Optical Modulation via Coupling of Distributed Semiconductor Heterojunctions in a Si -ITO-Based Subwavelength Grating

Abstract

A mechanism of optical intensity modulation is proposed by utilizing the electro-optic coupling in distributed semiconductor heterojunctions of p-type silicon ($\mathrm{Si}$) and n-type indium tin oxide (ITO) in the form of the subwavelength grating in a rib waveguide. The coupled multiple semiconductor heterojunctions of $\mathrm{Si}$-ITO are made to exhibit efficient optical intensity modulation via electrically tunable permittivity of ITO. The subwavelength grating is a nanophotonic element that not only provides a way to couple multiple heterojunctions, but it also gives rise to efficient optical (fiber to chip) coupling at a wavelength of 1550 nm. Lateral coupling of distributed heterojunctions via depleted charge density distributed along vertical and horizontal directions enable the device to show a high extinction ratio of 24 dB. Also, electrical tuning of the coupling efficiency for an 80-\textmu{}m long device is reported, which exhibits the multifunctional nature of the proposed nanophotonic device. The proposed modulation scheme, with a modulation efficiency of 0.34 V/mm and energy consumption of 36 pJ, may open pathways for energy-efficient compact devices and circuits for large-scale optoelectronic integration. The proposed mechanism of optical modulation takes advantage of distributed semiconductor heterojunctions and enables electrically tunable inherent optical coupling with a nanophotonic element called a subwavelength grating, which further improves the modulation performance compared with a conventional $\mathrm{Si}$-ITO heterojunction.