Benjamin C. Olbricht

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.

Tri-component Diels–Alder polymerized dendrimer glass exhibiting large, thermally stable, electro-optic activity

A novel, thermally curable, tri-component organic glass for electro-optic applications was designed and synthesized. The system employed the Diels–Alder cycloaddition reaction to effect efficient cross-linking. The first component was a dendrimer containing multiple electro-optic chromophore substituents surrounded by an outer periphery possessing diene functionality. The second was a furan-protected, bis-dienophile electro-optic chromophore, introduced in order to function as a nonlinear optically-active cross-linking agent. The initial glass-transition temperature of the material was tuned by the addition of the third component, an optically-inert, maleimide-based dienophile cross-linking agent. Tuning of this mixture allowed optimum poling temperature to coincide with optimum thermal conditions for promotion of the Diels–Alder cross-linking reaction. The electro-optic properties of the material were evaluated in real-time, using a reflection-based single-beam ellipsometry apparatus that was modified to perform in situ signal monitoring. The high electro-optic activity observed (r33 of 150 pm V−1), was thermally stable up to 130 °C (a 48 °C improvement over similar uncross-linked materials). After processing, materials were insoluble in acetone, retained 90% of their original r33 after 15 months at room temperature, and performed well in accelerated operational testing at 85 °C in air.

Direct electron beam writing of electro-optic polymer microring resonators

Electro-optic polymer waveguides in electron beam sensitive polymethyl methacrylate (PMMA) polymer matrix doped with organic nonlinear chromophores could be directly patterned by electron beam exposure with high resolution and smoothness. The polymer in the exposed regions was removed with standard electron beam resist developer and without damaging the chromophore containing polymer waveguides. Feature sizes on the order of 100 nm could be clearly resolved. High quality microring resonators made of YL124/PMMA electro-optic polymer were successfully fabricated with this technique. The measured resonance extinction ratios were more than 16 dB and quality factors were in the range of 10(3) approximately 10(4).

Benzocyclobutene barrier layer for suppressing conductance in nonlinear optical devices during electric field poling

We measured the electro-optic (EO) coefficients (r33) of thin-film devices made from several monolithic, high number density organic EO chromophores with and without additional charge barrier layers. We found that a cross-linkable benzocyclobutene layer was very effective in suppressing unwanted, leakage current, keeping the effective poling voltage nearly identical to the applied voltage. This barrier layer proved to be superior to a titanium dioxide (TiO2) barrier layer. The suppression of the leakage current in combination with a new chromophore enabled the construction of EO devices that had r33 values in the range of 400–500 pm V−1 with poling fields ≥ 85 V μm−1.

Measuring Order in Contact-Poled Organic Electrooptic Materials with Variable-Angle Polarization-Referenced Absorption Spectroscopy (VAPRAS)

Organic nonlinear electrooptical (ONLO) chromophores must be acentrically ordered for the ONLO material to have electrooptic (EO) activity. The magnitude of the order is characterized by the acentric order parameter, , where β is the major Euler angle between the main axis of the chromophore and the poling field which imposes the acentric order. The acentric order parameter, which is difficult to measure directly, is related to the centrosymmetric order parameter, defined as = ½(3-1), through the underlying statistical distribution. We have developed a method to determine centrosymmetric order of the ONLO chromophores when the order is low (i.e., < 0.1). We have extended the method (begun by Graf et al. J. Appl. Phys. 1994, 75, 3335.) based on the absorption of light to determine the centrosymmetric order parameter induced by a poling field on a thin film sample of ONLO material. We find that the order parameters, analyzed by two different methods, are similar and also consistent with theoretical estimates from modeling of the system using coarse-grained Monte Carlo statistical mechanical methods.

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.

Advances in Organic Materials for Optical Modulation

A new class of organic electro-optic materials, binary chromophore organic glasses, are introduced and shown to provide a route to materials exhibiting useable electro-optic coefficients of greater than 300 pm/V, optical loss of 2 dB/cm or less, and material glass transition temperatures on the order of 200degC. Design principles and device applications are discussed.

Microring resonators fabricated by electron beam bleaching of chromophore doped polymers

Decomposition of chromophore molecules under direct electron beam irradiation reduces the refractive index of chromophore containing polymers. The induced refractive index contrast between the exposed and unexposed regions is high enough for waveguide bends of small radius and thus microring resonator devices. This electron beam bleaching of chromophore-containing polymers provides a fabrication approach for nonlinear polymer optical waveguide devices. Fabrication of high quality microring resonators with critical feature size on the order of 100nm was demonstrated with this technique in an electro-optic polymer that contains YL124 chromophores.