Trion Valley Coherence in Monolayer Semiconductors

TL;DR

This paper demonstrates the generation and detection of trion valley coherence in monolayer MoSe2 using ultrafast spectroscopy, revealing its persistence for hundreds of femtoseconds and highlighting different limiting mechanisms from exciton coherence.

Contribution

It introduces the first experimental observation of trion valley coherence in monolayer TMDs and analyzes its fundamental decoherence mechanisms.

Findings

Trion valley coherence persists for a few hundred femtoseconds.

Mechanisms limiting trion coherence differ from those of excitons.

Strategies are proposed to extend valley coherence times.

Abstract

The emerging field of valleytronics aims to exploit the valley pseudospin of electrons residing near Bloch band extrema as an information carrier. Recent experiments demonstrating optical generation and manipulation of exciton valley coherence (the superposition of electron-hole pairs at opposite valleys) in monolayer transition metal dichalcogenides (TMDs) provide a critical step towards control of this quantum degree of freedom. The charged exciton (trion) in TMDs is an intriguing alternative to the neutral exciton for control of valley pseudospin because of its long spontaneous recombination lifetime, its robust valley polarization, and its coupling to residual electronic spin. Trion valley coherence has however been unexplored due to experimental challenges in accessing it spectroscopically. In this work, we employ ultrafast two-dimensional coherent spectroscopy to resonantly generate and detect trion valley coherence in monolayer MoSe$_2$ demonstrating that it persists for a few-hundred femtoseconds. We conclude that the underlying mechanisms limiting trion valley coherence are fundamentally different from those applicable to exciton valley coherence. Based on these observations, we suggest possible strategies for extending valley coherence times in two-dimensional materials.