24 August 2020 Thresholdless Soliton Formation in Photonic Moiré Lattices

Photonic moiré pattern

The observation of soliton formation with a power threshold dictated by geometry in photonic moiré lattices reported in Nature Photonics. Take two identical layers of semi transparent material that have the same structure, put them one on top of the other, rotate them and look at them from above, and hexagonal patterns start to emerge. They are known as moiré patterns or moiré lattices.

Moiré lattices are used every day in applications such as art, textile industry, architecture, as well as image processing, metrology and interferometry. They have been a matter of major interest in science, since they are easily produced using coupled graphene–hexagonal boron nitride monolayers, graphene–graphene layers and graphene quasicrystals on a silicon carbide surface and have proven to generate different states of matter upon rotating or twisting the layers to a certain angle, opening to a new realm of richer physics to be investigated. A few years ago scientists at MIT let by Prof Pablo Jarillo-Herrero found a new type of unconventional superconductivity in twisted bilayer graphene that forms a moiré lattice. Since then, an explosion of new physics has occurred, which include several landmark contributions by the ICFO team led by Prof Efetov that unveiled a new zoo of unobserved states in such structures. (Read more: News 1, News 2, News 3 .)

In a different realm of Physics, a team of scientists in a long-standing collaboration between ICFO researchers Prof Yaroslav Kartashov and Prof Lluis Torner, the former post-doctoral researcher in the same group Prof Fangwei Ye (currently full professor at the Shanghai Jiao Tong University, where the experiments were conducted), and Prof Vladimir Konotop in Lisbon, reported early this year in Nature the observation of the transition from delocalization to localization in two-dimensional patters, afforded by the properties of the moiré structures with fundamentally different geometries (periodic, general aperiodic, and quasicrystal).

Now moiré lattices optically-induced in a photorefractive nonlinear crystal have been employed to observe the formation of optical solitons under different geometrical conditions controlled by the twisting angle between the constitutive sublattices. The behavior of the soliton formation threshold was confirmed to be directly linked to the band structure of the moiré lattices resulting from the different twisting angles of the sublattices and, in particular, of the band-flattening associated to the geometry of the lattices. Similar phenomena are anticipated to occur in moiré patterns composed of sublattices of other crystallographic symmetries and in other physical systems where flat-bands induced by geometry arise. The results were published yesterday in Nature Photonics.