Doris S. Leung

Defeating tradeoffs for nonlinear optical chromophores

Materials to be used for optoelectronic, photorefractive, or frequency doubling applications must have high nonlinearities, good thermal stabilities, and low optical loss (high transparency). Organic compounds synthesized for incorporation into poled nonlinear polymers typically exhibit tradeoffs between nonlinearity and each of the other two qualities. By judicious use of arylamino donor groups and cyano-containing acceptor groups, a small set of chromophores that are both highly nonlinear and stable at high temperatures has been prepared. By selecting delocalized bridging moieties that are either tuned for optimum hyperpolarizability or exhibit two charge-transfer excited states, highly transparent chromopohores with excellent nonlinearities can be prepared. The results suggest that thermal stability and nonlinearity are jointly achieved by modifying donor and acceptor groups, while transparency and nonlinearity are jointly achieved by modifying bridging groups.

DCM-polyimide system for triple-stack poled polymer electro-optic devices

We report the background leading to the development of the first all-polyimide system (cladding/core/cladding) suitable for fabrication of electro-optic waveguide devices on silicon substrates. The cladding layers are spun from a low optical loss, commercially available polyimide that is suitable for multilayer stacks. The electro-optic material consists of this same polyimide as host to a commercially available guest chromophore and is based upon our prior work on thermoplastic polyimides. The synthesis and purification of this chromophore and an analog is discussed. We also present the materials and process development methodology with the results for this polymer system and demonstrate it by fabrication of an all-polyimide Mach- Zehnder modulator operating at 830 nm. CMOS-compatible switching using a device based on the new material has been demonstrated.

Defeating Tradeoffs for Nonlinear Optical Materials

Abstract In order to be useful for electro-optic devices, nonlinear optical chromophores must be thermally stable, highly transparent, and have large hyperpolarizabilities. Although these qualities are to some extent mutually exclusive, a small group of chromophores has been prepared that meets the various criteria for applicability.

Progress in optoelectronic polymers and devices

Optoelectronic devices based on organic materials are uniquely suited to applications requiring high rf bandwidth. There have been significant advances in lithium niobate technology, but fundamental frequency-sensitivity tradeoffs are generally required in device design. Using a guest-host electro-optic polymer system, we have demonstrated a Mach- Zehnder modulator with a switching voltage of 3.5 V and interaction length of 2.6 cm. Anisotropic V-groove etching for fiber attachment provides a path to low-cost packaging of these devices. Materials and process optimization are expected to enhance device performance, allowing more compact, sensitive devices. Issues related to electro-optic device development are discussed in this paper, and an update on our development of new chromophores for use in electro-optic polymers is given.

Development of Chromophores Based on the Dicyanomethylene Acceptor Group

We have used DCM/polyimide guest-host films for a series of device demonstrations, including demonstrations of high speed, low voltage digital and analog switching. Although useful for such demonstrations, DCM tends to out diffuse at polyimide cure temperatures, motivating the search for related compounds with similar activity, transparency, processibility, and greater thermal stability. We have designed, synthesized, and characterized a series of chromophores with related molecular structures. Like DCM, all of these chromophores share the dicyanomethylene [>C=C(CN) 2 ] acceptor group. Some of these compounds are conventional donor-acceptor chromophores, while others fall into the class of lambda-shaped donor-acceptor-donor (DAD) compounds. An acceptor-donor-acceptor (ADA) chromophore has also been prepared and characterized. The donor-acceptor analog studies utilize a variation on the Knoevenagel condensation reaction used to make DCM, substituting isophorone for 4-pyranone to avoid the formation of mixtures. These reactions lead to the DCI series of compounds. We will describe progress in our investigations of these series of compounds.

Molecular studies and plastic optical fiber device structures for nonlinear optical applications

Summarized are two project areas: First, the development of a quantitative structure property relationship for analyzing thermal decomposition differential scanning calorimetry data of electro-optic dyes is presented. The QSPR relationship suggest that thermal decomposition can be effectively correlated with structure by considering the kinds of atoms, their hybridization, and their nearest neighbor bonded atoms. Second, the simple preparation of clad plastic optical fibers (POF) is discussed with the intention of use for nonlinear optical applications. We discuss preparation techniques for single core and multiple core POF, and present some recent data on index profiles and the optimization of thermal stability in acrylate-based POF structures.