Ultrafast Science, Volume. 1, Issue 1, 9820716(2021)
Polarization Flipping of Even-Order Harmonics in Monolayer Transition-Metal Dichalcogenides
We present a systematic study of the crystal-orientation dependence of high-harmonic generation in monolayer transition-metal dichalcogenides, WS2 and MoSe2, subjected to intense linearly polarized midinfrared laser fields. The measured spectra consist of both odd- and even-order harmonics, with a high-energy cutoff extending beyond the 15th order for a laser-field strength around ~1 V/nm. In WS2, we find that the polarization direction of the odd-order harmonics smoothly follows that of the laser field irrespective of the crystal orientation, whereas the direction of the even-order harmonics is fixed by the crystal mirror planes. Furthermore, the polarization of the even-order harmonics shows a flip in the course of crystal rotation when the laser field lies between two of the crystal mirror planes. By numerically solving the semiconductor Bloch equations for a gapped-graphene model, we qualitatively reproduce these experimental features and find the polarization flipping to be associated with a significant contribution from interband polarization. In contrast, high-harmonic signals from MoSe2 exhibit deviations from the laser-field following of odd-order harmonics and crystal-mirror-plane following of even-order harmonics. We attribute these differences to the competing roles of the intraband and interband contributions, including the deflection of the electron-hole trajectories by nonparabolic crystal bands.
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Yuki Kobayashi, Christian Heide, Hamed Koochaki Kelardeh, Amalya Johnson, Fang Liu, Tony F. Heinz, David A. Reis, Shambhu Ghimire. Polarization Flipping of Even-Order Harmonics in Monolayer Transition-Metal Dichalcogenides[J]. Ultrafast Science, 2021, 1(1): 9820716
Category: Research Articles
Received: Apr. 8, 2021
Accepted: Jun. 25, 2021
Published Online: Sep. 15, 2023
The Author Email: Ghimire Shambhu (shambhu@stanford.edu)