Efficient Low-Voltage Phase Shifter with Inkjet-Printed Liquid Crystal on a Silicon on Insulator Platform

Abstract

A new electro-optic phase shifter device architecture consisting of two lateral rail electrodes in doped silicon close to a waveguide with a liquid crystal cladding is demonstrated. Starting with a completed silicon photonics wafer of IMEC's iSiPP50G platform (including modulators, detectors, and metallization), the back-end-of-line stack is opened up locally down to the waveguides. Liquid crystal is deposited in the recesses using inkjet printing. The narrow gaps between the rail electrodes and the waveguide core allow for actuation with a low voltage, and increase the overlap with the actuated liquid crystal. The demonstrated device geometry has low carrier absorption losses even though the side-rails are doped. This allows an increased driving frequency, eliminating phase flicker. A phase shift of 2 π for 2.7 V is obtained within 100 µm, going up to 6 π at 10 V with an insertion loss of 1 dB. Models suggest a power consumption <1 nW. The performance of this phase shifter is unmatched by alternative techniques that either require a higher voltage, have a larger optical loss or consume more electrical power. A novel purely digital driving scheme is demonstrated enabled by the unique device architecture, simplifying the required driver electronics.