Nanoscale field effect optical modulators based on depletion of epsilon-near-zero films

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The field effect in metal-oxide-semiconductor (MOS) capacitors plays a key role in field-effect transistors (FETs), which are the fundamental building blocks of modern digital integrated circuits. Recent works show that the field effect can also be used to make optical/plasmonic modulators. In this paper, we report the numerical investigation of field effect electro-absorption modulators each made of an ultrathin epsilon-near-zero (ENZ) film, as the active material, sandwiched in a silicon or plasmonic waveguide. Without a bias, the ENZ films maximize the attenuation of the waveguides and the modulators work at the OFF state; on the other hand, depletion of the carriers in the ENZ films greatly reduces the attenuation and the modulators work at the ON state. The double capacitor gating scheme with two 10-nm HfO2 films as the insulator is used to enhance the modulation by the field effect. The depletion requires about 10V across the HfO2 layers. According to our simulation, extinction ratio up to 3.44dB can be achieved in a 500-nm long Si waveguide with insertion loss only 0.71dB (85.0% pass); extinction ratio up to 7.86dB can be achieved in a 200-nm long plasmonic waveguide with insertion loss 1.11dB (77.5% pass). The proposed modulators may find important applications in future on-chip or chip-to-chip optical interconnection.