AMORPHOUS THIN FILMS FOR MECHANICALLY FLEXIBLE, MULTI-MATERIAL INTEGRATED PHOTONICS

Abstract

Flexible integrated photonics is a new technology which only started to burgeon in the past few years, opening up emerging applications ranging from flexible optical interconnects to conformal sensors on biological tissues. One of the most important factors dictating the performance of these flexible devices is the material choice. Organic polymers are generally considered to be compatible with flexible substrates. However, the low refractive indices of polymers (compared to semiconductors) cannot provide the strong optical confinement necessary for compact photonic integration. Besides polymers, semiconductor NanoMembranes (NMs), thin slices of single crystal semiconductors with sub-micron thickness, are being actively pursued for photonic device integration on flexible substrates. Unlike their rigid bulk counterparts, NMs can be tightly bent without cracking, since surface strain induced by bending linearly scales with the membrane thickness. To make photonic devices, NMs structures are usually first patterned on a rigid substrate such as silicon. The fabricated structures are then picked up by a PDMS rubber stamp and transferred on to the final flexible substrate.This multi-step hybrid process limits processing yield and throughput. Therefore, we turned to amorphous glasses – the material of choice for optics given their exceptionally low optical attenuation. In flexible photonics, using these non-crystalline materials also enables a monolithic fabrication route as they can be directly deposited on to flexible substrates without resorting to epitaxial growth. Specifically, we focus on chalcogenide glass materials and amorphous TiO₂, as both can be deposited at relatively low temperature (250°C or less) compatible with flexible substrate integration [1-5].