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Polymer Photonics

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Abstract

The accessibility of ultrashort high-repetition rate lasers has led to the study of many nonlinear optical (NLO) phenomena and their applications in photonics. Organic and polymeric materials have attracted enormous interest as materials from nonlinear optics because of their tailorability and easy processability. In this chapter we deal with the design and structure–property relationships in materials for second- and third-order NLO effects. The phenomena that are explored include Pockel’s effect, optical Kerr effect, and two-photon absorption (TPA). The factors contributing to the design of highly active NLO materials are discussed with examples to demonstrate various approaches.

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Thesis
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Chapter
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Chapter
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We describe the use of two-photon absorption in a photorefractive crystal for recording bit data in multilayered optical memory. A short-pulse near-infrared laser is used for generating the photorefractive effect by two-photon absorption. We succeeded in recording and reading seven layers of data in a LiNbO 3 crystal with a lateral resolution (distance between bits) of 5 µ m and an axial resolution (distance between layers) of 20 µ m .
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Electro-optic (EO) modulators encode electrical signals onto fiber optic transmissions. High drive voltages limit gain and noise levels. Typical polymeric and lithium niobate modulators operate with halfwave voltages of 5 volts. Sterically modified organic chromophores have been used to reduce the attenuation of electric field poling–induced electro-optic activity caused by strong intermolecular electrostatic interactions. Such modified chromophores, incorporated into polymer hosts, were used to fabricate EO modulators with halfwave voltages of 0.8 volts (at a telecommunications wavelength of 1318 nanometers) and to achieve a halfwave voltage-interaction length product of 2.2 volt-centimeters. Optical push-pull poling and driving were also used to reduce halfwave voltage. This study, together with recent demonstrations of exceptional bandwidths (more than 110 gigahertz) and ease of integration (with very large scale integration semiconductor circuitry and ultra-low-loss passive optical circuitry) demonstrates the potential of polymeric materials for next generation telecommunications, information processing, and radio frequency distribution.
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WE have designed and built a prototype of a scanning microscope for thick objects of low reflectivity and low optical contrast. As light source, we used a 5 mW He-Ne continuous wave laser (Spectra-Physics Model 120). An improved prototype is under construction; in a year or two we expect to have a microscope suitable for advancing studies of brain cells in living animals1-4.
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The absorption and emission spectroscopy and nonlinear optical absorption of a series of a,o-dithienyl polyenes were studied in chloroform and n-octane solutions. Dithienyl polyenes containing butyl substituents were also studied. Steady state fluorescence, time resolved emission, fluorescence lifetime and quantum yield measurements were made. The fluorescence quantum yields were significantly lower than those of the corresponding a,w-diphenyl polyenes. Among the dithienyl polyenes, 3,3'-substituted polyenes exhibited stronger fluorescence than those of the 2,2'-substituted. Nonlinear optical absorption experiments showed evidence of two photon absorption. In some polyenes, two photon absorption was followed by excited state absorption.
Conference Paper
We have developed compact millimeter thick optical fiber arrays with nonlinear optical liquid guiding cores. These image transmitting fiber arrays are capable of passive optical limiting action against frequency agile short (picosecond- nanosecond) laser pulses in the visible spectrum. We present the results of further studies of their limiting performance. We also discuss how a recently discovered extraordinarily large nonlinear optical response of methyl-red doped nematic liquid crystal film may be employed for limiting action on longer pulse or cw lasers with nanowatt threshold power.
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The design, synthesis, characterization, and understanding of new molecular and macromolecular assemblies with large macroscopic optical nonlinearities represents an active field of research at the interface of modern chemistry, physics, and materials science. Challenges in this area of photonic materials typify an important theme in contemporary chemistry: to create new types of functional materials by the rational construction of supramolecular assemblies exhibiting preordained collective phenomena by virtue of "engineered" molecule-molecule interactions and spatial relationships. This review surveys several approaches to, and the microstructural and optical properties of, second-order nonlinear optical materials built from noncentrosymmetric assemblies of chromophores having large molecular hyperpolarizabilities. Such types of materials can efficiently double the frequency of incident light, exhibit other second-order nonlinear optical effects, and contribute to the knowledge base needed for new photonic device technologies. Systems described include chromophore macromolecule guest host matrices, chromophore-functionalized glassy macromolecules, thermally crosslinked chromophore-macromolecule matrices, and intrinsically acentric self-assembled chromophoric superlattices.
