Abstract
The band-edge optical response of transition metal dichalcogenides, an emerging class of atomically thin semiconductors, is dominated by tightly bound excitons localized at the corners of the Brillouin zone (valley excitons). A fundamental yet unknown property of valley excitons in these materials is the intrinsic homogeneous linewidth, which reflects irreversible quantum dissipation arising from system (exciton) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Here we use optical two-dimensional Fourier transform spectroscopy to measure the exciton homogeneous linewidth in monolayer tungsten diselenide (WSe2). The homogeneous linewidth is found to be nearly two orders of magnitude narrower than the inhomogeneous width at low temperatures. We evaluate quantitatively the role of exciton–exciton and exciton–phonon interactions and population relaxation as linewidth broadening mechanisms. The key insights reported here—strong many-body effects and intrinsically rapid radiative recombination—are expected to be ubiquitous in atomically thin semiconductors.
Highlights
The band-edge optical response of transition metal dichalcogenides, an emerging class of atomically thin semiconductors, is dominated by tightly bound excitons localized at the corners of the Brillouin zone
While bulk transition metal dichalcogenides (TMDs) have been investigated over a few decades, recent advances in isolation of atomically thin layers have opened a new regime of semiconductor physics at the ultimate two-dimensional (2D) limit[1,2,3]
Excitons in monolayer TMDs exhibit robust electronic and valley coherence[8,9] as well as coupled spin and valley pseudospin degrees of freedom[1,2] arising from strong spinorbit coupling and time-reversal symmetry[9]. While these seminal experiments have demonstrated exciting new properties of TMDs and their potential for novel optoelectronic devices, much remains to be learned about the unique exciton physics in these materials
Summary
The band-edge optical response of transition metal dichalcogenides, an emerging class of atomically thin semiconductors, is dominated by tightly bound excitons localized at the corners of the Brillouin zone (valley excitons). Excitons in monolayer TMDs exhibit robust electronic and valley coherence[8,9] as well as coupled spin and valley pseudospin degrees of freedom[1,2] arising from strong spinorbit coupling and time-reversal symmetry[9] While these seminal experiments have demonstrated exciting new properties of TMDs and their potential for novel optoelectronic devices, much remains to be learned about the unique exciton physics in these materials. Microscopic calculations predict a similar exciton radiative lifetime for a perfect monolayer WSe2 crystal, suggesting that all decoherence mechanisms compete on a sub-picosecond timescale in samples with minimal defects Such dephasing and relaxation dynamics deviate drastically from those found in conventional semiconductors[11], where radiative recombination is considerably slower than pure dephasing processes. Characterization of the exciton linewidth and dephasing mechanisms would facilitate optimal cavity designs for optical mode matching and efficient light emission
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