Direct observation of internal quantum transitions and femtosecond radiative decay of excitons in monolayer WSe_2

TL;DR

This study provides the first direct experimental observation of internal excitonic transitions and femtosecond radiative decay in monolayer WSe2, revealing detailed exciton dynamics crucial for optoelectronic applications.

Contribution

It introduces a novel phase-locked mid-infrared spectroscopy method to directly probe internal excitonic states in monolayer WSe2, advancing understanding of exciton structure and decay processes.

Findings

Observation of 1s-2p excitonic resonance

Record-fast radiative decay within 150 fs

Differentiation of radiative and Auger recombination processes

Abstract

Atomically thin two-dimensional crystals have revolutionized materials science. In particular, monolayer transition metal dichalcogenides promise novel optoelectronic applications, due to their direct energy gaps in the optical range. Their electronic and optical properties, however, are complicated by exotic room-temperature excitons, whose fundamental structure and dynamics has been under intense investigation. While interband spectroscopy probes energies of excitons with vanishing centre-of-mass momenta, the majority of excitons has remained elusive, raising questions about their unusual internal structure, symmetry, many-body effects, and dynamics. Here we report the first direct experimental access to all relevant excitons in single-layer WSe2. Phase-locked mid-infrared pulses reveal the internal orbital 1s-2p resonance, which is highly sensitive to the shape of the excitonic envelope functions and provides accurate transition energies, oscillator strengths, densities and linewidths. Remarkably, the observed decay dynamics indicates a record fast radiative annihilation of small-momentum excitons within 150 fs, whereas Auger recombination prevails for optically dark states. The results provide a comprehensive view of excitons and introduce a new degree of freedom for quantum control, optoelectronics and valleytronics of dichalcogenide monolayers.