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Akira Otomo1 Takahiro Kaji1 Toshiki Yamada1

1, National Institute of Information and Communications Technology, Kobe, , Japan

Electro-optic (EO) polymers have attracted attention for applying to ultrafast modulators for long-haul optical communication as well as short range optical interconnection, electromagnetic wave sensors, terahertz (THz) wave generators and detectors, etc. Large EO coefficient and modulation bandwidth are the advantageous features of EO polymers and are used to evaluate the performance of EO polymers. Although absorption coefficient (α) and EO coefficient (r) are known as in a trade-off relation for EO polymers, the absorption has been assumed to be small enough to be neglected. That is because the absorption at NIR wavelength region is less than the measurable limit in the thin film absorption measurement. However, propagation losses of waveguides made with recently developed EO polymers having very large EO activities are more than a few dB/cm and it is suspected that the polymers have significant absorption. We measured the absorption at NIR wavelength region by making thick films of EO polymers and revealed that the magnitude of absorption is small but definitely not negligible. Therefore, we re-evaluate EO polymers by the figure-of-merit for modulators including absorption (FOMMOD = n3r/α) and determine the molecular structure of EO polymers optimized for each of the specific device applications such as ultrafast modulators for telecommunication, Si hybrid modulators for data communication, phase modulator array for LiDAR.
EO polymers also show the large FOM for THz wave generation (FOMTHz = nopt6r2/16nTHz), long coherence length and relatively small absorption coefficients in a broadband THz frequency region, compared to the other inorganic and organic nonlinear optical materials such as lithium niobate (LiNbO3), zinc telluride (ZnTe), and DAST. We evaluated refractive index and absorption coefficient of our EO polymers at THz frequency region and confirmed these advantages. In order to make efficient THz generation and detection devices, EO polymer waveguides have to be formed with the claddings of THz-wave low-loss materials. However, the conventional fabrication process of the EO polymer waveguide devices requires the conductive polymer-based claddings as well as the electrodes for poling the EO chromophores, and the conductive cladding materials or the electrodes strongly absorb the THz waves. We developed a novel fabrication process that the poling procedure was made before forming the waveguide. Then we successfully fabricated the EO polymer waveguides on the substrate of a cyclic olefin polymer (COP), whose absorption coefficients are very small in the THz region, with complete elimination of the poling electrodes. Such novel waveguides can be made only with the recently developed thermally stable EO polymers whose EO activity is kept unchanged even after experiencing high temperature around 100 deg C during the waveguide fabrication processes.

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