High Electro-optic Coefficient Research Articles

Design and fabrication of electro-optic waveguides with self-assembled superlattice films

This paper presents a strategy for fabricating low loss waveguide devices based on high electro-optic (EO) coefficient self-assembled superlattice (SAS) films, a new sort of polymeric films grown on SiO 2 film and coated with other polymeric films to form multi-layer EO waveguides structure without electric field poling. Firstly, the optical propagation loss induced by the absorption of electrodes is simulated and optimized to obtain both the low optical loss and the low drive voltage. Then this paper gives the scanned electron microscopic (SEM) images of the fabricated devices, the simulated and experimental images of the single guided mode, and the measured optical propagation loss of the EO waveguide devices of 1.0 dB/cm. Finally, the very great agreement between the simulated and measured results of propagation loss of devices is observed.

Very large electro-optic coefficients from in situ generated side-chain nonlinear optical polymers

A synthetic/processing method has been developed for achieving highly efficient nonlinear optical polymers. A maleimide-containing chromophore with large molecular nonlinearity was covalently attached onto a anthrathene-containing polymer to generate in situ a side-chain polymer via the Diels-Alder reaction. The poling and covalent side-chain attachment processes can be performed simultaneously in a solid state, resulting in high electro-optic (E-O) coefficients (r33 values up to 50pm∕V at 1310nm) and temporal stability (85% retention of the original dipole alignment after isothermal heating at 85°C for more than 500h). A very high chromophore loading level (39wt%) can be achieved by adding more chromophore into this side-chain polymer without causing any obvious phase separation. Poling of this chromophore-doped side-chain polymer (total dye content: 34wt%) showed a significantly increased E-O coefficient, from 50to110pm∕V.

Increased thermal stability of a poled electro-optic polymer using high-molar-mass fractions

Polymer fractionation yields materials with higher average molar masses and thus substantially increased glass transition temperatures. We apply this technique to a photoaddressable polymer that exhibits high electro-optic coefficients, in order to enhance the thermal stability of the poled material. The degree of orientation of the dipolar chromophores is investigated with both pyroelectric and electro-optic measurements. The pyroelectric measurements at elevated temperatures show no significant contribution of beta relaxation as compared to alpha relaxation.

Photorefractive Four-Wave Mixing Switches Utilizing Pockels Effect -Longitudinal and Lateral Switches-

We carried out detailed calculations for photorefractive wave-mixing switches based on one of three crystals with high electro-optic coefficients, namely, BaTiO3, Strontium Barium Niobate (SBN (0.75)), and Potasium Sodium Strontium Barium Niobate (KNSBN). A comparison of results for the three crystals shows that a 0_-cut BaTiO3 crystal is suitable for a longitudinal switch and requires a voltage of about 80 for a 2-mm-thick crystal to induce sufficient phase mismatch. The electrodes must be transparent for the incident and diffracted beams. A 45_-cut SBN (0.75) crystal, however, is suitable for a lateral switch and requires a voltage of about 150 for a 1-mm-wide crystal. The electrodes do not need to be transparent.

Cross-Linkable Zwitterionic Polyimides with High Electrooptic Coefficients at Telecommunication Wavelengths

A series of nonlinear optical (NLO) polyimides were prepared by grafting 4−16 wt % zwitterionic pyridinium tricyanoquinonedimethane chromophore with a large negative hyperpolarizability (β0 = −590 × 10-30 esu) onto the host polyimides through an ester bond formation. To enhance the temporal stability of the poled NLO polymers, a new thermally reactive group, i.e., 5-aminobenzocylcobutenone, was introduced into the NLO polymer systems to effect the cross-linking during the thermal poling process. The resulting NLO polyimides showed good solubility in polar aprotic solvents such as N,N-dimethylformamide and could form uniform films by casting or spin coating without any noticeable chromophore aggregation or phase separation as examined under an optical microscope. These NLO polyimides had glass transition temperatures in the range of 170−213 °C and were thermally stable up to 270 °C. The electrical resistivity of NLO polyimides was found to be on the order of 1015 Ω·cm at 80 °C and dependent on the applied voltages. As the temperature increased, the resistivity dropped monotonically to 2.4 × 1011 Ω·cm at 200 °C. With a 10 wt % chromophore loading (or a number density of 15 × 1019 cm-3) in the polymer, the electrooptic (EO) coefficient (r33 at 1550 nm) reached 45 pm/V. No substantial decrease in EO coefficients was observed after the test cell was kept in the dark at 85 °C under nitrogen after 1200 h nor when the test cell was irradiated with 1550 nm light (50 μW/∼4 mm2) for 10 h at ambient temperature in air.

