  Electro-optic signal modulation provides a key functionality in modern
technology and information networks. Photonic integration has enabled not only
miniaturizing photonic components, but also provided performance improvements
due to co-design addressing both electrical and optical device rules. However,
the millimeter-to-centimeter large footprint of many foundry-ready photonic
electro-optic modulators significantly limits on-chip scaling density. To
address these limitations, here we experimentally demonstrate a
coupling-enhanced electro-absorption modulator by heterogeneously integrating a
novel dual-gated indium-tin-oxide (ITO) phase-shifting tunable absorber placed
at a silicon directional coupler region. Our experimental modulator shows a 2
dB extinction ratio for a just 4 um short device at 4 volt bias. Since no
material nor optical resonances are deployed, this device shows spectrally
broadband operation as demonstrated here across the entire C-band. In
conclusion we demonstrate a modulator utilizing strong index-change from both
real and imaginary part of active material enabling compact and high-performing
modulators using semiconductor foundry-near materials.
