In optical communication systems, electro-optic (EO) modulators are used to encode electrical input data signals onto fiber optic transmission lines. The dominant EO material in the presently applied technology is lithium niobate. Commercially available lithium niobate modulators are limited in their bandwidth to 40 GHz (GigaHertz) due to the velocity mismatch between the electrical drive signal and the optical transmission signal. For the next generation optical communication systems with data rates > 40 Gb/s (Gigabits per second) a better performing material is required. During the 1980s it became clear that organic materials might be a better choice for use in nonlinear optical applications. A lot of organic chromophores exhibit extremely high and fast nonlinearities, much better than those observed in inorganic crystals. In addition, due to the versatility of organic synthesis, their nonlinear optical properties can be custom-tailored depending on the desired application. At this moment, poled polymers seem to be closest to commercial applications due to the several advantages that this approach offers. Such materials are cheap, easy to fabricate, have good film-forming properties for making waveguides, and are compatible with existing semiconductor technologies.
The tricyanovinylidene diphenylaminobenzene (TCVDPA) chromophore has been identified as one of the most promising chromophores for polymeric electro-optic waveguide applications. EFISH measurements showed a 20% higher figure of merit µβ/Mw as compared to the dimethylaminonitrostilbene (DANS) chromophore. In addition, TCVDPA has also been identified as the most photostable structure. In the current work a direct waveguide definition of the host polymer SU-8, a negative photoresist, containing this chromophore by masked UV exposure followed by development is demonstrated. This was possible by utilizing the low absorption window of the chromophore in the UV region that allows crosslinking of the host polymer by exposing to UV light followed by thermal curing. A series of derivatives of TCVDPA have been synthesized either by modification of the chromophore shape with bulky substituents in order to prevent unwanted intermolecular electrostatic interactions, or by introduction of epoxy polymerizable groups to make it suitable for photocrosslinking.
Fig.1: Structure of TCVDPA chromophore and possible derivatives.