Radially polarized light beams from spin-forbidden dark excitons and trions in monolayer WSe₂

Abstract

The rich optical properties of transition metal dichalcogenide monolayers (TMD-MLs) render these materials promising candidates for the design of new optoelectronic devices. Despite the large number of excitonic complexes in TMD-MLs, the main focus has been put on optically bright neutral excitons. Spin-forbidden dark excitonic complexes have been addressed for basic science purposes, but not for applications. We report on spin-forbidden dark excitonic complexes in ML WSe 2 as an ideal system for the facile generation of radially polarized light beams. Furthermore, the spatially resolved polarization of photoluminescence beams can be exploited for basic research on excitons in two-dimensional materials.

Interplay of excitonic complexes in p -doped WSe2 monolayers

“…In the holedoped regime (V g < 0.7 V), the spectra contain emission from the positive bright trion (X + , 1.698 eV), the positive dark trion (X + d , 1.663 eV) and a broad band at low energy (1.640 eV). The radially polarized signal beams from the dark excitonic states clearly irrefutably confirms their nature 51 . Note that the low intensity of the dark states' photoluminescence cannot be used as a measure of their population, due to the different radiation patterns of dark and bright excitonic complexes 29 .…”

Unveiling Valley Lifetimes of Free Charge Carriers in Monolayer WSe2

“…Although this process is spin-forbidden in case of dipole transitions, in-plane symmetry nevertheless dictates that it is possible via the coupling to photons with in-plane propagation and out-of-plane linear polarization, 36 which is demonstrated in several PL studies. 35,36,[44][45][46][47][48][49] This kind of spin-flip recombination is reducing the number of carriers in the K-valley (compare Figs. 2c and 2d), whereas the electron number in the K'valley is increased by the leftover photo-exited electron, creating a net valley polarization of conduction band electrons (see Fig.…”

Valley lifetimes of conduction band electrons in monolayer WSe2

“…Time-resolved Kerr rotation traces has been recorded for photon energies resonantly exciting trion arXiv:2007.14712v1 [cond-mat.mes-hall] 29 Jul 2020 states. 7,8 In Figs. 2b,c, we show the gate-dependent Kerr rotation amplitudes and the respective lifetimes at 40 K. When entering the conduction band at around +10 V gate voltage there is a linear increase in the Kerr rotation amplitude.…”