Suppressed out-of-plane polarizability of free excitons in monolayer WSe$_{2}$

TL;DR

This study demonstrates that free excitons in monolayer WSe2 exhibit extremely suppressed out-of-plane polarizability under strong electric fields, indicating strong electron confinement and robustness of excitons in 2D materials.

Contribution

The paper provides the first experimental measurement showing suppressed out-of-plane polarizability of free excitons in monolayer WSe2, contrasting with previous reports on other 2D materials.

Findings

Out-of-plane polarizability is less than 10^{-11} Dm/V

Stark shift remains below 0.4 meV at high electric fields

Free excitons are highly robust against surface potential fluctuations

Abstract

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