Trion valley coherence in monolayer semiconductors

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 MoSe2 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.

Disorder-dependent valley properties in monolayer WSe2

“…The VC drops to zero at the lower-energy edge of the exciton resonance which spectrally overlaps with the trion resonance. Trions cannot exhibit VC in PL spectra due to photon-spin entanglement [21,27]. Interestingly, we observe a monotonic increase of the VC across the inhomogeneously broadened exciton resonance with ρ VC varying from 0 to 0.4 as shown in Fig.…”

Interlayer charge transport controlled by exciton–trion coherent coupling

“…First, trions carry a net charge, allowing us to manipulate and detect trions through electrical probing. Second, the radiative lifetime of trion has been reported in a broad range from few ps to tens of ps [11,12,13,14], but nonetheless, longer than that of exciton [11,15,16,17], providing us time for electrical manipulation before it radiatively recombines in a spontaneous manner. However, many of the perceived applications of trions have not yet been demonstrated experimentally.…”

Gate-tunable trion switch for excitonic device applications

“…Each of the trion consists of a bright exciton in one valley electrostatically bound with an electron in the lower spin-split conduction band from the opposite valley. Since inter-layer transfer is ultra-fast (∼sub-ps [12,[27][28][29]), which is faster than trion radiative decay (∼tens of ps [?, 2,9,10,19,[30][31][32]), the whole negatively charged trion can be transferred to the bottom monolayer graphene driven by the built-in electric field, as illustrated in Figure 2d. To account for charge neutrality, the top layer graphene injects an electron to the WS 2 , completing the circuit.…”

“…The monolayer semiconducting transition metal dichalcogenides (MoS 2 , WS 2 , MoSe 2 , and WSe 2 ) exhibit strongly bound two-dimensional excitons with a binding energy on the order of few hundreds of meV, making these ultra-thin monolayers an excellent test bed for excitonic manipulation even at room temperature [1][2][3][4][5]. The neutral excitons (X 0 ) show excellent valley polarization and valley coherence properties that can be readily probed through initialization by circularly and linearly polarized photons, respectively, followed by detection through a circular or linear analyzer [6][7][8][9][10][11]. However, controlling these excitonic states electrically remains a challenge due to the charge neutral nature of these excitons.…”

“…Trions can be valley polarized when initiated through a circularly polarized light [7]. Trion can also carry valley coherence information when resonantly initialized through linearly polarized light [9].Thus trions are excellent candidates for gate controlled excitonic device applications and also for transferring the valley information into electrical domain. In this work, we demonstrate a fast, gate-and light-tunable vertical trion switch where the trion is coherently initialized through resonant excitation followed by ultra-fast inter-layer transfer and electrical detectionthus generating a gate controlled photocurrent governed by inter-layer trion transport.…”