定阶段空间电荷场的矢量解析解。在综合考虑空间电荷场的各向异性以及晶体有效电光系数的各向异性后, 给出了双中心全息记录的优化记录方向。结果表明, 对(Fe, Mn):LiNbO3晶体633 nm寻常光记录, 优化记录方向主要由有效电光系数决定, 光栅波矢与光轴夹角为22°, 方位角为30°;对(Fe, Mn):LiNbO3晶体633

单块晶体集成的N×N纵横开关网络及其控制算法

设计了一种单块晶体集成的n×n纵横开关(Crossbar)网络。通过综合考虑晶体的双折射和全内双反射现象,以及晶体的电光效应,将构成n×n纵横开关网络的所有单元器件都集成到一块具有电光效应的双折射晶体上。同时,给出了该网络的控制算法,通过对开关工作状态的控制,可以实现任意输入输出通道之间的无阻塞连接。这种单块晶体集成的纵横开关网络具有能量损耗低、无阻塞、易安装、抗干扰等优点,适合于全光网络发展的需要。

基于电光材料的光学相控阵技术研究进展

综述了基于电光材料的光学相控阵(OPA)的研究进展。介绍了光学相控阵技术的基本原理以及不同电光材料(铌酸锂电光晶体,AlGaAs光波导,液晶和掺镧锆钛酸铅(PLZT)电光陶瓷)光学相控阵技术的基本构想和涉及的关键技术;着重介绍近年来基于PLZT电光陶瓷材料光学相控阵技术的发展情况以及在这方面的最新研究成果,包括单级相控阵、级联相控阵、不同电极结构(表面电极和上下电极结构)相控阵技术等;最后简要介绍了光学相控阵技术在激光雷达等军事领域中的应用。