Electro-Optical Chromophores and Polymers for Short-Wavelength NIR Applications

With the increasing influence of photonics in communication and sensing, especially in integrated circuits, functional optical materials are at the center of this development. The ability to translate an external impulse into an optical signal and vice versa as exhibited by piezo- and elasto-optic, magneto-optic, acousto-optic and electro-optic materials is utilized in modulators, sensing resonators and beam steering devices. In the field of electro-optics (EO), which has long been dominated by periodically poled LiNbO₃, a shift towards specialized applications has driven demand for alternatives with differing properties. Organic electro-optic chromophores exhibit a Pockels effect through the intramolecular movement of electrons along the molecular axis in a donor-acceptor structure upon external stimuli in an electric field. As a part of composite polymer materials, they are compatible with common spin-coating and molding methods and can be integrated into waveguides. This ease of fabrication and the high electro-optical activity r₃₃ when compared to LiNbO₃, at specific wavelengths lead to an accessible and low-voltage alternative.<br/><br/>Electro-optic polymer materials are developed to operate at 850 nm (shortwave infrared window) and 980 nm (Nd/YAG) which are two significant wavelengths at the shorter end of the NIR spectrum. The active component of such materials are electro optical (EO) chromophores, which are integrated in a suitable host polymer, mostly amorphous polycarbonate (APC) and poly (methyl methacrylate) (PMMA) and electro-poled to achieve parallel orientation. A range of molecular parameters such as the dipole moment µ, hyperpolarizability β, the optical absorption and miscibility with the host polymer need to be taken into account. Due to this, the synthesis of EO chromophores is accompanied by DFT calculations which guide the main scaffold and the attachment of side groups. Chromophores with structural variations and their effect on absorption, miscibility and EO activity are shown. Through the utilization of bulky side groups, intermolecular interactions are reduced for more efficient external electro-poling. A copolymer of bulky isobornyl methacrylate and methyl methacrylate has been found to give significantly higher glass transition temperatures and temperature stabilities of the poled state alongside good miscibility. A lower refractive index compared with the more common polycarbonates opens up broader applications in waveguide cladding.<br/><br/>Chromophores with anchor groups have also been synthesized for covalent integration of the chromophores into polymethacrylate and polycarbonate chains. This allows for significantly more stable orientations and prevents agglomeration as the chromophores are held in place by their connection to the main chain.