The ultrafast onset of exciton formation in 2D semiconductors

Abstract

The equilibrium and non-equilibrium optical properties of single-layer transition metal dichalcogenides (TMDs) are determined by strongly bound excitons. Exciton relaxation dynamics in TMDs have been extensively studied by time-domain optical spectroscopies. However, the formation dynamics of excitons following non-resonant photoexcitation of free electron-hole pairs have been challenging to directly probe because of their inherently fast timescales. Here we use extremely short optical pulses to non-resonantly excite an electron-hole plasma and show the formation of two-dimensional excitons in single-layer MoS2 on the timescale of 30 fs via the induced changes to photo-absorption. These formation dynamics are significantly faster than in conventional 2D quantum wells and are attributed to the intense Coulombic interactions present in 2D TMDs. A theoretical model of a coherent polarization that dephases and relaxes to an incoherent exciton population reproduces the experimental dynamics on the sub-100-fs timescale and sheds light into the underlying mechanism of how the lowest-energy excitons, which are the most important for optoelectronic applications, form from higher-energy excitations. Importantly, a phonon-mediated exciton cascade from higher energy states to the ground excitonic state is found to be the rate-limiting process. These results set an ultimate timescale of the exciton formation in TMDs and elucidate the exceptionally fast physical mechanism behind this process.

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Cited by in corpus (35)

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• Efficient phonon cascades in hot photoluminescence of WSe${}_2$ monolayers
• Ultrafast dynamics of bright and dark excitons in monolayer WSe${}_2$ and heterobilayer WSe${}_2$/MoS${}_2$
• Charge transfer dynamics in MoSe${}_2$/hBN/WSe${}_2$ heterostructures
• Time-dependent screening explains the ultrafast excitonic signal rise in 2D semiconductors
• Exciton-phonon interaction calls for a revision of the "exciton" concept
• Hot carrier extraction from 2D semiconductor photoelectrodes
• Excitons and carriers in transient absorption and time-resolved ARPES spectroscopy: an abinitio approach
• Sub-ps thermionic electron injection effects on exciton-formation dynamics at a van der Waals semiconductor/metal interface
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• Ultrafast electron localization and screening in a transition metal dichalcogenide
• Excitonic response in TMD heterostructures from first-principles: impact of stacking, twisting, and interlayer distance
• Strain-induced activation of chiral-phonon emission in monolayer WS${}_2$
• High-harmonic spectroscopy of strongly bound excitons in solids
• Spatiotemporal dynamics of moiré excitons in van der Waals heterostructures
• Shaping Exciton Polarization Dynamics in 2D Semiconductors by Tailored Ultrafast Pulses
• Time-domain study of coupled collective excitations in quantum materials
• Dynamic screening of quasiparticles in WS${}_2$ monolayers
• Intertube excitonic coupling in nanotube van der Waals heterostructures
• Non-Hermitian theory of valley excitons in two-dimensional semiconductors
• FEL stochastic spectroscopy revealing silicon bond softening dynamics
• Direct observation of ultrafast defect-bound and free exciton dynamics in defect-engineered WS${}_2$ monolayers
• Carrier-envelope phase control of ultrafast photocurrents in layered MoS${}_2$
• Properties and challenges of hot-phonon physics in metals: MgB${}_2$ and other compounds
• Ultrafast Dynamics of Spin-Orbit Entangled Excitons Coupled to Magnetic Ordering in van der Waals Antiferromagnet NiPS3
• Non-adiabatic suppression of 3D excitonic screening in black phosphorus by mid-infrared pulses
• Tracking the photoinduced dynamics of a dark excitonic state in single-layer WS${}_2$ via resonant Autler-Townes splitting
• Increased formation of trions and charged biexcitons by above-gap excitation in single-layer $WS{e}_2$
• Excitonic effects in the photocarriers dynamics of two-dimensional materials
• Ultrafast Charge Transfer Dynamics at the MoS${}_2$/Au Interface Observed via Optical Spectroscopy under Ambient Conditions
• Two-color Kerr microscopy of two-dimensional materials with sub-picosecond time resolution
• Exciton self-trapping causes picoseconds recombination in metal-organic chalcogenides hybrid quantum wells