Andreas F. Tillack

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.

Systematic Generation of Anisotropic Coarse-Grained Lennard-Jones Potentials and Their Application to Ordered Soft Matter

We have developed an approach to coarse-grained (CG) modeling of the van der Waals (vdW) type of interactions among molecules by representing groups of atoms within those molecules in terms of ellipsoids (rather than spheres). Our approach systematically translates an arbitrary underlying all-atom (AA) representation of a molecular system to a multisite ellipsoidal potential within the family of Gay-Berne type potentials. As the method enables arbitrary levels of coarse-graining, or even multiple levels of coarse-graining within a single simulation, we describe the method as a Level of Detail (LoD) model. The LoD model, as integrated into our groups Metropolis Monte Carlo computational package, is also capable of reducing the complexity of the molecular electrostatics by means of a multipole expansion of charges obtained from an AA force field (or directly from electronic structure calculations) of the charges within each ellipsoid. Electronic polarizability may additionally be included. The present CG representation does not include transformation of bonded interactions; ellipsoids are connected at the fully atomistic bond sites by freely rotating links that are constrained to maintain a constant distance. The accuracy of the method is demonstrated for three distinct types of self-assembling or self-organizing molecular systems: (1) the interaction between benzene and perfluorobenzene (dispersion interactions), (2) linear hydrocarbon chains (a system with large conformational flexibility), and (3) the self-organization of ethylene carbonate (a highly polar liquid). Lastly, the method is applied to the interaction of large (∼100 atom) molecules, which are typical of organic nonlinear optical chromophores, to demonstrate the effect of different CG models on molecular assembly.

Toward optimal EO response from ONLO chromophores: a statistical mechanics study of optimizing shape

Organic nonlinear optical (ONLO) chromophores are key components in electro-optic (EO) devices, particularly on chip. They have the potential to have footprints compatible with silicon-based devices. Materials based on ONLO chromophores are extremely easy to process, being plastics. The development of better chromophores requires the study of how strong the EO properties are of individual chromophores and how well they can be organized in a host material and, ultimately, how densely they can be packed in a neat material. We now assess how well the existing chromophores perform as neat materials and compare with how well the optimal chromophore could perform. By comparison with idealized structures the nature of the packing of such structures is examined. Furthermore, we consider how to augment core chromophores to improve the order and the packing density to improve the EO performance for bulk materials. The optimal chromophore for a neat material requires efficient utilization of its volume; groups pendant to a chromophore core can be designed to improve the overall order under poling, but eventually become self-defeating when they become too large. Adding pendant groups can lead to desired chemical and physical material properties although they may not necessarily improve the EO performance. Now, simulations show how pendant groups can be placed strategically to optimize EO performance. Estimates are developed for the theoretical maximum EO performance and compared to hypothetical and existing molecules. These comparisons lead to design principles that can be applied in molecular synthesis. Determining the effect of local environment (beyond just the general dielectric effect) is the next major challenge for quantum mechanics/molecular mechanics theory.

Simple Model for the Benzene Hexafluorobenzene Interaction

While the experimental intermolecular distance distribution functions of pure benzene and pure hexafluorobenzene are well described by transferable all-atom force fields, the interaction between the two molecules (in a 1:1 mixture) is not well simulated. We demonstrate that the parameters of the transferable force fields are adequate to describe the intermolecular distance distribution if the charges are replaced by a set of charges that are not located at the atoms. The simplest model that well describes the experimental distance distribution, between benzene and hexafluorobenzene, is that of a single ellipsoid for each molecule, representing the van der Waals interactions, and a set of three point charges (on the axis perpendicular to the arene plane) which give the same quadrupole moment as do the all atom charges from the transferable force fields.

Poling-induced birefringence in OEO materials under nanoscale confinement

Standard models for evaluating the electro-optic (EO) response of organic materials typically assume that the refractive index of the material in the absence of a RF modulation field is isotropic and homogeneous. Such assumptions work very well for low-concentration guest-host materials in bulk devices. However, current generation organic EO materials at high densities and under nanoscale confinement can show sufficient birefringence to affect device performance. We use computer simulations and spectroscopic experiments to characterize and predict changes in the index of refraction under poling. We also demonstrate that poling-induced birefringence can lead to a non-linear relationship between the apparent EO coefficient and poling field strength.

Multi-scale theory-assisted nano-engineering of plasmonic-organic hybrid electro-optic device performance

Multi-scale (correlated quantum and statistical mechanics) modeling methods have been advanced and employed to guide the improvement of organic electro-optic (OEO) materials, including by analyzing electric field poling induced electro-optic activity in nanoscopic plasmonic-organic hybrid (POH) waveguide devices. The analysis of in-device electro-optic activity emphasizes the importance of considering both the details of intermolecular interactions within organic electro-optic materials and interactions at interfaces between OEO materials and device architectures. Dramatic improvement in electro-optic device performance--including voltage-length performance, bandwidth, energy efficiency, and lower optical losses have been realized. These improvements are critical to applications in telecommunications, computing, sensor technology, and metrology. Multi-scale modeling methods illustrate the complexity of improving the electro-optic activity of organic materials, including the necessity of considering the trade-off between improving poling-induced acentric order through chromophore modification and the reduction of chromophore number density associated with such modification. Computational simulations also emphasize the importance of developing chromophore modifications that serve multiple purposes including matrix hardening for enhanced thermal and photochemical stability, control of matrix dimensionality, influence on material viscoelasticity, improvement of chromophore molecular hyperpolarizability, control of material dielectric permittivity and index of refraction properties, and control of material conductance. Consideration of new device architectures is critical to the implementation of chipscale integration of electronics and photonics and achieving the high bandwidths for applications such as next generation (e.g., 5G) telecommunications.

Hybrid electro-optics and chipscale integration of electronics and photonics

Taken together, theory-guided nano-engineering of organic electro-optic materials and hybrid device architectures have permitted dramatic improvement of the performance of electro-optic devices. For example, the voltage-length product has been improved by nearly a factor of 104 , bandwidths have been extended to nearly 200 GHz, device footprints reduced to less than 200 μm2 , and femtojoule energy efficiency achieved. This presentation discusses the utilization of new coarse-grained theoretical methods and advanced quantum mechanical methods to quantitatively simulate the physical properties of new classes of organic electro-optic materials and to evaluate their performance in nanoscopic device architectures, accounting for the effect on chromophore ordering at interfaces in nanoscopic waveguides.

Alternative bridging architectures in organic nonlinear optical materials: comparison of π- and χ-type structures

Organic nonlinear optical (ONLO) chromophores are used to make electro-optic devices. Traditional ONLO chromophores use a π-conjugated bridge to couple the electron acceptor and donor moieties. We have explored whether other types of conjugation can be used to make high-performance ONLO chromophores. We have found that cross-conjugated bridge structures, when other parameters are kept the same, can exhibit comparable hyperpolarizabilities. Experimental hyperpolarizabilities of prototypical cross-conjugated chromophores, measured by hyper-Raleigh scattering, are comparable with their π-conjugated analogues, in contrast with the prediction of several electronic structure calculation methods. This opens new synthetic routes to other types of chromophores, which may provide enhanced performance.