Spotlight on “Direct fabrication of silicon photonic devices on a flexible platform and its application for strain sensing”

Everybody working in photonics has the perception that the natural substrate for integrated devices is a bit of a rigid material, which is normally provided by a semiconductor wafer or by a crystal. This picture comes from well established fabrication methodologies, but it is not necessarily what has to be. Although being somewhat surprising, optical materials like semiconductors, that are brittle and fragile as a bulk, become soft and flexible when shrunk to nanometer scale. With this in mind, we can really imagine the concept of silicon waveguides, fabricated on a flexible substrate, which can be elastically deformed with neither cracks nor damages, as if they were made of plastic themselves. Material properties tell us that there are no fundamental barriers to make silicon photonics on a plastic sheet, but how to do it in practice? Several strategies have been explored so far, in which photonic devices are patterned on a sacrificial solid substrate and then transferred to a flexible substrate. Yet, all these approaches suffer from severe limitations on the size and geometry of the manufactured devices, and require extremely smooth surfaces for chip bonding and extremely sophisticated alignment equipment. The treasure map disclosing a viable route to fabricate silicon photonic devices directly on a flexible substrate seemed far away from being discovered, at least until the work by L. Fan and coworkers. They found a simple and reproducible way to integrate complex photonic structures on a plastic film without the need of transferring them from another rigid substrate. Simple to say, but to succeed in their goal they had to address and solve a number of critical problem