Enhanced Polling and Infiltration for Highly-Efficient Electro-Optic Polymer-Based Mach-Zehnder Modulators

TL;DR

This paper presents a novel fabrication approach for ultra-narrow slot waveguide Mach-Zehnder modulators using electro-optic polymers, achieving high modulation efficiency, reduced current, and improved phase shift with minimal added capacitance.

Contribution

The authors develop two innovative fabrication processes and a TiO₂ surface treatment to enhance infiltration, polling efficiency, and device miniaturization in electro-optic polymer modulators.

Findings

VπL below 1.2 V·mm achieved experimentally

Simulated VπL of 0.35 V·mm obtained

Peak current reduced by a factor of 3 with TiO₂ layer

Abstract

An ultra-narrow slot waveguide is fabricated for use in highly-efficient, electro-optic-polymer-based, integrated-optic modulators. Measurement results indicate that $V_\pi L$'s below 1.2 V.mm are possible for balanced Mach-Zehnder modulators using this ultra-narrow slot waveguide on a silicon-organic hybrid platform. Simulated $V_\pi L$'s of 0.35 V.mm have also been obtained. In addition to adapting standard recipes, we developed two novel fabrication processes for achieving miniaturized devices with high modulation sensitivity. To boost compactness and decrease the overall footprint, we use a fabrication approach based on air bridge interconnects on thick, thermally-reflowed, MaN 2410 E-beam resist protected by an alumina layer. To overcome the challenges of high currents and imperfect infiltration of polymers into ultra-narrow slots, we use a carefully designed, atomically-thin layer of TiO$_2$ as a carrier-barrier to enhance the polling efficiency of our electro-optic polymers. Additionally, finite-difference time-domain simulations are employed to optimize the effect of the thin layer of TiO$_2$. As compared to other, non-optimized, cases, our peak measured current is reduced by a factor of 3; scanning electron microscopy images also demonstrate that we achieve almost perfect infiltration. The anticipated increase in total capacitance due to the TiO$_2$ layer is shown to be negligible. In fact, applying our TiO$_2$ surface treatment to our ultra-narrow slot, allows us to obtain an improved phase shift efficiency ($\partial n / \partial V$) of $\sim$94% for a 10 nm TiO$_2$ layer.