Denise H. Bale

Rational Enhancement of Second-Order Nonlinearity: Bis-(4-methoxyphenyl)hetero-aryl-amino Donor-Based Chromophores: Design, Synthesis, and Electrooptic Activity

Two new highly hyperpolarizable chromophores, based on N,N- bis-(4-methoxyphenyl) aryl-amino donors and phenyl-trifluoromethyl-tricyanofuran (CF3-Ph-TCF) acceptor linked together via pi-conjugation through 2,5-divinylenethienyl moieties as the bridge, have been designed and synthesized successfully for the first time. The aryl moieties on the donor side of the chromophore molecules were varied as to be thiophene and 1-n-hexylpyrrole. The linear and nonlinear optical (NLO) properties of all compounds were evaluated in addition to recording relevant thermal and electrochemical data. The properties of the two new molecules were comparatively studied. These results are critically analyzed along with two other compounds, reported earlier from our laboratories and our collaborators, that contain (i) aliphatic chain-bearing aniline and (ii) dianisylaniline as donors, keeping the bridge (2,5-divinylenethienyl-), and the acceptor (CF3-Ph-TCF), constant. Trends in theoretically (density functional theory, DFT) predicted, zero-frequency gas-phase hyperpolarizability [beta(0;0,0)] values are shown to be consistent with the trends in beta HRS(-2omega;omega,omega), as measured by Hyper-Rayleigh Scattering (HRS), when corrected to zero-frequency using the two-level model (TLM) approximation. Similarly, trends in poling efficiency data (r33/E(p)) and wavelength dispersion measured by reflection ellipsometry (using a Teng-Man apparatus) and attenuated total reflection (ATR) are found to fit the TLM and DFT predictions. A 3-fold enhancement in bulk nonlinearity (r33) is realized as the donor subunits are changed from alkylaniline to dianisylaminopyrrole donors. The results of these studies provide insight into the complicated effects on molecular hyperpolarizability of substituting heteroaromatic subunits into the donor group structures. These studies also demonstrate that, when frequency dependence and electric-field-induced ordering behavior are correctly accounted for, ab initio DFT generated beta(0;0,0) is effective as a predictor of changes in r33 behavior based on chromophore structure modification. Thus DFT can provide valuable insight into the electronic structure origin of complex optical phenomena in organic media.

Organic electro-optics: Understanding material structure/function relationships and device fabrication issues

Realization of large electro-optic (EO) activity for dipolar organic chromophore-containing materials requires the simultaneous optimization of chromophore first hyperpolarizability (β), acentric order , and number density (N). As these parameters are inter-related, correlated quantum and statistical mechanical calculations are required to understand the dependence of macroscopic electro-optic activity upon chromophore structure and intermolecular electrostatic interactions. Correlated time-dependent density functional theory (TD-DFT) and pseudo-atomistic Monte Carlo (PAMC) calculations are used in an attempt to understand the dependence of linear and nonlinear optical properties on dielectric permittivity, optical frequency, and a variety of spatially-anisotropic interactions that can be nano-engineered into the macroscopic material structure. Structure/function relationships are considered for three classes of organic electro-optic materials: (1) Chromophore/polymer composite materials; (2) chromophores covalently incorporated into passive organic host materials; (3) chromophores incorporated into chromophore-containing host materials—a new class of materials referred to as binary chromophore organic glasses (BCOGs). Issues associated with processing these materials into device structures, including those relevant to the integration with silicon photonics, are discussed. The purpose of this article is to address issues critical to ascertaining the viability of organic electro-optic (OEO) materials for next generation telecommunications, computing, and sensing applications.

Reduced dimensionality in organic electro-optic materials: theory and defined order.

Identification of electronic intermolecular electrostatic interactions that can significantly enhance poling-induced order is important to the advancement of the field of organic electro-optics. Here, we demonstrate an example of such improvement achieved through exploitation of the interaction of coumarin pendant groups in chromophore-containing macromolecules. Acentric order enhancement is explained in terms of lattice-symmetry effects, where constraint of orientational degrees of freedom alters the relationship between centrosymmetric and acentric order. We demonstrate both experimentally and theoretically that lattice dimensionality can be defined using the relationship between centrosymmetric order and acentric order. Experimentally: Acentric order is determined by attenuated total reflection measurement of electro-optic activity coupled with hyper-Rayleigh scattering measurement of molecular first hyperpolarizability, and centrosymmetric order is determined by the variable angle polarization referenced absorption spectroscopy method. Theoretically: Order is determined from statistical mechanical models that predict the properties of soft condensed matter.

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.

Photostability studies of π-conjugated chromophores with resonant and nonresonant light excitation for long-life polymeric telecommunication devices

Theoretical and experimental studies of molecular photodegradation in π-conjugated chromophores with resonant and nonresonant excitation relative to the lowest-energy electronic transition of the chromophore are performed. The limitations of previous photodegradation models are discussed, and new models that overcome these limitations and provide more accurate estimates of chromophore photostability are presented. In particular, the necessity of considering multiple degradation pathways in the analysis of photobleaching studies is shown. Photostability studies of a dihydrofuran thiophene-bridged dicyanomethylene based chromophore (FTC) employing 1.55-μm excitation reveal that the photoinitiated decay kinetics are biphasic. We present what we believe to be a new, double-pathway photodegradation model capable of describing this behavior. Through investigations employing the singlet-oxygen quencher bis(dithiobenzil)nickel, photooxidation is shown to be one of the photodegradation pathways, and the ability of a quencher to inhibit chromophore photooxidation is quantified. The studies presented here provide insight into the mechanism of photochemical degradation of π-conjugated chromophores for devices operating in the visible and at telecommunication wavelengths.

Dielectric dependence of the first molecular hyperpolarizability for electro-optic chromophores.

Experimental and computational studies of the solvent dependence of the first molecular hyperpolarizability (β) for two donor-bridge-acceptor chromophores (CLD-1 and YLD156) are presented. Hyper-Rayleigh scattering (HRS) measurements are performed with 1907 nm excitation in a series of solvents with dielectric constants ranging from ~2 (toluene) to ~36 (acetonitrile). For both chromophores an approximately 2-fold increase in β is observed by HRS over this range of dielectric constants. Computational studies employing a polarized continuum model to represent the solvent are capable of reproducing this experimental result. The experimental and computational results are compared to the predictions of the widely employed two-state model (TSM) for β. Surprisingly, for the chromophores studied here the TSM predicts that β should decrease with increasing dielectric constant over the range investigated. The results presented here demonstrate that the TSM provides neither a quantitative nor qualitative description of the solvent dependence of β for CLD-1 and YLD156. The enhancement of β with increased dielectric constant suggests that modification of the dielectric surrounding the chromophore is one path by which the performance of nonlinear optical devices employing these chromophores may be significantly enhanced.

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.

Electro-optic thin films of organic nonlinear optic molecules aligned through vacuum deposition

Nonlinear optical molecules can be vacuum deposited into uniform thin films using thermal evaporation. Alignment order can be achieved during thin film deposition by an in-plane electrical field poling using electrodes patterned on the substrate. Electro-optic (EO) coefficients, r33 and r13 are independently measured using Youngs interferometry technique. Thin-films of N-benzyl-2-methyl-4-nitroaniline (BNA) can exhibit an EO coefficient, r33, comparable to that of BNA single crystals. EO coefficients of BNA at different poling fields, wavelengths, and frequencies are investigated.