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 fie...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 oddorder 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.展开更多
基金supported by the US Department of Energy,Office of Science,Basic Energy Sciences,Chemical Sciences,Geosciences,and Biosciences Division through the AMOS program.F.L.was supported by a Terman Fellowship and startup funds from the Department of Chemistry at Stanford University.Y.K.acknowledges support from the UrbanekChodorow Fellowship from Stanford University.C.H.acknowledges support from the W.M.Keck Foundation and a Humboldt Research Fellowship.
文摘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 oddorder 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.
