Cluster Tool Assembly of Stacked Disordered Metasurfaces: A Lithography-Free Route to Complex Photonics

The nanoscale control of light is an essential element of modern advanced optics, with subwavelength-sized structures employed to manipulate its propagation, reflections, and even colour. Known as metasurfaces, these engineered surfaces are already crucial to sensing, displays, and advanced imaging technologies. A recent study by an international team of researchers led from France and Switzerland, led by Amit Sharma (Sr. Scientist, Swiss Cluster AG, Spiez, Switzerland) and Angelos Xomalis (Laboratory for Mechanics of Materials and Nanostructures, Empa,Thun 3602, Switzerland & Norwegian University of Science and Technology, Norway), introduces a unified model for optical stacks that enables advanced applications like dynamic color design and a new visual authentication method we call chromo-encryption. (Authors and affiliations at the end of this article).

A new approach to metasurface design

Traditional metasurfaces rely on precisely ordered arrays of nanoscale structures. While this precision allows for predictable optical behaviour, ordered structures often create strong interference patterns that are governed by the angle and wavelength of incident light. Instead of carefully arranging nanoparticles into periodic patterns, the researchers in this new study fabricated cascading disordered plasmonic metasurfaces: submicron multilayers of randomly distributed gold nanoparticles separated by ultra-precise dielectric spacers. When several of these multilayers are stacked together, they form a complex optical system that crucially offers control over diffuse (scattered) light. In stacking the layers in a multilayer structure, the internal light field that excites each nanoparticle layer is modified, thus enabling the surface's optical response to be engineered. These cascading disordered plasmonic metasurfaces can therefore provide optical control that is not possible via conventional thin-film coatings.

Scalable, lithography-free nanofabrication

One of the undoubted highlights of the work is the development of a ground-breaking lithography-free and wafer-scale fabrication method for creating stacked disordered metasurfaces. At the heart of this success was the Swiss Cluster SC-1 ALD-PVD Cluster System, which allowed the researchers to combine atomic layer deposition (ALD), physical vapour deposition (PVD), and thermal annealing processes in a single controlled environment. 

The fabrication procedure followed a repeatable cycle of:

1. ALD of Å-level precision dielectric spacers.
2. PVD of highly uniform metal films.
3. Thermal annealing to transform the metal films into large-area nanoparticle layers via dewetting.

The process is illustrated below in the figure from the paper, where alternating oxide layers and thin metal films are deposited and then transformed into nanoparticle layers through thermal treatment.

The SC-1’s modular, gate-valve-isolated design allowed a tight feedback loop to be maintained between process parameters and the resulting nanoparticle size and density, giving total reproducibility across multiple iterations and transforming a complex lab innovation into a scalable manufacturing reality.

Modelling Light in Complex Nanostructures

Understanding how light behaves inside such disordered plasmonic systems is challenging. The researchers therefore developed theoretical models to predict specular reflection and diffuse scattering behaviour, even at large angles of incidence. These models revealed the colour of diffusely scattered light depends strongly on the coherence of the illumination. In other words, the same structure can appear differently depending on whether it is illuminated by direct light (such as a focused beam) or diffuse light (ambient illumination).

Chromo-encryption in action

To showcase the concept, the team created a centimetre-scale “chromo-encryption” device composed of three metasurface layers. The stacks appeared similar under diffuse illumination but displayed notably distinct colours under direct light illumination. With the optical response dependent on the illumination conditions, it is possible for information to be encoded in the structure such that it only becomes visible under specific lighting environments. This approach could enable new anti-counterfeiting labels and optical security technologies.

A Platform for Future Nanophotonics

By stacking disordered nanoparticle layers, this team of researchers have created a versatile platform capable of producing optical effects that are otherwise unattainable with conventional thin-film coatings or single-layer metasurfaces. 

With the Swiss Cluster SC-1 enabling precise multilayer engineering, stacked disordered metasurfaces may open new possibilities in optical security, sensing, and photonic devices. It is hoped that this technological advancement will pave the way for a new generation of miniaturised nanophotonic systems across energy, photovoltaic, and display applications. 

Reference

Dive into the future of optics: Assembling and Modeling Stacked Disordered Metasurfaces https://doi.org/10.1021/acs.nanolett.5c03172

Authors
Miao Chen − LP2N, Institut d’Optique Graduate School, CNRS, Université de Bordeaux, Talence 33400, France
Amit Sharma − Laboratory for Mechanics of Materials and Nanostructures, Empa, Swiss Federal Laboratories for Materials Science and Technology, Thun 3602, Switzerland; Swiss Cluster AG, Thun 3602, Switzerland
Johann Michler − Laboratory for Mechanics of Materials and Nanostructures, Empa, Swiss Federal Laboratories for Materials Science and Technology, Thun 3602, Switzerland; orcid.org/0000-0001-8860-4068
Xavier Maeder − Laboratory for Mechanics of Materials and Nanostructures, Empa, Swiss Federal Laboratories for Materials Science and Technology, Thun 3602, Switzerland Philippe Lalanne − LP2N, Institut d’Optique Graduate School, CNRS, Université de Bordeaux, Talence 33400, France

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