Flexible Dielectric Glass Metasurfaces

The new method employs a natural process already used in fluid mechanics: dewetting. (Source: V. Navikas, EPFL)

Optical circuits are set to revo­lutionize the performance of many devices. Not only are they 10 to 100 times faster than electronic circuits, but they also consume a lot less power. Within these circuits, light waves are controlled by extremely thin meta­surfaces that concen­trate the waves and guide them as needed. The meta­surfaces contain regularly spaced nano­particles that can modulate electro­magnetic waves over sub-micrometer wavelength scales.

Meta­surfaces could enable engineers to make flexible photonic circuits and ultra-thin optics for a host of appli­cations, ranging from flexible tablet computers to solar panels with enhanced light-absorption charac­teristics. They could also be used to create flexible sensors to be placed directly on a patient’s skin, for example, in order to measure things like pulse and blood pressure or to detect specific chemical compounds. The catch is that creating meta­surfaces using conven­tional litho­graphy is a fasti­dious, several-hour-long process that must be done in a clean room. But EPFL engineers from the Labora­tory of Photonic Materials and Fiber Devices (FIMAP) have now developed a simple method for making them in just a few minutes at low temperatures or sometimes even at room tempera­ture with no need for a clean room. The EPFL’s School of Engi­neering method produces dielectric glass meta­surfaces that can be either rigid or flexible.

The new method employs a natural process already used in fluid mechanics: dewetting. This occurs when a thin film of material is deposited on a substrate and then heated. The heat causes the film to retract and break apart into tiny nano­particles. “Dewetting is seen as a problem in manufacturing, but we decided to use it to our advantage,” says Fabien Sorin, the head of FIMAP. With their method, the engineers were able to create dielectric glass meta­surfaces – rather than metallic meta­surfaces – for the first time. The advantage of dielectric meta­surfaces is that they absorb very little light and have a high refractive index, making it possible to effec­tively modulate the light that propa­gates through them.

To construct these meta­surfaces, the engineers first created a substrate textured with the desired archi­tecture. Then they deposited a material – in this case, chalco­genide glass – in thin films just tens of nanometers thick. The substrate was subse­quently heated for a couple of minutes until the glass became more fluid and nano­particles began to form in the sizes and positions dictated by the substrate’s texture.

The engineers’ method is so efficient that it can produce highly sophis­ticated meta­surfaces with several levels of nano­particles or with arrays of nanoparticles spaced 10 nm apart. That makes the meta­surfaces highly sensitive to changes in ambient conditions such as to detect the presence of even very low concen­trations of bio­particles. “This is the first time dewetting has been used to create glass meta­surfaces. The advantage is that our meta­surfaces are smooth and regular, and can be easily produced on large surfaces and flexible substrates,” says Sorin. (Source: EPFL)

Reference: T. D. Gupta et al.: Self-assembly of nanostructured glass metasurfaces via templated fluid instabilities, Nat. Nano., online 11 February 2019; DOI: 10.1038/s41565-019-0362-9

Link: Photonic Materials and Fiber Devices Laboratory, Institute of Materials, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland

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