Engineering Exciton Interactions in van der Waals Heterostructures

Abstract

Light-matter interaction is fundamental to both classical and quantum nonlinear optics. Yet photons do not interact except for extreme conditions: e.g.strong spatial confinement. With strong interactions, photon blockade can occur, wherein a photon coupled to the system inhibits the absorption of subsequent photons, analogous to Coulomb blockade. Realizing such strong interactions typically requires a high photon density and an interaction strength determined by the material properties. 2D materials with tightly bound excitons exhibit strong light–matter interactions and sizable optical nonlinearities arising from exciton–exciton interactions.In this thesis, we demonstrated that introducing free carriers into homotrilayer WSe$_2$ can significantly enhance these interactions. The carrier bonded excitons---Fermi polarons exhibit large optical nonlinear behavior due to exciton-carriers interactions. By coupling these Fermi polarons to a 1D microcavity, we realized polaron--polaritons with distinct nonlinear behavior. The photon numbers needed for inducing the nonlinear shift can be further reduced with improved cavity quality. In addition, the expected longer polariton lifetimes and enhanced polariton nonlinear shift could push the polariton to the blockade regime. Another route towards the photon blockade is by confining excitons below subwavelength scales, thus strengthening exciton--exciton interactions. We realize such confinement by imprinting the electrostatic superlattice of twisted hBN onto the adjacent monolayer MoSe2. PFM and Optical measurements reveal excitons trapped in a nanoscale, 1D potential at moiré domain boundaries. Our results establish van der Waals heterostructures as a solid platform for engineering nonlinear optical response with applications to classical and quantum optoelectronics.Our methods provides a road map points toward on-chip, solid-state single-photon sources and low-power optical logic grounded in excitonic platforms.