Epsilon-Near-Zero Si Slot-Waveguide Modulator

We experimentally demonstrate a broadband electro-absorption modulator exploiting indium tin oxide (ITO) as the active switching material. Si strip waveguides are fabricated and covered with 8 nm of HfO2 and 15 nm of ITO to form metal-oxide-semiconductor capacitor (MOS-C) based modulators. The mobile carrier density in the ITO film is controlled using a postanneal treatment to tune its permittivity epsilon to a near-zero value at the operation wavelength of 1550 nm. Using simulations and experiments, we demonstrate that realizing an epsilon-near-zero (ENZ) can enhance the modulation performance as it increases the overlap of the guided mode with the active ITO layer. We then show even greater benefits of this approach with Si waveguides featuring a central slot filled with ITO. Leveraging the ENZ effect, we achieve a notable 3 dB modulation depth of optical signals in a nonresonant waveguide structure with a length of 20 mu m. The results provide insight into the design of very compact modulators for chip-scale optical links.