Suppressed Out-of-Plane Polarizability of Free Excitons in Monolayer WSe₂

Abstract

Monolayer semiconductors are atomically thin quantum wells with strong confinement of electrons in a two-dimensional (2D) plane. Here, we experimentally study the out-of-plane polarizability of excitons in hBN-encapsulated monolayer WSe₂ in strong electric fields of up to 1.6 V/nm (16 MV/cm). We monitor free exciton photoluminescence peaks with increasing electric fields at a constant carrier density, carefully compensating for unintentional photodoping in our double-gated device at 4 K. We show that the Stark shift is smaller than 0.4 meV despite the large electric fields applied, yielding an upper limit of polarizability α _z to be ∼10^-11 Dm/V. Such a small polarizability, which is nearly two orders of magnitude smaller than the previously reported value for MoS₂, indicates strong atomic confinement of electrons in this 2D system and highlights the unusual robustness of free excitons against surface potential fluctuations.