Observation of ~100% valley-coherent excitons in monolayer MoS2 through giant enhancement of valley coherence time

Abstract

In monolayer transition metal dichalcogenide semiconductors, valley coherence degrades rapidly due to a combination of fast scattering and inter-valley exchange interaction. This leads to a sub-picosecond valley coherence time, making coherent manipulation of exciton a highly challenging task. Using monolayer MoS2 sandwiched between top and bottom graphene, here we demonstrate fully valley coherent excitons by observing ~100% degree of linear polarization in steady state photoluminescence. This is achieved in this unique design through a combined effect of (a) suppression in exchange interaction due to enhanced dielectric screening, (b) reduction in exciton lifetime due to a fast inter-layer transfer to graphene, and (c) operating in the motional narrowing regime. We disentangle the role of the key parameters affecting valley coherence by using a combination of calculation (solutions of Bethe-Salpeter and Maialle-Silva-Sham equations) and a careful choice of design of experiments using four different stacks with systematic variation of screening and exciton lifetime. To the best of our knowledge, this is the first report in which the excitons are found to be valley coherent in the entire lifetime in monolayer semiconductors, allowing optical readout of valley coherence possible.

References in corpus (7)

• Valley polarization in MoS2 monolayers by optical pumping
• Observation of giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor
• Probing Excitonic Dark States in Single-layer Tungsten Disulfide
• Exciton band structure of monolayer MoS2
• Ultrafast Manipulation of Valley Pseudospin
• Superior valley polarization and coherence of 2s excitons in monolayer WSe2
• Fundamental exciton linewidth broadening in monolayer transition metal dichalcogenides

Cited by in corpus (8)

• Photonics in Flatland: Challenges and Opportunities for Nanophotonics with 2D Semiconductors
• Magnetically tunable exciton valley coherence in monolayer WS${}_2$ mediated by the electron-hole exchange and exciton-phonon interactions
• Laser-field detuning assisted optimization of exciton valley dynamics in monolayer WSe${}_2$: Geometric quantum speed limit
• Motion of 2D exciton in momentum space leads to pseudospin distribution narrowing on the Bloch Sphere
• Signatures of valley drift in the diversified band dispersions of bright, gray, and dark excitons in MoS2 monolayers under uni-axial strains
• Polarized and narrow excitonic emission from graphene-capped monolayer WS${}_2$ through resonant phonon relaxation
• Quantum valley pseudospin controlled by strain
• Ellipticity-Controlled Bright-Dark Coherence Transition in Monolayer WSe2