Highly-Efficient, Ultra-Compact and Polarization-Insensitive Electro-Absorption Modulator Driven by Hybrid Silicon-Indium Tin Oxide-Based MOS Capacitors

Abstract

Polarization-insensitive modulation plays a key role to be compatible with the current fiber communication systems and the further high-capacity multiplexing transmissions on-chip. Here, we propose a highly-efficient, ultra-compact and polarization-insensitive electro-absorption modulator (EAM), which is driven by the hybrid silicon-indium tin oxide (ITO) based metal-oxide-semiconductor (MOS) capacitors. The hybrid silicon-ITO waveguide is formed by embedding an ITO layer in the silicon waveguide center and depositing a second ITO layer on the waveguide surface, thus the light-matter interaction can be strongly enhanced inside the silicon waveguide based on the epsilon-near-zero (ENZ) effect of ITO rather than the commonly employed outside cladding region. By choosing the optimum ENZ condition of ITO layers at the designated wavelength and optimizing the waveguide dimensions, polarization-insensitive modulation is realized with a high modulation efficiency (ME) of ~3.18 dB/ $\mu \text{m}$ (ME discrepancy $\mu \text{m}$ ) for both polarizations, which is superior to the previous reports. Benefiting from this excellent modal modulation feature, we further design a polarization-insensitive EAM in a length of only $8~\mu \text{m}$ . The modulation depth and insertion loss are higher than 25 dB and lower than 0.24 dB at $1.55~\mu \text{m}$ for both polarizations, respectively. Moreover, the corresponding wavelength bandwidth and electric properties are also analyzed including fabrication procedures. The proposed EAM is expected to be applied in the polarization multiplexing transmissions on-chip.