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The second youth of optics

When trying to miniaturize optical instruments to the utmost, the physical phenomena that come into play become complex, fascinating and particularly interesting. Welcome to the world of nanophotonics.

Number 33

Vue au microscope électronique à balayage de microstructures suspendues dans le vide pour des applications de filtrage spectral
Electron scanning microscope view of microstructures suspended in a vacuum for spectral filtering applications.
 		 While some scientists build ever larger instruments, like the Large Hadron Collider (LHC) in Geneva in order to discover new particles, others, on the contrary, try to build ever smaller ones. Riad Haïdar and his team in Onera's Theoretical and Applied Optics Department are a case in point. Their objective is to miniaturize optical systems.


La nano-photonique dans la nature : les ailes de papillons sont des réseaux micro-structurés. Ces réseaux, véritables pièges à photons, diffractent la lumière. C’est ce qui donne leur couleur aux papillons. De gauche à droite : une aile de Morpho ménalaus, une vue microscopique de l'aile, et le schéma de leur microstructure. La nano-photonique dans la nature : les ailes de papillons sont des réseaux micro-structurés. Ces réseaux, véritables pièges à photons, diffractent la lumière. C’est ce qui donne leur couleur aux papillons. De gauche à droite : une aile de Morpho ménalaus, une vue microscopique de l'aile, et le schéma de leur microstructure.
Nanophotonics in nature: Butterfly wings are micro-structured networks. These networks, real photon traps, diffract the light. This is what gives butterflies their colors.
From left to right: a Morpho menalaus wing, a microscopic view of the wing and the diagram of their microstructure.

 Over the last fifteen years, optics have undergone developments comparable to those that created the phenomenal success of electronics half a century ago: miniaturization, integration of complex systems and introduction into the activities and needs of the general public. This kind of optics no longer has much to do with that of mirrors and lenses, whose sizes could be counted in centimeters at the least. To mark the difference, we now talk about photonics – the science of the photon, just as electronics is that of the electron. Even better, researchers are now able to design devices with sizes of a few hundred nanometers, and we then talk about nanophotonics.

This new discipline brings considerable possibilities because photons naturally travel faster and easily farther than electrons. However, when you get to this scale, the laws of optics are no longer quite the same as those that we know and call for new theoretical developments. "When the characteristic size of components is around the same as the wavelength of the light in question, everything becomes more complex", indicated Riad Haidar. In the case of infrared light, this wavelength is around a micrometer.
A Frenchman (born in Norway), Thomas Ebbesen, demonstrated this new physics. In the early 1990s, he discovered the "photon screen". This is an opening smaller than the wavelength pierced in a metal film and surrounded by nanometric concentric engravings. A hole, pierced under these conditions, has an extraordinary optical transmission power, several orders of magnitude greater than that predicted by classical theory. Furthermore, its diffraction can be controlled by activating the "surface plasmons", collective motions of electrons on the surface of a conductive material. How can these plasmons be used to control the light? "The theoretical challenges are considerable. We are developing models in order to better understand the physical phenomena at work," explained Riad Haidar. "We are also defining which solutions based on nanophotonics can meet needs in terms of optical instrumentation." This research is being carried out in collaboration with the Optical Institute's Photonics and Nanostructures and Charles Fabry Laboratories within the framework of Precision, a very dynamic scientific and technological collaboration platform run by Jérôme Primot. L’expérience historique de Thomas Ebbesen : les tamis à photons. Transmission extraordinaire à travers un réseau de micro-trous.
The historical experiment of Thomas Ebbesen: photon screens. Extraordinary transmission through a network of micro-holes.
 

Les plasmons de surface (en rose et en bleu) sont des ondes électromagnétiques qui se créent à la surface d’un métal. Ces plasmons peuvent faciliter la transmission des photons grâce à des phénomènes de résonance (en rose)
Surface plasmons (in pink and blue) are electromagnetic waves created on the surface of a metal. These plasmons can make the transmission of photons easier due to resonance phenomena (in pink).

Among the applications of these miniature optical systems, we can mention spectral filtering systems intended to let through certain wavelengths and block others. "We have made membranes around one hundred nanometers thick, suspended between two edges," the researcher described.  "These regularly spaced silicon carbide "micro-bridges," two or three micrometer wide, are a good spectral filtering system."


Onera is studying the filtering properties of metallic nanostructures. Electron scanning microscope view of "micro-bridges" (dark grey) suspended in a vacuum (black areas). Each strip is about 5µm wide.

Another important application is the shaping of light beams, in particular for intense laser radiation. "Our team has developed an original concept for analyzing the surface of the wave, which has already given very good results," indicated Riad Haidar. "And we think that metallic nanostructures will help us improve its performance further. These mechanisms can also be used to change the shape of the beam at will, for example, by selecting certain wavelengths, or by making certain areas more or less intense."

Photovoltaic cells, which convert sunlight into electricity, could also benefit from the progress in nanophotonics. Nano or micro-antennae could trap the light near the solar cell and produce a better yield. Similarly, plasmon micro-antennae could improve the sensitivity of infrared detectors. Onera has proposed a project called Antares (plasmon resonance antennae) on this theme to the National Research Agency.

 

Cécile Michaut, scientific reporter.
 

 

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