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arXiv · 1806.11318

Direct observation of ultrafast exciton formation in monolayer WSe$_2$

Philipp Steinleitner·Philipp Merkl·Philipp Nagler·Joshua Mornhinweg·Christian Schüller·Tobias Korn·Alexey Chernikov·Rupert Huber
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Abstract

Many of the fundamental optical and electronic properties of atomically thin transition metal dichalcogenides are dominated by strong Coulomb interactions between electrons and holes, forming tightly bound atom-like excitons. Here, we directly trace the ultrafast formation of excitons by monitoring the absolute densities of bound and unbound electron-hole pairs in monolayers of WSe$_2$ following femtosecond non-resonant optical excitation. To this end, phase-locked mid-infrared probe pulses and field-sensitive electro-optic sampling are used to map out the full complex-valued optical conductivity of the non-equilibrium system and to discern the hallmark low-energy responses of bound and unbound pairs. While free charge carriers strongly influence the infrared response immediately after above-bandgap injection, up to 60% of the electron-hole pairs are bound as excitons already on a sub-picosecond timescale, evidencing extremely fast and efficient exciton formation. During the subsequent recombination phase, we still find a large density of free carriers in addition to excitons, indicating a non-equilibrium state of the photoexcited electron-hole system.

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Philipp Steinleitner, Philipp Merkl, Philipp Nagler, Joshua Mornhinweg, Christian Schüller, Tobias Korn, Alexey Chernikov, Rupert Huber. 2018-06-29. Direct observation of ultrafast exciton formation in monolayer WSe$_2$. https://arxiv.org/abs/1806.11318

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