Ultra-compact silicon nanophotonic modulator with broadband response

TLDR

Electro‑optic modulators are essential for optical communication, yet silicon‑based devices require millimeter‑scale footprints because of weak nonlinearity, and resonator‑based approaches trade bandwidth for strength, while emerging materials like graphene still need long interaction lengths. The study aims to demonstrate an on‑chip, ultra‑compact electro‑optic modulator that maintains high bandwidth and modulation strength. The modulator uses a plasmonic metal‑oxide‑semiconductor waveguide that concentrates the optical field in a nanometer‑thin ITO layer, whose free‑carrier concentration is electrically tuned to increase loss. Experimentally, the device achieves a record 1 dB/µm extinction ratio across a 1 µm bandwidth and integrates seamlessly into silicon‑on‑insulator platforms, offering a path toward broadband, compact, efficient communication links.

Abstract

Abstract Electro-optic modulators have been identified as the key drivers for optical communication and signal processing. With an ongoing miniaturization of photonic circuitries, an outstanding aim is to demonstrate an on-chip, ultra-compact, electro-optic modulator without sacrificing bandwidth and modulation strength. While silicon-based electro-optic modulators have been demonstrated, they require large device footprints of the order of millimeters as a result of weak non-linear electro-optical properties. The modulation strength can be increased by deploying a high-Q resonator, however with the trade-off of significantly sacrificing bandwidth. Furthermore, design challenges and temperature tuning limit the deployment of such resonance-based modulators. Recently, novel materials like graphene have been investigated for electro-optic modulation applications with a 0.1 dB per micrometer modulation strength, while showing an improvement over pure silicon devices, this design still requires device lengths of tens of micrometers due to the inefficient overlap between the thin graphene layer, and the optical mode of the silicon waveguide. Here we experimentally demonstrate an ultra-compact, silicon-based, electro-optic modulator with a record-high 1 dB per micrometer extinction ratio over a wide bandwidth range of 1 μm in ambient conditions. The device is based on a plasmonic metal-oxide-semiconductor (MOS) waveguide, which efficiently concentrates the optical modes’ electric field into a nanometer thin region comprised of an absorption coefficient-tuneable indium-tin-oxide (ITO) layer. The modulation mechanism originates from electrically changing the free carrier concentration of the ITO layer which dramatically increases the loss of this MOS mode. The seamless integration of such a strong optical beam modulation into an existing silicon-on-insulator platform bears significant potential towards broadband, compact and efficient communication links and circuits.