High-speed hybrid plasmonic electro-optical absorption modulator exploiting epsilon-near-zero effect in indium-tin-oxide

Abstract. Using transparent conducting oxides, such as indium-tin-oxide (ITO), for optical modulation has attracted research interest because of their epsilon-near-zero (ENZ) characteristics at the telecom wavelengths. Utilizing ITO in multilayer structure modulators, optical absorption of the active ITO layer can be electrically modulated over a large spectrum range. Although they show advances over common silicon electro-optical modulators (EOMs), they suffer from high insertion losses (ILs). To reduce ILs and device footprints without sacrificing bandwidth and modulation strength, slot waveguides are promising options because of their high optical confinement. We present the study and design of an electro-optical absorption modulator based on an electrically tuning ITO carrier density inside an MOS structure. The device structure is based on the dielectric slot waveguide with an ITO plasmonic waveguide modulation section. By changing the dimensions, the effective refractive indices for the slot mode and the off-state mode of the plasmonic section can be matched. When applying electric field to the plasmonic section (on-state), carriers are generated at the ITO-dielectric interface that results in changing the layer where the electric field is confined from a transparent layer into a lossy layer. A finite difference time-domain method with perfect matching layer absorbing boundary conditions is taken up to simulate and analyze this design. An extinction ratio of 15.5 dB is achieved for a 10-μm-long modulation section, at the telecommunications wavelength (1.55  μm). This EOM has advantages of simple design, easy fabrication, compact size, compatibility with existing silicon photonics platforms, as well as broadband performance.