Published the Oct. 22, 2018

Adjust the reflectance of a glass while preserving its transparency quality

This article was first published in the CNRS – La lettre Innovation (in French) in September 2018, under « Partenariat ».

Researchers from the Centre for Nanoscience and Nanotechnology1, the Institut Fresnel2 and the PSA Group have designed an innovative glass support that can display virtual images even in unfavourable ambient lighting conditions while remaining transparent. It could lead to applications of augmented reality, especially in the automotive sector.

With an head-up system, the driver of a car no longer needs to look down to consult his dashboard or his GPS: the information is displayed directly in front of the windshield, without forcing to leave the eyes from the road and while preserving a clear vision through the glazing. To achieve this type of display, which also concerns aeronautics and other applications of "augmented reality", it requires a support that reflects the image that is projected while remaining perfectly transparent. The innovative process developed by researchers at the Centre for Nanoscience and Nanotechnology1 and Institut Fresnel2, in collaboration with the PSA Group, aims to improve the performance of an augmented reality display increasing its reflectance for a given wavelength, without impairing its transparency. In addition, it allows to see the data displayed regardless of the angle of incidence (the position of the head of the observer), and overcomes the problems of diffraction or light scattering that could impair vision in transparency.

To achieve augmented reality support, researchers have developed an original pathway3: the creation of nanostructures - silver nanoparticles of controlled dimensions - on the surface of a glass plate. These nanoparticles locally create a plasmonic resonance phenomenon4, responsible for increasing the reflectance of the glass around the chosen wavelength. For the support to remain transparent, the particles must be sufficiently spaced. A periodic network has the disadvantage of generating diffraction, which affects the visual quality. The researchers opted for a "correlated disorder" arrangement, in which the distance between nearest particles is almost constant. "The correlated disorder arrangement makes it possible to overcome diffraction problems without creating diffusion, which is also detrimental to transparency vision," said Béatrice Dagens, CNRS Senior Research at the Centre for Nanoscience and Nanotechnology.

Tests have validated these performances on a display surface of approximately 1 cm2 and the technology will soon be validated for larger areas, in particular using nano-printing techniques.

The Centre for Nanoscience and Nanotechnology and the PSA Group have also decided to expand their collaboration by creating a joint research laboratory in the field of optoelectronics and photonics. Several teams from the Centre for Nanoscience and Nanotechnology will participate in this "OpenLab", called PhOVeA, for Photonics and Optoelectronics for Vehicles and Automotives.

1 CNRS / University Paris-Sud
2 CNRS / Aix-Marseille University / Central School Marseille
3 Correlated Disordered Plasmonic Nanostructures Arrays for Augmented Reality, Hervé Bertin, Yoann Brûlé, Giovanni Magno, Thomas Lopez, Philippe Gogol, Laetitia Pradere,Boris Gralak, David Barat, Guillaume Demésy, and Beatrice Dagens. ACS Photonics, Mai 2018, DOI: 10.1021/acsphotonics.8b00168
4 The phenomenon of plasmonic resonance results from the interaction between an electromagnetic wave and conduction electrons on the surface of a metal. Here, the interaction between the incident wave and the silver nanoparticles, for a precise wavelength, creates an oscillation coupled with the electrons (a "plasmon"). The nanoparticle then behaves like an antenna that re-emits light: the reflection of the incident light is increased.

Picture: Dagens/C2N