Scott R. Hammond

Molecular engineering of nanoscale order in organic electro-optic glasses

The rational design of bulk nanoscale order in organic electro-optic materials, where the strong dipole–dipole interactions tend to dominate over the weaker forces exploited for self-assembly processes, remains an attractive yet elusive goal. Towards this end, a series of pseudo-discotic dipolar nonlinear optical chromophores have been synthesized and fully characterized. Theoretical guidance and an iterative molecular design process have succeeded in engineering long-range nanoscale order in organic electro-optic glasses. Small-angle thin-film X-ray diffraction experiments demonstrate a self-assembled lamellar morphology in a majority of these materials. Cryogenic crystallography, using a synchrotron X-ray source, afforded the structure of a representative system. This structure, in concert with thin-film X-ray diffraction, atomic force microscopy, UV-vis-NIR absorption spectroscopy, and refractive index experiments elucidated the nanoscale order in the films. Application of these materials in electro-optics is discussed.

Electro-optic coefficients of 500 pm/V and beyond for organic materials

Theoretical guidance, provided by quantum and statistical mechanical calculations, has aided the recent realization of electro-optic coefficients of greater than 300 pm/V (at 1.3 microns wavelength). This articles attempts to provide physical insight into those recent results and to explore avenues for the further improvement of electro-optic activity by structural modification, including to values of 500 pm/V and beyond. While large electro-optic coefficients are a necessary condition for extensive practical application of organic electro-optic materials, they are not a sufficient condition. Adequate thermal and photochemical stability, modest to low optical loss, and processability are important additional requirements. This article also examines such properties and suggests routes to achieving improved auxiliary properties.

Synthesis and lyotropic liquid crystalline behaviour of a taper-shaped phosphonic acid amphiphile

A taper-shaped phosphonic acid, 3,4,5-tris(dodecyloxy)phenylmethylphosphonic acid (1), was synthesized; its lyotropic liquid crystalline (LLC) behaviour and its ability simultaneously to order and acid-dope polyaniline were examined. It was found that the ability of 1 to form LLC phases in the presence of several hydrophilic solvents is restricted by strong intermolecular interactions between the phosphonic acid head groups (presumably H-bonding). The amphiphile exhibits poor miscibility with pure water and even with strong H-bonding organic solvents such as DMF. However, it forms a lamellar mesophase in the presence of aqueous acid. Upon deprotonation of the phosphonic acid head group with NaOH, the resulting disodium salt of the amphiphile is able to form a well defined lamellar phase with pure water. The propensity of 1 to form lamellar phases is somewhat unusual since its tapered molecular shape should direct it to form an inverted hexagonal LLC phase. These results suggest that intermolecular head group interactions are more important in determining the overall LLC behaviour of this phosphonic acid amphiphile than are the hydrophobic character and shape of the organic tail system. Compound 1 was also found to be sufficiently acidic to act as an acid dopant for the conjugated polymer polyaniline in the emeraldine base form. LLC acid 1 induces the resulting polymeric salt to form an electrically conductive LLC complex with an extended lamellar microstructure. The bulk conductivity of the resulting nanostructured polyaniline salt was found to be only in the semiconducting regime (10-5 Scm-1), due to an unfavourable polyaniline chain conformation in the LLC complex.

Modeling the optical behavior of complex organic media: from molecules to materials.

For the past three decades, a full understanding of the electro-optic (EO) effect in amorphous organic media has remained elusive. Calculating a bulk material property from fundamental molecular properties, intermolecular electrostatic forces, and field-induced net acentric dipolar order has proven to be very challenging. Moreover, there has been a gap between ab initio quantum-mechanical (QM) predictions of molecular properties and their experimental verification at the level of bulk materials and devices. This report unifies QM-based estimates of molecular properties with the statistical mechanical interpretation of the order in solid phases of electric-field-poled, amorphous, organic dipolar chromophore-containing materials. By combining interdependent statistical and quantum mechanical methods, bulk material EO properties are predicted. Dipolar order in bulk, amorphous phases of EO materials can be understood in terms of simple coarse-grained force field models when the dielectric properties of the media are taken into account. Parameters used in the statistical mechanical modeling are not adjusted from the QM-based values, yet the agreement with the experimentally determined electro-optic coefficient is excellent.

Acentric lattice electro-optic materials by rational design

Quantum and statistical mechanical calculations have been used to guide the improvement of the macroscopic electro-optic activity of organic thin film materials to values greater than 300 pm/V at telecommunication wavelengths. Various quantum mechanical methods (Hartree-Fock, INDO, and density functional theory) have been benchmarked and shown to be reliable for estimating trends in molecular first hyperpolarizability, β, for simple variation of donor, bridge, and acceptor structures of charge-transfer (dipolar) chromophores. β values have been increased significantly over the past five years and quantum mechanical calculations suggest that they can be further significantly improved. Statistical mechanical calculations, including pseudo-atomistic Monte Carlo calculations, have guided the design of the super/supramolecular structures of chromophores so that they assemble, under the influence of electric field poling, into macroscopic lattices with high degrees of acentric order. Indeed, during the past year, chromophores doped into single- and multi-chromophore-containing dendrimer materials to form binary glasses have yielded thin films that exhibit electro-optic activities at telecommunication wavelengths of greater than 300 pm/V. Such materials may be viewed as intermediate between chromophore/polymer composites and crystalline organic chromophore materials. Theory suggests that further improvements of electro-optic activity are possible. Auxiliary properties of these materials, including optical loss, thermal and photochemical stability, and processability are discussed. Such organic electro-optic materials have been incorporated into silicon photonic circuitry for active wavelength division multiplexing, reconfigurable optical add/drop multiplexing, and high bandwidth optical rectification. A variety of all-organic devices, including stripline, cascaded prism, Fabry-Perot etalon, and ring microresonator devices, have been fabricated and evaluated.

