Theoretical calculations have demonstrated that the ratio of second and third degree order parameters can define lattice
dimensionality and furthermore, that an increased ratio of second to third degree order parameters represents reduced
lattice dimensionality. As a result, the third degree order parameter (i.e. acentric order parameter) is increased, causing
an increase in electro-optic activity with reduced lattice dimensionality. Experimentally, specific spatially-anisotropic
interactions associated with coumarin moieties and Frechet-type (arene/perfluoroarene) dendrons have been incorporated
into chromophore systems and have been shown to lead to lattices of reduced dimensionality, resulting in increased
values of the acentric order parameter and therefore, electro-optic activity. Reductions in lattice dimensionality can also
arise from guest chromophore-host chromophore interactions in binary chromophore organic glasses and from laserinduced
ordering of host lattice chromophores observed in the laser-assisted electric field poling of azo-dye-containing
host lattices. These interactions in various chromophore systems including investigation of EO and order properties are
Photonic technologies hold the promise of transformative advances in the telecommunications, aerospace, and
computing industries. In particular, organic materials displaying electro-optic coefficients an order of magnitude larger
than the current industry standard inorganic materials (r33 ≥ 300 pm/V) may hold the key to the development of cheap,
high bandwidth, and highly integratable electro-optic devices. The pertinent linear and nonlinear optical properties of
such organic second-order nonlinear optical materials are highly dependant on their dielectric properties as well as the
extent of dipolar order that can be created. Here, theoretical approaches to the simulation of highly active organic
electro-optic materials are described. Such simulations assist understanding and may be used to predict optical properties
facilitating the rational design process. Improved ordering schemes, such as laser-assisted electric-field-poling may help
in the translation of large chromophore hyperpolarizability values into large r33. Recent results also suggest that
incorporation of these improved organic materials into new hybrid organic/silicon device designs may lead to
dramatically reduced device operational voltages and create opportunities for the development of new device
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.
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.
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.
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.
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.
The potential of organic electro-optic materials for large electro-optic activity and fast response to applied electric fields (leading to 100 GHz device bandwidths) is important and increasingly well-recognized. In this communication, we demonstrate how quantum and statistical mechanical calculations can be used to guide the systematic improvement of both molecular first hyperpolarizability (β) and macroscopic electro-optic activity (r). Femtosecond time-resolved, wavelength-agile Hyper-Rayleigh Scattering (HRS) measurements have been used to measure β values relative to chloroform and to avoid confusion associated with two photon contributions. Electro-optic coefficients have been characterized by simple reflection (Teng-Man method), attenuated total reflection (ATR), and Mach Zehnder interferometry. "Constant bias" modifications of these techniques have been used to permit investigation of optimized poling conditions. Organic electro-optic materials also afford unique advantages for the fabrication of conformal and flexible devices, for the integration of disparate materials, and for exploitation of novel manufacturing technologies such as soft lithography. Both stripline and ring microresonator structures have been fabricated by soft lithography. The integration of organic electro-optic materials with silicon photonics (both split ring microresonators and photonic bandgap circuitry) has been demonstrated.
In pursuit of greater understanding of structure property relationships in NLO chromophores, a series of molecules consisting of three aromatic rings was synthesized. The relative positions of benzene and thiophene rings in these molecules were varied. Theoretical calculations also suggest that the use of a slightly electron deficient heteroaromatic, such as thiazole, can increase β through the concept of an electronic gradient. The use of this heteroaromatic in the correct orientation can compensate for the energetic barrier that benzene presents during charge-transfer. Hyper-Raleigh Scattering (HRS) measurements on three of these “gradient bridge” type chromophores show that benzene located at the donor end provided the highest hyperpolarizability. The poor solubility of these three-ring systems severely limited their processability and gave considerable synthetic challenges. The difference in theoretical and experimental trends of β are discussed.
Density functional theory calculations were used to develop understanding of the effects of differing substitution patterns in multiply donor and acceptor substituted nonlinear optical chromophores. A novel series type substitution design was presented and evaluated. Calculations were performed for a number of structures and hyperpolarizability values were compared. The data obtained showed an increase in molecular first hyperpoloarizability in multifunctional chromophores based on this series type design as compared with linear molecules constructed from the same donor-bridge-acceptor components. This data was then used to direct the synthesis of novel nonlinear optical chromophores.