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An optical modulator is a very essential component in optoelectronic communication system as it generates an optical data stream out of an applied electric signal. Designing devices with subwavelength dimensions turned out to be a challenge constrained by the ‘diffraction limit’. Plasmonic-based devices had overcome this issue, reaching miniature footprint but introducing high propagation losses. Alternative materials such as conducting transparent oxides (TCOs) act as an optical dielectric material with an excess of free carriers. By adjusting the gating potential on such materials, an epsilon-near-zero state can be reached and the material exhibits a metal-like behavior, in terms of the optical losses. A very popular TCO material is indium tin oxide (ITO). In this work, ITO is utilized in electro-optic modulator design based on a directional coupler device with a slot waveguide platform. The design introduces an ITO/HfO2 combination in the coupler section waveguide. An extinction ratio more than 9 dB and an insertion loss less than 0.9 dB are achieved at a telecommunication standard wavelength of 1.55 μm. Additionally, an average energy per bit (E) of 4.355 fJ/bit as well as an operating bandwidth up to 1 THz, under optimal conditions, are attained.