Indium-Tin-Oxide for High-performance Electro-optic Modulation

TL;DR

This paper reviews the use of Indium Tin Oxide (ITO) for ultra-compact, high-performance electro-optic modulation, highlighting its strong light-matter interaction, potential for GHz speeds, and significant footprint reduction compared to silicon modulators.

Contribution

It introduces novel insights into ITO-based modulators, including cavity enhancement, speed analysis, and footprint advantages, addressing open questions about permittivity modulation mechanisms.

Findings

Waveguide inline cavity improves extinction ratio by tenfold.

Device speed is resistance-limited, enabling hundreds of GHz operation.

ITO modulators have three orders of magnitude smaller footprint than silicon MZI modulators.

Abstract

Advances in opto-electronics are often led by discovery and development of materials featuring unique properties. Recently the material class of transparent conductive oxides (TCO) has attracted attention for active photonic devices on-chip. In particular Indium Tin Oxide (ITO) is found to have refractive index changes on the order of unity. This property makes it possible to achieve electro-optic modulation of sub-wavelength device scales, when thin ITO films are interfaced with optical light confinement techniques such as found in plasmonics; optical modes are compressed to nanometer scale to create strong light-matter-interactions. Here we review efforts towards utilizing this novel material for high-performance and ultra-compact modulation. While high performance metrics are achieved experimentally, there are open questions pertaining the permittivity modulation mechanism of ITO. Furthermore, we show that a footprint-saving waveguide inline cavity can enhance obtainable extinction-ratio to insertion-loss ratios by about one order of magnitude over non-cavity based version. Moreover, we offer a speed analysis that shows that the device is resistance limited, but not capacitance or drift-carrier limited. Interestingly, two bias options exist for ITO and we find that a side-connection enables devices that should in principle enable several hundred of GHz fast devices, using our routinely achievable ITO film resistivities. Finally, we offer a brief discuss about footprint savings of compact ITO modulators showing a 3-orders of magnitude smaller footprint over Silicon photonic MZI-based modulators.