« Dielectric optically-active metamaterials: are they achievable through bottom-up chemistry ? »

Glenna L. Drisko

CNRS researcher
ICMCB – UMR5026
Bordeaux

The advent of metamaterials consisted of a flurry of activity around plasmonic resonators. Fifteen years of research has confirmed that plasmonic-based materials absorb too much light to be used in optically-active metamaterials. High refractive index materials theoretically provide an alternative to plasmonics. Semiconductors boast the highest refractive index across the visible spectrum among naturally occurring materials. Silicon is generally favored for optical applications due to its natural abundance, omnipresence in the electronics industry, and relatively easy synthesis compared to other semiconductors. However, even silicon is extremely difficult to fabricate using bottom-up chemistry. It is relatively easy to produce 10 nm silicon particles, however these are too small for optically-active meta-atoms and such small particles oxidize completely to low-refractive index SiO2. This begs asking, what silicon forms can we realistically expect to produce? And are there alternatives to silicon?

Metamaterials produced via bottom-up synthesis methods require the preparation of individual resonant particles, referred to as meta-atoms, and then the assembly of these particles into a 2D or 3D material. My group has approached the synthesis of silicon meta-atoms by examining the effect of the nature of the solvent, silicon precursor, reducing agent and processing conditions on the silicon particle diameter, polydispersity and crystallinity. Through these lessons, we have come closer to ideal silicon meta-atoms. We have also begun research into less optically fantastic, but more easily obtained TiO2 nanoparticles. I will present the synthesis methods, optical measurements and future prospects of these dielectric meta-atoms. I will also present the plasmonic-based metamaterials produced previously in the team, to showcase what can be achieved using bottom-up chemistry.

« Nanodevices for Bio-inspired Computing »

Julie GROLLIER

CNRS/Thales, Palaiseau, France

In the last five years, Artificial Intelligence has made striking progress, now defeating humans at subtle strategy games, such as Go, and even Poker. However, these algorithms are running on traditional processors which have a radically different architecture than the biological neural networks they are inspired from. This considerably slows them down and requires massive amounts of electrical power, more than ten thousand times what the brain typically needs to function. This energy dissipation is not only becoming an environmental issue, but it also sets a limit to the size of neural networks that can be simulated. We are at a point where we need to rethink the way we compute, and build hardware chips directly inspired from the architecture of the brain. This is a challenge. Indeed, contrarily to current electronic systems, the brain is a huge parallel network closely entangling memory and processing.
In this talk, I will show that, for building the neuromorphic chips of the future, we will need to emulate functionalities of synapses and neurons at the nanoscale. I will review the recent developments of memristive nano-synapses and oscillating nano-neurons, the physical mechanisms at stake, and the challenges in terms of materials. Finally, I will present the first achievements of neuromorphic computing with novel nanodevices and the fascinating perspectives of this emerging field.