Suppressed plasmonic mode coupling for efficient electro-optic lithium niobate modulator.

Integrated lithium niobate electro-optic (EO) modulators have received strong attention due to the unique material properties of LN, which include low optical loss, high refractive index, and strong Pockels effect. The trade-off between electro-optic bandwidth (BW) and half-wave voltage (V_π) in LN modulators has been widely studied; however, the trade-off between the voltage-length product (V_π · L) and optical insertion loss (IL) has received less attention. Nevertheless, it is important for efficient EO modulators. The V_π · L-IL trade-off originates from the balance between the applied electric field intensity and the absorption losses in metallic electrodes, which are both determined by the distance between electrodes and optical waveguides. Here, we find out tha the absorption loss is highly dependent on the electrode width as a result of the mode coupling between the dielectric waveguide and the metal-dielectric plasmonic modes. And we overcome this trade-off by using a special electrode shape that can suppress the mode coupling. As a result, we numerically demonstrate a 5-fold propagation loss reduction (at the same V_π · L) and a 16% V_π · L reduction (at the same loss) compared to a conventional electrode design. We also show that the proposed design does not degrade the frequency response of the modulator, and a more than 50 GHz 3-dB BW can be achieved with an electrode length of 0.8 cm. Our design principle could be used to achieve high-efficiency EO modulators with low insertion loss. Furthermore, the design principle could also be applied to other optical devices with metal-dielectric waveguide structure, such as EO or thermally tuned phase shifters, filters, and optical resonators, to improve their performance.