Thin Film Ferroelectrics for Guided Wave Devices

Thin film ferroelectrics are being developed for guided wave optical devices. Of particular interest is BaTiO3 because of its high electro-optic coefficient. Epitaxial thin films have been deposited by metalorganic chemical vapor deposition that are suitable for electro-optic devices. Thin film electro-optic modulators have been fabricated and their optical properties characterized. Factors that determine the ultimate bandwidth of these devices are discussed.

Electroceramics Research in India

Electroceramics research is driven by the technology development needs and the device applications in the fields of microelectronics, communications, automation, energy conservation, MEMS and others. Ferroelectric ceramics have been technically exploited because of their unique properties such as high dielectric permittivity, high piezoelectric & electromechanical coupling and discovery of electrical poling process. Under certain circumstances they show high optical transparency and electro-optic coefficients. High permittivity barium titanate based piezoelectric ceramics and their polymer composites show unusual properties for a wide range of applications in sonars, ultrasonic cleaners, micro-accelerometers, hydrophones, surface acoustic wave filters, delay lines, and microactuators etc. A number of organizations in India viz. academic institutions, research laboratories and industries are involved in the preparation, characterization and device fabrication of the electroceramic materials. This paper presents an overview of the ongoing work and the future prospects.

Endless Fiber-to-Fiber Polarization Controller Based on Ceramic Programmable Waveplates

In this letter, we report a fast and endless polarization controller based on lead lanthanum zirconate titanate (PLZT) ceramic programmable waveplates. A set of four volume electrodes placed in a cylindrical arrangement are drilled in an electrooptic ceramic plate. By using the Kerr effect, and taking advantage of the high electrooptic coefficient and fast response time of the PLZT, we thus, obtain a fast rotatable and variable waveplate. Then, cascading two of these waveplates produces an endless polarization controller.

All-dielectric miniature wideband rf receive antenna

An integrated Mach-Zehnder interferometer made of electro- optic (EO) polymers with a dimension of 331.2 cm was fabricated as a wideband rf receive antenna. When an electric field is applied to the interferometer arm(s) made of EO material, a phase delay is generated that results in a net imbalance in the interferometer, and thus a change in the output intensity. This output intensity change, which contains electric field strength and temporal profile information, is then read by a photo- detector and processed. This antenna was tested in a m-strip traveling wave electromagnetic cell at frequencies from 100 MHz to 1.04 GHz. The test results show the antenna had a good linear response over a 70-dB power range. The lowest possible measured Emin was about 4 mV/m (or 2.1 pW/cm 2 ) at 1-kHz resolution bandwidth with a laser power of 0.4 mW (24 dBm) measured after the sensor. The measured E-field signal increases with increasing laser power, which indicates that signifi- cant sensitivity improvement can be easily obtained by lowering passive losses. The antenna sensitivity can be further improved by lowering the device insertion loss, optimizing the photodetector and detection cir- cuitry, and using EO polymers with higher EO coefficients. © 2004 Society

Pulsed laser deposition of PLZT films: structural and optical characterization

Transparent lanthanum-modified lead zirconate titanate (PLZT) thin films with electro-optic properties were deposited on indium-doped tin oxide (ITO) coated glass by pulsed laser deposition (PLD). Stoichiometric films with the crystallographic perovskite structure required for electro-optical behaviour, and a composition ratio close to 9/65/35 were obtained. The microstructural, optical and electro-optic properties of these films were studied for different substrate temperatures, T s, and reactor oxygen pressure, P O 2 . The films annealed at 700 °C in oxygen at atmospheric pressure are highly (1 1 0) perovskite orientated and free of the undesired pyrochlore phase. The optical constants ( n and k as a function of wavelength) of the films were obtained using variable angle spectroscopic ellipsometry (VASE) and spectrophotometry in the UV-Vis–NIR regions. They appear to be strongly depending on the deposition conditions. A high quadratic electro-optic coefficient, close to that of the bulk value, was obtained at 632.8 nm by single-beam (ellipsometric) configuration.

