Materials and devices for mechanically flexible integrated photonics
Date
2016
Authors
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Publisher
University of Delaware
Abstract
Flexible integrated photonics is a new technology that has only started to burgeon in the past few years, which enables a wide cross-section of emerging applications ranging from flexible optical interconnects to conformal sensors on biological tissues. Such devices are traditionally fabricated using pattern transfer methods. However these techniques are relatively complicated and limit the fabrication yield. Direct patterning of a-Si devices on a polymer substrate has recently been reported; nevertheless, the optical quality of the a-Si film is compromised due to the low deposition temperature. This thesis includes our work in material development, micro-mechanical design and device engineering towards enabling novel flexible integrated photonic systems.
First, we pioneered a monolithic approach to realize flexible, high-index-contrast chalcogenide glass photonics with significantly improved processing throughput and yield. Noting that the conventional multilayer bending theory fails when laminates have large elastic mismatch, we derived a mechanical theory accounting for multiple neutral axes in one laminated structure to accurately predict its strain-optical coupling behavior. Through combining monolithic fabrication and local neutral axis designs, we fabricated devices that boast record optical performance (Q = 460,000) and excellent mechanical flexibility enabling repeated bending down to sub-millimeter radius without measurable performance degradation. The capabilities of these devices far surpass those of current ‘state-of-the-art’ designs. Moreover, we demonstrated that our flexible glass device technology offers a simple, effective fabrication route for 3-D photonic structures including vertical-stack resonator filters, interlayer waveguide couplers, and woodpile photonic crystals.
Furthermore, we investigated TiO2 as a material candidate for biocompatible and flexible integrated photonics. Amorphous TiO2 films were deposited using a low temperature (< 250 °C) sol-gel process and exhibited a low optical loss of 3 dB/cm. Structural and optical properties of the films were systematically characterized. We also fabricated and tested TiO2 optical waveguides and resonators monolithically integrated on flexible polymer substrates, measuring resonator quality factors as high as 20,000. Despite the inherent mechanical rigidity of the TiO2 material, we experimentally demonstrated repeated folding down to a < 0.3 mm radius of devices fabricated with the developed multi-neutral-axis mechanical design without degrading their optical performance. Finally, we showed through in-vitro tests that the TiO2 devices are cytocompatible. These results pave the way towards emerging applications of flexible photonics technology such as epidermal sensing, optical imaging, optogenetic modulation, and data communications.