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A series of new star-shaped donor-π-acceptor (D-π-A) molecules containing the 2,4,6-tri(thiophen-2-yl)-1,3,5-triazine unit were synthesized and characterized. The 1,3,5-triazine group, as a strong electron-accepting center, is connected to three electron-donating end groups through π-conjugated bridges. As a result of the coexistence of the electron acceptor and donor, these compounds show reversible or quasireversible redox behavior. Through changing the peripheral end group the optical properties can be modified. All compounds exhibit two-photon absorption activity in the range of 720–880 nm and show large two-photon absorption cross sections that are closely related to the intramolecular charge transfer and π-conjugated length of the molecule.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)
Article
A scheme based on Diels-Alder 'click reaction' to functionalize maleimide-containing nonlinear optical (NLO) chromophores onto polymers that possess pendant anthryl-containing diene moieties was described. The 'click-reaction enables to create different architectures of macromolecules by changing the attaching modes of the chromophore onto polymers. Three different macromolecular architectures were created by covalently attaching the center site of the chromophore bridge to a polymer backbone or a dendritic moiety. The results show that these polymers exhibit high dielectric strength, excellent optical quality, good processability, and reliable poling behavior.
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It is shown experimentally that the complete replacement of benzene rings with thiophene rings in a donor–acceptor substituted stilbene significantly enhances its second-order nonlinear optical hyperpolarizability (β); results obtained with a series of thiophene-substituted donor–acceptor compounds with different conjugation lengths reveal a power law dependence of βµ on the molecular conjugation length.
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Natural photosynthetic systems collect sunlight using a vast array of light-harvesting chromophores that channel the absorbed energy to a single reaction center. Recently, it has been realized that dendritic macromolecules can exhibit similar properties, though on a more modest scale. The preparation of dendritic structures and assemblies composed of numerous light-collecting chromophores that transfer their energy to a single energy ‘sink’ at the core has been achieved in a number of diverse and creative ways. These novel structures are being used as model systems in light-emitting diodes, signal amplifiers, fluorescent sensors, frequency converters, and other photonic devices.
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The non-linear optical polarization of an isolated atom or molecule is treated, giving careful consideration to secular and resonant terms in the perturbation expansion. The Method of Averages introduced by Bogoliubov and Mitropolsky is used. The case where resonance-induced excited state populations are negligible, which is relevant to a wide range of non-linear optical experiments, is examined in detail for polarizations through third order in the perturbing fields. This yields concise expressions which are valid for any combination of applied field frequencies, including static fields.
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The electronic and vibronic contributions to the two-photon absorption of a series molecules with multi-branched structures have been studied using ab initio response theory. The results indicate that the electronic coupling between different branches alone cannot explain the experimental finding of a strong enhancement of the two-photon absorption cross section over the single branch structure, whereas it is predicted that vibronic contributions can play an important role in this respect. It is shown that for multi-branched molecules the use of circularly polarized light can increase the two-photon absorption cross section by a factor of 1.5 over linearly polarized light excitation.
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Electro-optic (EO) modulators are typically made from inorganic materials such as LiNbO3, but replacing them with organic EO materials, that is, ones with optical properties that change in response to an electric field, could be a promising alternative because they offer large bandwidth, ease of processing and relatively low cost. Here we incorporate a doped, crosslinked organic EO polymer into hybrid polymer/sol-gel waveguide modulator devices with exceptional performance. The half-wave voltages of the resulting Mach-Zehnder (MZ) and phase modulators at 1550 nm are 1 V and 2.5 V, respectively. The unique properties of the sol-gel cladding materials used in the hybrid structure result in a 100% device poling efficiency, leading to respective in-device EO coefficients of 138 pm V-1 and 170 pm V-1 in the MZ and phase modulators. These results are the first to show in-device EO coefficients that are five to six times larger than those of the benchmark inorganic material.
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Bisthienylethenes (BTE) are one of the most promising families of photochromic compounds used in molecular switches because of their fatigue resistance and thermally irreversible properties. Moreover, such molecular switches hold tremendous potential applications in molecular electronic and photonic devices such as optical data storage media, optical limiters, logic gates, molecular wires and sensors. Herein we highlight some recent developments of photochromic bisthienylethene derivatives with large two-photon absorption (TPA) or excellent two-photon excited fluorescence (TPEF) which can be potentially used in three-dimensional (3D) optical data storage media, two-photon switches and photon electronic devices. What's more, we examine some promising approaches on bisthienylethene derivatives such as supramolecular self-assembled structures, coordination with metal and ligands, capping on nanoparticles or assemblies on metal surfaces, which would tune the photochromic properties of photochromic switches for further applications.
Article
The degenerate two- and three-photon absorption properties of a multi-branched chromophore were investigated in the femtosecond regime utilizing white-light continuum and nonlinear transmission techniques. The experimental results show that the studied multi-branched structure exhibits relatively strong and broad two- and three-photon absorption bands in the near infra-red (IR) region. It is demonstrated that a highly conjugated molecular structure based on a symmetrically substituted skeleton would possess promoted molecular nonlinear absorptivities within the studied spectral region. Both two- and three-photon absorption-based optical power-limiting properties in the femtosecond time domain of this model compound were also characterized. The results indicate that a multi-chromophoric structure with expanded π-conjugation could be an effective multi-photon absorber and might be used as a single-component material system for quick-responsive and broadband optical-suppressing related applications, especially when under ultrashort laser pulses.
