Organic electro-optic materials: theory and experiment, devices, and applications [Invited]

Organic electro-optic (OEO) materials require molecular units (chromophores) with large first-order hyperpolarizability that are organized in a non-centrosymmetric (acentric matrix) fashion. Unlike typical inorganic electro-optic materials, OEO materials relevant to devices are usually not naturally occurring crystalline materials but rather must be prepared from chemically synthesized molecular units organized into acentric matrices by appropriate material processing strategies. Theory-guided design based on first-principles quantum and statistical mechanics is crucial to the design and development of new desired materials. When intermolecular electronic interactions are important, integrated quantum-statistical mechanical calculations can be required. Development of materials has been made even more challenging as materials must be adapted to widely different meso- to nanoscale (with respect to waveguide width/electrode separation) subwavelength device architectures that place significantly different demands with respect to acceptable material properties and material processing options. Recent device architectures that are receiving greater focus include hybrid silicon photonic devices, hybrid plasmonic devices, and hybrid meta-surface devices—all characterized by critical sub-wavelength dimensions. The focus on applications is also dynamic, with emerging applications (in telecom, datacom, sensing, spatial light modulation, displays, Lidar, and autonomous vehicles/robotics, supercomputing/AI/machine learning, etc.) receiving ever-increasing attention. Relevant operational temperatures have been expanded dramatically, now ranging from 4 to 400 K, reflecting emerging applications. The diversity of device architectures and applications makes simplistic (universal) specification of required material properties difficult, so a more nuanced consideration of material properties is required moving forward in the development and implementation of new materials. It is the objective of this review to bring useful perspective to the rapidly evolving fields of electro-optics and chip-scale integrated photonics (photonic integrated circuits (PICs)), particularly with respect to utilization of OEO materials. The emphasis of this review is not a comprehensive review of the literature or a discussion of emerging commercial activities but rather on providing an understanding of the factors that influence the development of OEO materials, devices, and applications. The reader is referred to the cited literature for critical insight into detailed aspects of research and development relevant to OEO materials. Consideration of the details of material/device development and characterization is critical to understanding variation of results reported in the literature; however, reported metrics can depend strongly on measurement conditions, theoretical (unit) conventions, as well as the details of materials and devices so that detailed review of all factors affecting reported results is not possible in a finite review. The reader is directed to the cited literature utilizing the general and cautionary insights provided by this review.