Optimizing electro-optic activity in chromophore/polymer composites and in organic chromophore glasses

The motivation for use of organic electro-optic materials derives from (1) the inherently fast (sub-picosecond) response of π-electron systems in these materials to electrical perturbation making possible device applications with gigahertz and terahertz bandwidths, (2) the potential for exceptionally large (e.g., 1000 pm/V) electro-optic coefficients that would make possible devices operating with millivolt drive voltages, (3) light weight, which is a concern for satellite applications, and (4) versatile processability that permits rapid fabrication of a wide variety of devices including conformal and flexible devices, three dimensional active optical circuitry, hybrid organic/silicon photonic circuitry, and optical circuitry directly integrated with semiconductor VLSI electronics. The most significant concerns associated with the use of organic electro-optic materials relate to thermal and photochemical stability, although materials with glass transition temperatures on the order of 200°C have been demonstrated and photostability necessary for long term operation at telecommunication power levels has been realized. This communication focuses on explaining the theoretical paradigms that have permitted electro-optic coefficients greater than 300 pm/V (at telecommunication wavelengths) to be achieved and on explaining likely improvements in electro-optic activity that will be realized in the next 1-2 years. Systematic modifications of materials to improve thermal and photochemical stability are also discussed.

Organic electro-optic glasses for WDM applications

This communication primarily deals with utilizing organic electro-optic (OEO) materials for the fabrication of active wavelength division multiplexing (WDM) transmitter/receiver systems and reconfigurable optical add/drop multiplexers (ROADMs), including the fabrication of hybrid OEO/silicon photonic devices. Fabrication is carried out by a variety of techniques including soft and nanoimprint lithography. The production of conformal and flexible ring microresonator devices is also discussed. The fabrication of passive devices is also briefly reviewed. Critical to the realization of improved performance for devices fabricated from OEO materials has been the improvement of electro-optic activity to values of 300 pm/V (or greater) at telecommunication wavelengths. This improvement in materials has been realized exploiting a theoretically-inspired (quantum and statistical mechanics) paradigm for the design of chromophores with dramatically improved molecular first hyperpolarizability and that exhibit intermolecular electrostatic interactions that promote self-assembly, under the influence of an electric poling field, into noncentrosymmetric macroscopic lattices. New design paradigms have also been developed for improving the glass transition of these materials, which is critical for thermal and photochemical stability and for optimizing processing protocols such as nanoimprint lithography. Ring microresonator devices discussed in this communication were initially fabricated using chromophore guest/polymer host materials characterized by electro-optic coefficients on the order of 50 pm/V (at telecommunication wavelengths). Voltage-controlled optical tuning of the pass band of these ring microresonators was experimental determined to lie in the range 1-10 GHz/V or all-organic and for OEO/silicon photonic devices. With new materials, values approaching 50 GHz/V should be possible. Values as high as 300 GHz/V may ultimately be achievable.

Recent advances in organic electro-optic materials for ring micro-resonators and optical modulation

Recently developed organic electro-optic materials have demonstrated large increases in activity creating a drive towards utilizing organics in ring micro-resonators and modulators. These materials allow for extremely low drive voltages and fundamental response times within the terahertz region. Present synthetic efforts have efficiently incorporated molecules with large first molecular hyperpolarizabilities, β, into macromolecular systems producing unprecedented electro-optic coefficients, r33. Previously, incorporation of these large β molecules into macromolecular systems proved difficult due to phase separation or molecular aggregation within the processed films. Therefore, integration into workable devices was inconsistent and difficult. The new material systems however, have shown considerably enhanced film qualities, leading to improved device incorporation and fabrication. This paper will focus on current organic materials strategies and their incorporation into current ring micro-resonator devices and results.

A novel approach to achieve higher order using pseudo-discotic chromophores in electro-optic materials and devices

Organic materials have great potential for electro-optic devices. Achieving higher unidirectional ordering of chromophores is the main challenge in using organic materials in such devices. The prime reason for this decreased order is due to interchromophore interactions which lead to a decrease in the electro-optic coefficient, r33. A material consisting of a chromophore surrounded by dendrons and possessing an appropriate aspect ratio should facilitate the near Ferro-electric ordering as observed in the discotic liquid crystals. For that purpose, a prototype dendrimeric chromophore has been synthesized and will be used in Mach-Zehnder devices to achieve lower drive voltages.

Synthesis of Dendridic NLO Chromophores for the Improvement of Order in Electro-optics

Previous research in organic electro-optics has shown dramatic increases in the hyperpolarizablity of NLO chromophores. However, this large microscopic activity has not been translated to the macroscopic domain. The polymeric electro-optic (E-O) materials continue to lack the high noncentrosymmetric order of the poled chromophores within the matrix necessary for high E-O response ( r 33 ). This deficiency of order represents one major obstacle that must be overcome before E-O device commercialization can be achieved. This lack of order is partially due to the large dipole moments of high μβ chromophores, which cause the chromophores to align in a centrosymmetric fashion through intermolecular electrostatic interactions. However, quantum calculations show that when the aspect ratio between the width and length of the chromophore system is adjusted to be greater than 1.4:1 by adding bulky side groups around the center of the chromophore, it would prevent side on pairing of the chromophores. This would cause a decrease in the large areas of centrosymmetric aggregation and thus allow for easier poling of the system. Here we report the synthesis of a nanoscale NLO architecture in which dendritic moieties have been incorporated around the center of the chromophore to give a three dimensional structure in order to achieve the 1.4:1 aspect ratio and maximize the macroscopic order of the system.