Abstract
Quantum beats, periodic oscillations arising from coherent superposition states, have enabled exploration of novel coherent phenomena. Originating from strong Coulomb interactions and reduced dielectric screening, two-dimensional transition metal dichalcogenides exhibit strongly bound excitons either in a single structure or hetero-counterpart; however, quantum coherence between excitons is barely known to date. Here we observe exciton quantum beats in atomically thin ReS2 and further modulate the intensity of the quantum beats signal. Surprisingly, linearly polarized excitons behave like a coherently coupled three-level system exhibiting quantum beats, even though they exhibit anisotropic exciton orientations and optical selection rules. Theoretical studies are also provided to clarify that the observed quantum beats originate from pure quantum coherence, not from classical interference. Furthermore, we modulate on/off quantum beats only by laser polarization. This work provides an ideal laboratory toward polarization-controlled exciton quantum beats in two-dimensional materials.
Highlights
By using ultrafast two-pulse pump-probe spectroscopy, we observe quantum beats in a few-layer ReS2 and modulate the intensity of the quantum beats signal
We identify the energies of the anisotropic excitons by using static spectroscopy
Considering that researchers have observed only beatings between excitons and trions in 2D transition metal dichalcogenides (TMDs) so far, our observation is rather surprising because the anisotropic excitons show completely different optical selection rules and orientations
Summary
By using ultrafast two-pulse pump-probe spectroscopy, we observe quantum beats in a few-layer ReS2 and modulate the intensity of the quantum beats signal. Our theoretical investigations prove that our two-pulse pump-probe technique does not allow the interference hindering the observation of quantum beats, unlike the three-pulse four-wave mixing experiments.
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