Article
Two-photon absorption properties of a series of symmetrically substituted stilbenes and asymmetrically substituted stilbene-type derivatives with the same conjugated length have been investigated. The effective two-photon absorption cross sections, δTPA, as large as 62.0 × 10−48 cm4 s photon−1 for D-π-D molecules and 48.5 × 10−48 cm4 s photon−1 for D-π-A counterparts have been observed. The effect of these two types of chromophores on the peak position of the linear absorption, one-photon fluorescence as well as two-photon absorptivity is reported. Dipole moment change between the ground and the first excited states (Δµge), and the transition dipole moment between the first and second excited states (Mee′) have also been calculated. It was found that the asymmetrically substituted derivatives possess relatively large Δµge, whereas the symmetrical counterparts show an increase in Mee′. Although a large two-photon absorption resonance is due to the simultaneously high values of Mee′ and Δµge, correlated to intramolecular charge transfer, the former function is larger. These results obtained have demonstrated that the magnitude and the peak position of two-photon absorption depend not only on the amount but also on the direction of the intramolecular charge transfer.
Article
The synthesis of a new soluble conjugated porphyrin polymer 4 is reported. The MALDI TOF mass spectrum shows the presence of oligomers with up to 13 repeat units and GPC gives a Mn of 53 kDa. The electronic absorption spectra of this polymer exhibit an intense Q band at 800 nm in solution and 853 nm in the solid state, demonstrating a high degree of conjugation. Electroabsorption spectroscopy shows that thin films of 4 have lower resonant third order NLO susceptibility than our previous conjugated porphyrin polymer 2, whereas closed z-scan measurements indicate that the off-resonance real susceptibility, at 1064 nm, is exceptionally large for both polymers (χ(3)R = −2 × 10−16 m2 V−2). Open z-scan measurements were also made at 1064 nm, demonstrating that the two polymers exhibit similar nonlinear absorption at this wavelength (β = 1 cm GW−1 at 0.2 mM concentration).
Article
During the past five years there has been considerable progress in the design of organic materials for optical power limiting. Among the more promising of the new material approaches are new reverse saturable absorbers (RSA) which derive their limiting capability on the photogeneration of highly absorbing charge states. Equally intriguing are the new approaches to designing chromophores with large two-photon cross-sections which give access to highly absorbing transient excited states. It now seems possible to combine these two new optical limiting paradigms in single structures which may therefore be considered as bimechanistic optical power limiters.
Article
A new series of aromatic azobenzol compounds containing vinyl have been designed as monomers. The azobenzene-containing side-chain polymers containing azo NLO chromophore in each side chain have been synthesized via free radical polymerization. FT-IR, elemental analysis and 1H NMR were performed to characterize the azo monomers. The molecular weight of the polymers and their distribution were determined by gel permeation chromatography (GPC). The third-order NLO coefficient of azo monomers and their polymers were measured by degenerated four wave mixing (DFWM) technique. As a result, the enhancement of the molecular conjugation and the increase of the NLO chromophore concentration in the molecular chain contribute much to heightening the third-order NLO effect. The electronic effect of substituent on the azobenzol group and the push–pull electronic structure contributes much to enhancing the NLO property.
Article
We will report the synthesis and linear and nonlinear optical (NLO) properties of novel fluorine-containing second-order NLO polymers with high glass transition temperature (Tg). Using pentafluorobenzonitrile as a starting material, a monomer with electron-donating group for NLO active site was synthesized. This monomer was polymerized with four types of bisphenol analogues. Three types of electron-withdrawing groups for NLO activity were introduced into the polymers by post-functionalization. These NLO polymers show high Tgs, e.g., a polymer containing fluorene structure has Tg of 209 °C. An NLO polymer with fluorene structure and DR1 at 100 mol% content does not show polarized light dependence of the refractive indices and has very low optical propagation loss of 0.42 dB/cm at 1.3 μm. The polymer has χ(2)33 value of about 57 pm/V at 1.3 μm fundamental wavelength and the nonlinearity was maintained at 100 °C for more than 160 h.
Article
3D bit optical data storage has the ability to reach Tbytes on a recording medium no larger than a compact disc using two-photon excitation and multi-layer recording. The introduction of a poly-vinyl carbazole (PVK) based photorefractive polymer as the recording material allows the system to rewrite the recorded information via the photorefractive effect.[1,2] In this paper, we demonstrate the use of continuous wave illumination for three-dimensional (3D) bit optical data storage under two-photon excitation in a new poly(methylmethacrylate)-based (PMMA) photorefractive polymer. This achievement makes it possible to develop a cheap, compact, sub Tbyte rewritable optical data storage system to further extend the capabilities of compact disc and digital versatile disc technology.
