Researchers examine twisted bilayer graphene's intriguing interactions with light

In 'magic angle' graphene, especially near the angle of 1 degree, the electrons slow down dramatically, favoring interactions between the electrons. Such interactions give rise to a new type of superconductivity and insulating phases in twisted bilayer graphene. Along with many other fascinating properties discovered in the past three years, this material has proven to display extremely rich physical phenomena, but most importantly, it has shown to be an easily controllable quantum material. Up until now, the interaction between twisted bilayer graphene and light was shown to have fascinating outcomes on a theoretical level, but no experiment has so far been able to clearly show how this interaction works.

In a recent work, ICFO researchers Niels Hesp, Iacopo Torre, David Barcons-Ruiz and Hanan Herzig Sheinfux, led by ICREA Prof. at ICFO Frank Koppens, in collaboration with the research groups of Prof. Pablo Jarillo-Herrero (MIT), Prof. Marco Polini (University of Pisa), Prof. Efthimios Kaxiras (Harvard), Prof. Dmitri Efetov (ICFO) and NIMS (Japan), have found that twisted bilayer graphene can be used to guide and control light at the nanometer scale. This is possible thanks to the interaction between light and the collective movement of the electrons in the material.

By exploiting the properties of plasmons, in which electrons and light move together as one coherent wave, the scientists were able to observe that plasmons propagate while being strongly confined to the material, down to the nanoscale. Moreover, by observing the unusual collective optical phenomena occurring in the material, they were able to understand the type peculiar properties of the electrons. This observation of propagating light, confined to the nanoscale, can be used as a platform for optical sensing of gases and bio-molecules.

In their new work, the team used a near-field microscope, which allows probing the optical properties with a spatial resolution of 20 nanometers, a resolution that goes beyond the diffraction limit. In short, the scientists took two layers of graphene, put them one on top of the other, while twisting them close to the magic angle and then, at room temperature, illuminated the material with infrared light on a nano-sized spot. They saw that the plasmons behaved very differently from the usual plasmons, for example in metals or graphene, and this deviation is linked to peculiar motion of the electrons within the moiré superlattice of the bilayer graphene.

This work could represent the first step for nano-optical studies on the exotic phases of twisted bilayer graphene at low temperatures. In particular, it demonstrates that twisted bilayer graphene is a remarkable nanophotonic material, especially since it serves as an intrinsic (no external voltage is required) host of collective excitations.