Lithium niobate electro-optic sensor for examining conductive patterns on display panels

The fabrication of an electric field detector using an electro- optic LiNbO3 (LN) single crystal is studied for an application to check the electric field of a conductively patterned panel. When this electric field detector moves on the surface of the panel, a constant air gap, which is close enough for the given field intensity and resolution, must be main- tained so as not to damage the surface patterns. For the effective detec- tion of electric field change in this air gap state, an LN single crystal is selected because of its relatively high electro-optic coefficient, transmit- tance, and low dielectric constant. X-cut LN and Z-cut LN structures are selected to estimate the applicability of LN by simulation on the optical intensity variation and electric field distribution of various structures. At an air gap of 10 mm in the Z-cut LN structure, Vp is about 100 to 150 V and optical intensity variation with unit voltage (Q) is 6.7%/V larger than that of the X-cut LN structure. From these characteristics, the Z-cut LN structure proves to be applicable for the electric field detector because the variation of optical intensity (0.8 mW) is sufficient in the ac driving voltage region (620 V) of a real system. © 2003 Society of Photo-Optical

Surface conductivity of DR1-functionalized organic-inorganic sol-gel materials

In recent years, in looking for an important photorefractive application, several studies on electro-optic and photoconductive sol-gel responses have been done. A very important effort has been addressed to establish the appropriate induced-orientation procedure, in order to get the highest electro-optic coefficient. In this way a very high coefficient of 48 pm/V at 831 nm in sol-gel has been already found. Similarly, the importance of the non-linear chromophore concentration into the material electro-optic behavior has been studied. However, the influence of the orientation procedure and the chromophore concentration over the photoconductive response has not been performed. In this work we study the vacuum-surface-charge-transport under and without illumination after poling times of 10, 30 and 120 min on DR1-functionalized sol-gel thin films of 1.3 μm in thickness with a suitable concentration of DR1. We include the measures before poling for other chromophore concentrations. We found the largest density of photocurrent at 633 nm for a poling time of 30 min. We also measured the order parameter in order to follow the Corona induced orientation evolution as function of time for each case. The saturation found into this parameter and into the photoconduction show the existence of an optimal poling time.

LiNbO/sub 3/ optical waveguides formed in a new proton source

Proton-exchanged planar waveguides have been fabricated on Z-cut and X-cut lithium niobate crystals by using a new proton source formed by a mixture of benzoic and adipic acids. Waveguide index profiles and optical characteristics have been obtained at different values of the adipic-benzoic acid concentration ratio. The samples have been structurally characterized by Raman and infrared (IR) absorption spectroscopy and double-crystal X-ray diffraction. Good quality samples have been fabricated by using 30 mol% ratio dilution, showing very low scattering levels (<0.1 dB/cm), relatively high electrooptic coefficient (r/sub 33/=0.88 pm/V), and low relative percentage of interstitial protons (26%). The main factor limiting the waveguide optical properties is the substitutional-interstitial proton ratio, which can be easily controlled to produce good quality waveguides. A demonstration of the repeatability of the exchange process in the acid mixture is also provided.

A simple method for image restoration and image pre-processing using two-wave mixing in Bi12TiO20

Optical information processing is one possible application of photorefractive crystals. In contrast to other methods where photorefractive crystals with high electro-optic coefficients (e.g. BaTiO3) or an applied electric field for increasing the beam coupling efficiency were applied, we used a simple two-wave mixing (TWM) set-up with a Bi12TiO20 (BTO) crystal without applied electric field to restore the intensity profile of a distorted image and for edge amplification and detection.

Electric Poling and Relaxation of Thermoset Polyurethane Second-Order Nonlinear Optical Materials: Role of Cross-Linking and Monomer Rigidity

A high μβ isophorone-derived phenyltetraene chromophore (denoted CLD-5) was synthesized. The chromophore was modified with a hexyl group at the middle of the π-conjugate bridge to improve solubility and processability and was dihydroxy-functionalized for covalent incorporation into various cross-linked PU polymer systems. Its electrical poling and relaxation behavior in PU polymer thin films were studied. First, the chromophore was incorporated into conventional TDI/TEA polyurethane, and an electrooptic (EO) coefficient of 57.6 pm/V at 1.06 μm was obtained, which is 28% higher than that obtained from nonhexylated chromophore (CLD-2) in the same polyurethane system. Two new polyurethane systems, poly[(phenyl isocyanate)-co-formaldehyde] (PPIF)/triethanolamine (TEA) and PPIF/bisphenyl-1,1‘-dimethanol (BPDM), were designed to study the influence of cross-link density and monomer rigidity on electrical field poling of chromophore dipoles and relaxation behavior of poling-induced chromophore alignment. CLD-5/PPIF/TEA polymer has the highest cross-link density (3.07 mmol/g) among all the polyurethanes studied here. It gains 38 °C in thermal stability but loses 50% of EO activity as compared with the CLD-5/TDI/TEA polyurethane material, which has a cross-link density of 1.91 mmol/g. A higher EO coefficient (41 pm/V), lower optical loss of 2.56 dB/cm at 1.3 μm, and the highest dynamic stability (133 °C) were obtained for the CLD-5/PPIF/BPDM polyurethane EO material, which has the lowest cross-link density (1.27 mmol/g) and the most rigid monomers. The results indicate that excessive cross-linking deteriorates electric poling of long chromophores in a cross-linked polymer thin film. Therefore, cross-linking by itself is not necessarily a good approach to achieving high thermal stability of electrical field-induced chromophore alignment in polyurethane systems.