Article
Novel styryl derivatives with triphenylamine as donor and 1,3,5-triazine as acceptor were synthesized and their one- and two-photon properties were investigated. These molecules show large two-photon absorption (TPA) cross sections and strong two-photon fluorescence. TPA cross section is as large as 1.64 × 10-20 cm4/GW for the tri-branched molecule.
Article
The title host compounds form chiral inclusion complex crystals with various kinds of achiral guest compounds in which the achiral host molecules are arranged in a chiral form through halogen–halogen interactions between host molecules.
Article
A high mu beta isophorone-derived phenyltetraene chromophore (denoted CLD-5) was synthesized. The chromophore was modified with a hexyl group at the middle of the pi -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 mum 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), mere 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 degreesC 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 mum, and the highest dynamic stability (133 degreesC) 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, crosslinking by itself is not necessarily a good approach to achieving high thermal stability of electrical field-induced chromophore alignment in polyurethane systems.
Article
The corona poling process of electro-optic polymers has been investigated for rodlike aromatic polyimides doped with nonlinear optical chromophores. The dynamics of the chromophores in poling during and after imidization was observed by measuring second harmonic generation (SHG) from the polymer. In the course of poling during imidization, the SHG, observed before imidization, decreases to zero, reappears, and increases as the temperature increases. When poling was after imidization, the SHG intensity increased monotonically with increasing temperature. The disappearance of SHG in the poling during imidization is probably due to the known structural change in the polymer chains during imidization and the subsequent reorientation of the chromophores caused by formation of sheets of polyimides. The characteristics of the samples poled during and after imidization are nearly the same in both the extent of the nonlinear susceptibility and their thermal stability.
Article
Recent progress in developing highly efficient nonlinear optical dendrimers and polymers for high-performance electro-optic (EO) devices has been reviewed. Our efforts are focused on using nanoscale architectural control to tailor the size, shape, conformation, and functionality of NLO chromophores and macromolecules and Studying their effects on poling efficiency. The structures of these materials vary from a 3-D-shaped dendritic chromophore, multifunctional dendrimers with the center core connected to NLO chromophores and crosslinkable functional groups at the periphery, to side-chain-dendronized NLO polymers. All the poling results from these systems have shown dramatically enhanced EO properties (a factor of 2-3) compared to conventional NLO polymers.
Article
Fully conjugated π-expanded macrocyclic oligothiophenes with 60π–180π electron systems have been synthesized using modified McMurry coupling reaction as a key step. X-ray analyses of 60π and 72π systems revealed unique molecular and packing structures, reflecting planar cyclic structures with large inner cavities. All giant macrocycles exhibit multi-step reversible redox behavior with fairly low first oxidation potentials, reflecting their cyclic conjugation. Doping of macrocycles with iodine forms semiconductors owing to their π-donor properties and π–π stacking ability. Interestingly, 90π and 120π systems self-aggregate in the solid state to form red nanofibers. The structures of fibrous aggregates have been established by SEM and AFM analyses. Furthermore, two-photon properties of 72π, 108π, 144π, and 180π systems show that the increasing π-conjugation leads to an increase in the two-photon absorption cross-section with magnitudes as high as 100,000GM in the visible spectral region.
Article
Using the single beam z-scan technique with 30 ps pulses at 532 nm (close to the linear absorption region) the values of real and imaginary part of third-order optical nonlinear susceptibility χ(3) in solutions of thiophene oligomers have been determined for different numbers of repeat units (n=2–6). The values of the real part of χ(3) were found to be negative, the contribution of each repeat unit to this value increases with increasing chain length. In solutions of dioxane (10−3 mol/ℓ) we measured n2 values in the order of 10−13 esu, whereas two-photon absorption coefficients of about 0.1 cm/GW were found.
Article
We report the two-photon fluorescence excitation spectrum of poly(p-phenylenevinylene) over the energy range 1.1–2.0 eV. The onset for two-photon fluorescence excitation is at 1.3 eV with a strong peak near 1.475 eV. The peak locates the energy of the lowest even parity excited singlet at 2.95 eV, some 0.5 eV above the lowest dipole-allowed singlet transition (the fundamental ππ* absorption). This energetic ordering of excited states is in agreement with the predictions of theory and is consistent with the strong one-photon fluorescence seen for this polymer.
Article
Mean field theory is used to calculate the temperature-composition phase diagram and component order parameters of binary nematic mixtures. Experimental values for the mixture order parameter of a binary nematic mixture close to the nematic/isotropic transition have been obtained from refractive index measurements. The experimental results qualitatively confirm the predictions of the theory.