Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - FLOWTONICS (Solid-state flow as a novel approach for the fabrication of photonic devices)

Teaser

The development of advanced photon-based technologies offers exciting promises in fields of crucial importance for the development of sustainable societies such as energy and food management, security and health care. Innovative photonic devices will however reveal their true...

Summary

The development of advanced photon-based technologies offers exciting promises in fields of crucial importance for the development of sustainable societies such as energy and food management, security and health care. Innovative photonic devices will however reveal their true potential if we can deploy their functionalities not only on rigid wafers, but also over large-area, flexible and stretchable substrates. Indeed, providing energy harvesting, sensing, or stimulating abilities over windows, screens, food packages, wearable textiles, or even biological tissues will be invaluable technological breakthroughs. Today, however, conventional fabrication approaches remain difficult to scale to large area, and are not well adapted to the mechanical and topological requirements of non-rigid and curved substrates. In FLOWTONICS, we propose innovative materials processing approaches and device architectures to enable the simple and scalable fabrication of nano-structured photonic systems compatible with flexile and stretchable substrates. Our strategy is to direct the flow of optical materials through innovative and so far unexplored exploitation of the solid-state dewetting and thermal drawing processes. Our objectives are three-fold: (1) Study and demonstrate, for the first time, the strong potential of the dewetting of chalcogenide glasses layers for the fabrication of large area photonic devices; (2) Show that dewetting can also be exploited to realize photonic architectures onto engineered, nano-imprinted flexible and stretchable polymer substrates; (3) Demonstrate, for the first time, the use of the thermal drawing process as a novel tool to realize advanced flexible and stretchable photonic ribbons and fibers. These novel approaches can contribute to game-changing scientific and technological advances for the sustainable management of our resources and to meet our growing health care needs, putting Europe at the forefront of innovation in these crucial areas.

Work performed

Introduction
In the first period of the FLOWTONICS ERC Starting grant project, our rate of progress has been faster than expected, with three quarters of our objectives already reached, as we will describe in more details below. Beyond reaching our objectives, we have refined our approach to the scalable processing of soft and large-scale nanostructured photonic devices via fluid dynamic processes, and in particular the engineering of the interplay between viscosity and surface tension. The dewetting of thin amorphous films and the thermal drawing of polymer fibers are two seemingly very different processes. They can however both be described for a large part via an interplay between viscosity and surface tension. In this report, I will first show how we could achieve the control of solid-state dewetting of thin, high refractive index optical glass layers to realize high–quality optical metasurfaces over large-area rigid and soft substrates. We have in particular investigated how the interplay between viscous flow at the nanoscale, surface tension, and more broadly fluid instabilities, enables to tailor the size and shape of chaclogenide glasses nano-objects and hence control their optical properties. These results address all the objectives of parts 1 and part 2.1 of the action plan.
In a second part, I will present how we could apply similar principles to the process of thermal drawing – the same process used to fabricate optical fibers – to realize multi-material fibers and ribbons with advanced optical and electronic functionalities. In particular, I will show how we could fabricate thin and flexible fibers with sub-micrometer surface patterns by tailoring the materials surface tension. I will then describe how applying this understanding to electrically conducting polymer nano-composites can lead to intriguing fiber devices such as electromechanical one-dimensional distributed touch sensors. Turning to semiconductor-based fibers, as we proposed in the action plan, I will show how modifying the surface energy of semiconducting materials in solution can enable the fabrication of single-crystal nanowire-based optoelectronic fibers with unprecedented performance. These results address parts 2.2 and 2.3 of the action plan.
Finally, I will show how we could propose, for the first time, to tailor the viscosity of materials during thermal drawing by looking at rheological and microstructural attributes at a deeper level. I will in particular show how we could identify some elastomeric materials that can be drawn with flow properties similar to their thermoplastic counterparts. This opens novel opportunities for fiber-based devices in the fields of stretchable optics and electronics as well as robotics, biological probes and smart textiles. These results address the objectives of Part 3.1 and 3.2 of the action plan.

We have divided the description of our achievements into the three parts as they appear in the action plan.

Detailed description of achievements

Part 1: template dewetting of photonic structures
We have demonstrated the fabrication of advanced photonic structures and optical metasrufaces via the template dewetting of chalcogenide glasses. We have modeled the process, achieved it on a variety of hard and soft substrates, and demonstrated advanced optical properties such as sharp Fano resonances that we exploited for monolayer protein detection. These achievements fulfill the objectives set in Part 1 of the project, and part 2.1.

Let us be more specific about this first major achievement: top-down fabrications approaches rely on lithographic techniques that allow for high resolution and repeatability, but remain complex with multiple processes that are difficult to scale to large-area and non-rigid substrates. This is particularly true for sub-100 nm scale architectures where E-beam lithography remains cumbersome, time and resources intensive, which has hindered so far their deployment in practical applicati

Final results

The progress beyond the state of the art have been described in details in the previous section. Before the end of the project, we expect the following results in the three parts according to the action plan:

Part 1: template dewetting of photonic structures
We expect to deepen our understanding of the dewetting process by having a closer look at the surface science behind the disjoining pressure for the system of materials at play. Our second objective is to realize optoelectronic devices via dewetting as proposed in part 2.2. We finally would like to realize photonic circuits via template dewetting. One could indeed in principle realize waveguides, ring resonators etc.. with this approach.

Part 2: Large area and flexible photonic nanostructures
The proposed scheme to combine dewetting and thermal drawing to generate micro and nanowires in-fiber at prescribed positions has not been realized so far. Based on our improved understanding of dewetting, we will tackle the objective 2.3 from the action plan in the remaining of the project. If successful, our results would go significantly beyond state-of-the-art by integrating complex nanowire based device via a dynamic self-assembly process during fiber drawing. This could generate optoelectronic fibers with performance on par with their 2D counterparts. The integration of such fibers into smart fabrics will also be assessed.

Part 3: Nanostructured flexible and stretchable photonic fibers and ribbons
Finally, we will address part 3.1 of the action plan in the remaining of the project by applying the know-how and understanding developed in this first period. We will also push further the work on stretchable fibers by developing three areas:
- Microstructured and photonic bandgap stretchable fibers; we will show for the first time, and going well beyond state-of-the-art, the fabrication of soft, elastomer based fibers with sub-micrometer architectures. We will show in particular Bragg mirrors and antiresonnant structures to guide light in the hollow core of soft fibers, potentially drastically reducing transmission losses.
- We will demonstrate the thermal drawing of thermoplastic elastomer composites that are electrically conducting. This will open a breadth of novel opportunity for fiber based sensing and actuations in a myriad of applications.
- We will integrate semiconducting domains within soft fibers to make optical and optoelectronic soft fiber devices.