Design and synthesis of low refractive index polymers for modulation in optical waveguides

Design and synthesis of new electro-optic polymers are presented. These polymers have specific refractive index as low as the index of glass ion-exchange waveguides. With these materials, a novel hybrid structure can be designed for optical modulation in passive waveguide. The low index is obtained by copolymerization of an electro-optic monomer and two fluorinated comonomers. The result is a copolymer with an index close to 1.50, high electro-optic coefficient and good technical properties.

Progress toward Device-Quality Second-Order Nonlinear Optical Materials: 2. Enhancement of Electric Field Poling Efficiency and Temporal Stability by Modification of Isoxazolone-Based High μβ Chromophores

Isoxazolone (ISX) acceptor based chromophores terminated with hydroxyl functional groups were synthesized, spin cast into optical quality 1−2 μm thick films, poled in the presence of an electric field, and thermoset into polyurethane networks to lock in the poling-induced acentric chromophore order. Alkyl groups were introduced into the chromophore backbone (π-electron bridge) to increase chromophore solubility and to inhibit electrostatic interactions between these “high μβ” chromophores. Thermosetting polyurethanes containing alkyl-substituted chromophores exhibited much (1.7−1.9 times) higher electrooptic coefficients than polymers containing unsubstituted chromophores. Chromophores with hydroxyl groups on only the donor end of the molecule were processed into hardened “themoset” materials, exhibiting a dynamic thermal stability (temperature at which the macroscopic optical nonlinearity is first observed to decrease in a thermal ramping experiment) of poling-induced dipolar order of ∼80 °C, while chrom...

Design and fabrication of electro-optic polymer modulators and switches

Electro-optic (EO) polymers are interesting materials for realising active functions in integrated optics devices, since they can have relatively high EO coefficients and they can be easily combined or integrated with several (passive) materials. EO interaction can be exploited for obtaining modulation or switching in polymer-based devices using several principles (e.g. Mach–Zehnder, Digital Optical Switch or tuned coupling to surface plasmons), which have been investigated and tested using a number of different materials. We compare these principles in view of several aspects of practical importance: realistic values of EO coefficients and wavelength-dependent attenuation, optical, electrical and chemical compatibility of substrate, guiding and cladding layers, channel definition by etching (inverted) ridge waveguides or by photo-bleaching, local and global poling methods, polarisation-dependence, and the design of efficient high-bandwidth travelling wave electrodes. In most of the investigated devices, we employ a waveguiding structure based on silicon and its oxynitrides, exploiting the fact that the refractive index of silicon oxynitride can be accurately adjusted over a wide range by adjusting its composition. We will discuss the practical difficulties encountered and show the obtained results with phase- and intensity modulators and switches.

Structural and electro-optic properties in lead magnesium niobate titanate thin films

The structural and quadratic electro-optic properties in lead magnesium niobate titanate thin films have been systematically studied as a function of the fraction x of lead titanate. (110)-oriented, pyrochlore-free PMN–PT films with PT contents around morphotropic phase boundary compositions, which have a propagation loss as low as 3 dB/cm and a high electro-optical (EO) coefficient of 1.32×10−16 (m/V)2, have been fabricated on (1012)-cut sapphire substrates. The EO measurements show that the PMN–PT is a highly promising electro-optic material.

Nanocomposite of semiconducting ferroelectric antimony sulphoiodide dots-doped glasses

Antimony sulphoiodide (SbSI) is a semiconducting ferroelectric crystal. The SbSI possesses attractive properties, such as high pizoelectric constant, very high dielectric permittivity, and high electro-optic coefficient, but it is very difficult to grow a single crystal with bigger size. Currently, a method for synthesis of SbSI by liquid reaction has been developed in our laboratory. Nanocomposites of SbSI dots in sodium borosilicate glass as well as in organic modified silicate (ORMOSIL) matrix have been fabricated by the sol-gel technique. The dot size is controlled by processing conditions. The nanocomposites were characterized by x-ray diffraction and high resolution TEM. Quantum confinement effect was observed on optical spectra. Nonlinear optical property, the third order susceptibility χ(3). of the SbSI dots-doped glasses was measured by four wave mixing method at 532 nm. The value of χ(3) was found to be 6 χ 10−11 esu at this non-resonant wavelength.