We report charged exciton (trion) formation dynamics in doped monolayer transition metal dichalcogenides, specifically molybdenum diselenide (MoSe 2 ), using resonant two-color pump-probe spectroscopy. When resonantly pumping the exciton transition, trions are generated on a picosecond timescale through exciton-electron interaction. As the pump energy is tuned from the high energy to low energy side of the inhomogeneously broadened exciton resonance, the trion formation time increases by ~ 50%. This feature can be explained by the existence of both localized and delocalized excitons in a disordered potential and suggests the existence of an exciton mobility edge in transition metal dichalcogenides.

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.Keywords: atomically thin semiconductors, valley, trions, ultrafast dynamics PACS:The valley degree of freedom (DoF) indexes the crystal momentum of a local energy minimum within the electronic band structure, and has been proposed as an alternative information carrier, analogous to charge and spin [1]. In atomically thin transition metal dichalcogenides (TMDs), fundamental optical excitations, excitons (electron-hole pairs) and trions (charged excitons), are formed at the hexagonal Brillouin zone boundaries at the ( ′) points. As such, they inherit the valley index which is locked with electron spins in TMDs. Thus, exciton and trion resonances allow optical access and manipulation of the valley DoF in TMDs using circularly polarized light [2][3][4][5][6]. The exceptionally large binding energies of these quasiparticles (i.e. 200-500 meV for excitons and an additional binding energy of 20-40 meV for trions) further promise room temperature valleytronic applications [2,3,[7][8][9][10][11][12][13].High efficiency valley initialization and a long lifetime of valley polarization are preferred in valleytronic applications [14][15][16][17]. Initial experiments based on steadystate photoluminescence have shown the possibility of creating a near-unity valley polarization in MoS2 and WSe2 via exciton resonances [4,18]. Time-resolved measurements soon revealed that exciton valley polarization is quickly lost (~ 1 ps) due to intrinsic electron-hole exchange interaction [19]. The large initial exciton valley polarization observed in the steady-state PL results from the competition between the valley depolarization time (~ 1 ps) and the exciton population relaxation time (~ 100-200 fs) [13,20,21]. On the other hand, trions offer an interesting alternative route for optical manipulation of the valley index for a number of reasons. First, in contrast to the ultrafast exciton population relaxation time, trions exhibit an extended population relaxation time of tens of picoseconds in monolayer TMDs [22][23][24][25][26][27][28][29][30]. Secondly, trions as charged quasiparticles influence both transport and optical...

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