The Jen group’s goal in the research of organic electro-optics is to advance the science and engineering of organic second-order nonlinear optical materials and their hybrid systems, enabling the development of “transformative” electro-optic (EO) technologies for ultrafast information processing. Jen group has produced the state-of-the-art organic photonic materials with record-high EO coefficients (> 350 pm/V) and alignment stability (≥ 250°C). These accomplishments are achieved through innovative molecular design, synthesis, interface engineering, and device fabrication to overcome the challenging stability and performance issues of organic EO materials. The attractive features of Jen group’s high performance organic EO materials are extremely important for addressing the presently encountered limitations in silicon- and metal-based multilevel interconnects with respect to bandwidth and power consumption, and can potentially revolutionize the technologies for future telecommunications and computation.
Figure 1: Integrated research efforts from materials to device in EO thrust
Fig. 1 shows the strategic research effort that has been carried out at the Jen group in the design and synthesis of high performance organic electro-optic (OEO) materials and their hybrid systems for photonic devices. In this platform, the basic research is from the design and synthesis of OEO materials, which deliver the needed technology elements to drive the prototype device development and integration. A vast array of advanced EO devices and technologies has been demonstrated with collaborator and end-users. This research strategy has been proven to be a very efficient way to identify potential problems and opportunities in the rapid development process, and elicit more meaningful feedback of system requirements.
Rational Molecular Design of Highly Efficient Nonlinear Optical Chromophores
From the molecular level, the exceptional performance of OEO materials is derived from the non-centrosymmetric arrangement of large hyperpolarizability (b) chromophores. Most of the designs for large b chromophores have been based on optimizing conjugated “push-pull” molecules. Such chromophores consist of electron-donating and electron-withdrawing end groups interacting through a conjugated bridge (D-p-A) to create intramolecular charge transfer. In this regard, our research efforts have been focused on the design and synthesis of highly efficient nonlinear optical (NLO) chromophores with the combination of a few highly correlated parameters for photonic applications, such as large molecular b values, good near-IR optical transparency, excellent chemical and photostability, controlled charge-transfer ability, and improved processability in polymers (Fig. 2). We also systematically study how the local dielectric environment and poling-induced acentric ordering can affect the molecular polarization and EO performance of materials.
Figure 2: Design and synthesis of highly efficient dipolar chromophores for electro-optic applications.
Molecular Assembly and Click Chemistry for High-Efficiency OEO Materials
The material chemistry in the development of high performance OEO polymers is to translate the exceptional molecular properties of dipolar chromophores into high EO activities and low optical losses in poled films. There are two major issues that need to be addressed in order to produce highly ordered and thermally stable lattices with high concentrations of NLO chromophores:
- Creating a dominant dipole energy term mEcosq, where the angle q is between the molecular permanent dipole moment m and the poling field E, to achieve optimal poling-induced polar order (<cos3q>). This term needs to be balanced to overcoming strong dipole-dipole interactions between large b chromophores
- Preserving poling-induced polar order against processing and use conditions by physical and/or chemical lattice hardening.
Figure 3: Molecular assembly and “Click Chemistry” for High-Efficiency OEO Materials.
Fig. 3 shows examples of our approaches in using molecular assembly and click chemistry for the development of high-efficiency OEO materials. The self-assembling motifs include the reversible self-assembly of aromatic/perfluoroaromatic moieties and the molecular threading effect of triptycene moieties. The click chemistry, such Diels-Alder and 1,3-dipolar cycloaddition reactions, are also used as the most efficient synthetic tool for postfunctionalization and lattice-hardening of EO polymers. These endeavors have led to new paradigms of OEO material research, such as record-high EO coefficients (380 pm/V) from binary EO polymers and supramolecular dendrimers, and highly efficient and thermally stable temperature (up to 200-250 °C) EO polymers for CMOS-compatible interconnects.
Efficient Pyroelectric Poling of EO Polymers in Thin Films and Nanophotonic Waveguides
Generating noncentrosymmetric order of nonlinear optical (NLO) chromophores through electric field poling have been one of the most challenging issues in the research of organic polymeric EO materials. Currently the commonly used methods of poling are the contact poling and corona poling, and both techniques impose considerable limitation and challenges to the potential application of organic and polymeric EO materials. For example, in contact poling, severe charge injection from metal electrodes often results in large current that causes dielectric breakdown of the films. Significant challenges also exist in integrating these high activity materials into the silicon nano-photonic devices such as slotted waveguides and photonic crystals.
Figure 4: Generic scheme of pyroelectric poling by placing a pyroelectric-crystal/EO-polymer laminate on hot plate. Ps stands for the spontaneous polarization of pyro-crystal.
To address these challenging problems, we have recently developed a new protocol of pyroelectric poling, in which lithium tantalate (LT) and lithium niobate (LN) pyroelectric crystals can be used as a reliable voltage source for the poling of EO polymers. Heating or cooling the pyroelectric crystals can develop equal but opposite charges on both surfaces of the crystal that are normal to the polar axis. This effect, being widely used in pyroelectric detectors, originates from the perturbed equilibrium between the spontaneous polarization of the pyroelectric crystal and its surface screening charges. The unique advantage of this effect is that electric fields and potentials can be generated through modest temperature change of compact-size pyroelectric crystals, and no extra activation process is required. Upon rapid cooling of the pyroelectrics/EO polymer laminate, EO polymer films can be effectively poled by the pyroelectric crystals without the use of external DC voltage source (Fig. 4). The EO coefficients of stand-alone poled films are currently at the level of 150 pm/V. This simplified method can also induce a strong in-plane poling of EO polymer in hybrid silicon slot waveguides, leading to a record-high tunability of resonance wavelength shift (25 pm/V) in ring-resonator modulators. These studies demonstrate the feasibility of using electric field generation from pyroelectrics to activate the polarization of amorphous dielectric materials with finite resistivities, and open up new processing strategies of functional dielectrics and their hybrid systems for a broad spectrum of electronic and photonic applications.
High Performance EO Polymer Enabling Diverse Device Applications
The past several years have witnessed the significant progress in the research of hybrid OEO nanophotonic systems by using the high performance OEO materials from our research group (Table 1-3). Through these extensive research collaborations, OEO materials have been identified as one of the most promising active materials to ultimately achieve the combined system level performance for photonic devices. In particular, compared to other materials (such as ferroelectric crystals), these OEO materials offer intrinsic advantages, such as large optical nonlinearity, high-speed of operation, compatibility with other materials and substrates in the system, and exceptional processibility allowing complex configurations and arrays.
|Table 1. EO Polymer Modulators of Dielectric Slab Waveguides
|Table 2. Silicon EO Polymers Hybrid Nanophotonic Waveguides
|Table 3. EO Polymers for Electric Field Sensing and Terahertz Generation/Detection