Long-Lived Valley Polarization of Intra-Valley Trions in Monolayer WSe2

TL;DR

This study reveals that intra-valley trions in monolayer WSe2 exhibit exceptionally long-lived valley polarization, which is crucial for developing future valleytronic devices, by using polarization-resolved pump-probe spectroscopy.

Contribution

It demonstrates the long-lived valley polarization of intra-valley trions and the conditions necessary for its observation in monolayer WSe2.

Findings

Intra-valley trions show near-unity valley polarization with no decay over 25 ps.

Inter-valley trions exhibit a rapid decay of valley polarization (~4 ps).

Resonant excitation is essential for observing long-lived intra-valley trion polarization.

Abstract

We investigate valley dynamics associated with trions in monolayer tungsten diselenide (WSe2) using polarization resolved two-color pump-probe spectroscopy. When tuning the pump and probe energy across the trion resonance, distinct trion valley polarization dynamics are observed as a function of energy and attributed to the intra-valley and inter-valley trions in monolayer WSe2. We observe no decay of a near-unity valley polarization associated with the intra-valley trions during ~ 25 ps, while the valley polarization of the inter-valley trions exhibits a fast decay of ~ 4 ps. Furthermore, we show that resonant excitation is a prerequisite for observing the long-lived valley polarization associated with the intra-valley trion. The exceptionally robust valley polarization associated with resonantly created intra-valley trions discovered here may be explored for future valleytronic applications such as valley Hall effects.