Epsilon-Near-Zero Si Slot-Waveguide Modulator

Abstract

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