Coherent Electronic Coupling in Atomically ThinMoSe2

Abstract

We report the first direct spectroscopic evidence for coherent electronic coupling between excitons and trions in atomically thin transition metal dichalcogenides, specifically monolayer MoSe2. Signatures of coupling appear as isolated cross-peaks in two-color pump-probe spectra, and the lineshape of the peaks reveals that the coupling originates from many-body interactions. Excellent agreement between the experiment and density matrix calculations suggests the formation of a correlated exciton-trion state due to their coupling. PACS numbers: 73.20.Mf,78.47.jg Atomically thin transition metal dichalcogenides (TMDs) have emerged as an interesting class of twodimensional materials due to their unique optical properties. For example, some materials (e.g. MoS 2 ) exhibit a crossover from an indirect-to-direct gap semiconductor as the material thickness is reduced to one atomic layer [1], consequently enhancing the photoluminescence efficiency [2]. Valley-specific optical selection rules [3][4][5] dictated by time reversal and spatial inversion symmetries in monolayer structures have been observed, and such selection rules have been electrically controlled in bilayers [6]. These attributes make TMDs particularly intriguing for tunable and flexible valleytronic and photonic devices [7].A striking feature in the linear optical spectra of atomically thin TMDs is the presence of pronounced exciton resonances (Coulomb-bound electron-hole pairs), even at room temperature. The large binding energy of excitons, estimated to be hundreds of meV [8,9], as well as tightlybound trions (an exciton bound with an extra electron or hole) [10,11] arises from reduced dielectric screening in the thickness direction of the film [12]. It is reasonable to speculate that enhanced Coulomb interactions responsible for the large exciton and trion binding energies should also lead to strong coherent coupling among these quasiparticles. The presence or absence of coupling between excitonic states in conventional semiconductors is known to significantly influence energy transfer [13], photon emission statistics [14], and quantum-logic operations [15] in quantum wells, wires, and dots. It is particularly imperative to understand the properties of these intrinsically many-body states in TMDs as they are inevitably present in all optical devices such as photovoltaics [16,17], detectors [18], and modulators [19,20].The existence and manifestation of electronic coupling is challenging to study experimentally since nonlinear spectroscopy techniques are required. Previous experiments thus far have provided limited information on the effects of phase-space filling, exciton-carrier broadening effects [21,22], inter-excitonic scattering [23], valley relaxation dynamics [24], and biexciton formation [25] in TMDs.In this Letter, we provide clear evidence for coherent exciton-trion interactions in monolayer MoSe 2 using twocolor ultrafast pump-probe spectroscopy. In a high quality sample with spectrally well-resolved exciton and trion resonances, spectrosc...

Intervalley biexcitons and many-body effects in monolayerMoS2

“…Unlike traditional semiconductors, the Coulomb interaction in monolayer TMDs is unusually strong because screening is greatly suppressed and spatial overlap of the interaction is much larger. This enhances the stability of a variety of excitonic quasiparticles with extremely large binding energies, including excitons [9][10][11][12][13][14][15][16][17] , trions [18][19][20] , and exciton-trion complexes 21 .…”

Polarization and time-resolved photoluminescence spectroscopy of excitons in MoSe2 monolayers

“…This material is very promising for valley index manipulation [24,25] with bright, well separated emission lines for neutral (X 0 ) and charged excitons (trions T) [11] shown in Fig. 1, which allows to investigate the valley physics for these complexes individually at low temperature.…”

Emergence of electron coherence and two-color all-optical switching in MoS 2 based on spatial self-phase modulation

“…Among them molybdenum disulfide (MoS 2 ) and related layered quantum materials are particularly interesting due to their novel optical properties and potential valleytronics applications (4,5,8,9) at a thickness of monolayer and few layers (2,10,13). Layered materials share common physical properties rooted in their ubiquitous 2D quantum nature, for which achieving pure coherence among electrons (lattices) is of particular interest (14)(15)(16)(17)(18)(19). The presence of multiple domains is ubiquitous in many known 2D quantum materials, ranging from stripe-order cuprate superconductors to polycrystalline strongly correlated systems.…”