Abstract
Single layers of MoS2 and MoSe2 were optically pumped with circularly polarized light and an appreciable polarization was initialized as the pump energy was varied. The circular polarization of the emitted photoluminescence was monitored as a function of the difference between the excitation energy and the A-exciton emission at the K-point of the Brillouin zone. Our results show a threshold of twice the LA phonon energy, specific to the material, above which phonon-assisted intervalley scattering causes depolarization. In both materials this leads to almost complete depolarization within ~100 meV above the threshold energy. We identify the extra kinetic energy of the exciton (independent of whether it is neutral or charged) as the key parameter for presenting a unifying picture of the depolarization process.
Highlights
(c) Schematic of the single-particle band structure at K and K’ valleys
longitudinal acoustic (LA) phonons in these two systems have different energies (30 meV for MoS2 and 19 meV for MoSe2)[24,25], and we show that the threshold for the excess energy required to initiate the depolarization process clearly reflects the material specific phonon energy
We find that the key parameter for the depolarization process is the extra kinetic energy of the exciton – depolarization is due to intervalley scattering that begins to occur when the exciton energy exceeds a threshold corresponding to twice the LA phonon energy
Summary
(c) Schematic of the single-particle band structure at K and K’ valleys. The arrows denote the A (solid line) and B (dashed line) excitons. We probe the valley population dynamics in MoSe2 and MoS2 by selectively populating the K and K’ valleys with circularly polarized light while systematically varying the laser excitation energy. For both systems, the difference in the excitation energy and PL emission energy, Δ E = Epump − EPL, governs the depopulation of carriers in each valley. LA phonons in these two systems have different energies (30 meV for MoS2 and 19 meV for MoSe2)[24,25], and we show that the threshold for the excess energy required to initiate the depolarization process clearly reflects the material specific phonon energy. We find that the key parameter for the depolarization process is the extra kinetic energy of the exciton – depolarization is due to intervalley scattering that begins to occur when the exciton energy exceeds a threshold corresponding to twice the LA phonon energy
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