It all started with Nicolas Roy's trip to Stanford. Nicolas is a PhD student in the Department of Physics and a member of the NISM and NaXys Institutes. The purpose of the visit to Stanford was to develop expertise at UNamur on a new method of simulating twisted photonic crystals, recently published by the prestigious university. Following discussions during the stay at Stanford, avenues for collaboration emerged, notably that of continuing research related to one of their publications in order to try to make a device that allows the direction of the light beam to be manipulated as efficiently and compactly as possible.  The gamble paid off, as the theoretical study predicts a device measuring 6 microns (the size of a hair)!  What's more, it is very energy efficient.  In practical terms, it could be used to track satellites, for example, without moving the transmitter or receiver, which is complicated in a photonic circuit.  Another practical application is being studied for Meta, a company that wants to reduce the size of virtual reality headsets to a simple pair of glasses... 

During his PhD, and based on a Stanford team publication entitled "Theory for Twisted Bilayer Photonic Crystal Slabs", Nicolas reproduced the simulation method and developed an analytical model of the numerical simulations. The use of these inexpensive simulations has made it possible to find the photonic structures most capable of deflecting light in a controlled manner. The analytical model, in turn, provides an explanation for what has been observed, and thus a better understanding of what's going on. In short, it opens up prospects for simpler fabrication of future devices.

"Computational intelligence, combining machine learning and optimization/automation by algorithms, makes it possible to save human time by performing very numerous and rapid calculations. By way of comparison, the calculations that were carried out without the use of this method developed by the Stanford research team took several days. We now have simulations lasting 1 hour. The machine learning methods I've developed now make it possible to carry them out in less than a second!"

Nicolas Roy Researcher at the naXys Institute

A model, but for what?

The research teams collaborating on this study are working on twisted photonic crystals, i.e. two-dimensional materials formed, for example, from two superimposed and structured layers of silicon, and their interaction with light. 

It is a bit like a sandwich made of two slices of bread that can be slid over each other.   

 

Illustration caption: Schematic representation of the disoriented photonic device used to dynamically change the direction of light.

Représentation schématique du dispositif photonique désorientée servant à modifier la direction de la lumière de manière dynamique.

In designing an analytical model, Nicolas Roy also used a theory that has been known since the 1960s: lattice networks. A lattice network is a plane diffraction network with a sawtooth profile.  In concrete terms, it resembles the roofs of old factories.  The novelty he brought to this concept is that it allows us to understand the mechanism that controls the angle of the light beam's exit thanks to the twist between the two layers. In doing so, he identified that the system acted similarly to a lattice grating. The team, using meta-models, was able to concentrate the light in a very specific direction with 90% efficiency.

Mastering light

What is the purpose of this type of twisted structure? To control light and ultimately create systems that can slow it down or even stop it.

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Portrait Michaël Lobet

It's a remarkable feat for this speedster, light, which travels at over 300,000 km/s! It is the fastest speed that can be reached in the universe. Slowing it down is therefore no easy task. In this type of disoriented structure, light is trapped but its state is preserved: it is put ‘on pause’, so to speak.  In practical terms, we can imagine improving the characteristics of lasers or the performance of quantum computers. One important application would be to create optical memories, which would allow light bits to be stored without being destroyed and released at will. Or at least slow them down long enough to perform the mathematical operations necessary for all-optical computing. Another application is to take advantage of the slowing down of light to enhance light-matter interactions. This can be used to increase the efficiency of chemical reactions in photocatalysis, for example. These photocatalytic reactions are useful for water treatment or air treatment, for example, subjects on which Professors Olivier Deparis and Bao-Lian Su are working at the NISM institute.

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Professeur Michaël Lobet University of Namur

This twist technique therefore opens up many unexplored possibilities in photonics by adding a degree of control over light. The researchers are continuing their work in this area, continuing their fruitful collaboration with Professor Fan's team, Stanford University.  

It looks like there's no end in sight to the twisting at the University of Namur! 

The research teams involved

The Belgian team

The American team

  • Professor Shanhui Fan (Stanford University)
  • Dr Beicheng Lou

Thanks

The researchers thank UNamur, and more specifically the Department of Physics and the NISM Institute for funding Nicolas Roy's trip, the Institut naXys for its support in this project, the PTCI technology platform, whose supercomputers made this study possible, as well as the FNRS for funding the research mandates of Michaël Lobet and Alexandre Mayer.

Twist-Induced Beam Steering and Blazing Effects in Photonic Crystal Devices

To find out more