Silicon rib waveguide electro-absorption optical modulator using transparent conductive oxide bilayer

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We propose a novel ultra compact electro-absorption optical modulator based on a silicon rib waveguide and numerically demonstrate its performance. The proposed design employs two types of transparent conductive oxide (TCO) layers with different carrier densities to achieve both high modulation efficiency and low optical insertion loss. The thin TCO layer with high carrier density enables efficient modulation through the metal–oxide–semiconductor structure. On the other hand, the upper TCO layer with low carrier density allows low-resistance electrical contact for the top electrode without large optical loss. Using an indium tin oxide bilayer with optimized carrier densities, we numerically demonstrate a 4.3 dB extinction ratio and a 2.6 dB optical insertion loss with 1 µm device length. We estimate that the modulator operates under a low driving voltage of 1.3 V, exhibiting an ultra low energy consumption of 22.5 fJ/bit and a broad RC modulation bandwidth of over 40 GHz.