Chiral metasurface coupled to two-dimensional heterostructures for optical/valleytronics applications

Valleytronics is a new branch of semiconductor physics that has gained increasing interest in recent years, and it aims to manipulate the degree of freedom of the valley of electronic energy bands of certain materials to store or move information, much like electronics takes advantage of charge carriers for the same purpose. Ideal materials for valleytronics are bidimensional (2D) transition metal dichalcogenides (TMDs). One of the aspects of 2D TMDs that has caught most attention is the property of allowing a valley-selective excitation under interaction with circularly polarized light: illuminating the material with right-handed circularly polarized light promotes the formation of excitons (electron-hole pairs) only in one valley, vice versa, excitation under left-handed circularly polarized light promotes the creation of excitons only in the other valley, creating in this way a population imbalance between the two valleys. During radiative recombination, the handedness of the light is conserved so that it is possible to detect a difference in the photoluminescence emission in the two polarizations. This difference can be enhanced using a chiral metasurface coupled to the TMDs that preferentially enhances one of the two circular polarizations. This has already been experimentally verified for TMD monolayers. In this work, the same concept is applied to a vertical heterostructure of monolayers of two different TMDs (MoS2 and WSe2) with a type-II band alignment. Unlike single monolayers, heterostructures can host excitons whose electron and hole reside in different materials, ending up with an out-of-plane electric dipole moment, a lower binding energy, and a higher valley lifetime. The computational part of this work focuses on finite-difference time-domain (FDTD) simulations of a chiral metasurface designed to enhance the left-handed component of the signal emitted by interlayer excitons in a MoS2/WSe2 heterostructure. Meanwhile, the experimental part is dedicated to the fabrication and characterization of the heterostructures by exfoliation and thermal annealing after stacking the monolayers.