Abstract
We present a systematic study of the crystal-orientation dependence of high-harmonic generation in monolayer transition-metal dichalcogenides, WS 2 and MoSe 2 , 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 WS 2 , 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 MoSe 2 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.
Highlights
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
Our femtosecond midinfrared (MIR) beamline for the solid high-harmonic generation (HHG) experiments is based on a Ti:sapphire system that is followed by optical parametric amplification (OPA) and difference-frequency generation (DFG)
We have provided a comprehensive analysis for the crystalorientation dependence of anisotropic HHG from the two materials of the transition-metal dichalcogenides (TMDCs) family
Summary
Solid-state high-harmonic generation (HHG) has emerged as a novel spectroscopic tool to probe nonperturbative, ultrafast phenomena in a wide range of condensed matter systems [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]. The exciting possibilities include all-optical access to valence charge-density distributions [26], band structure [4, 27], transition dipole moments [28], and Berry curvature [29, 30], as well as to nonequilibrium laser-driven dynamics [31] These applications benefit from a detailed understanding of the microscopic mechanisms of HHG, which remains the subject of active experimental and theoretical investigation [32]. A series of recent experiments reported nonperturbative HHG from monolayer transition-metal dichalcogenides (TMDCs), where the roles of Berry curvature and band nesting were considered [11, 29, 34, 35] These atomically thin semiconductors have a graphenelike hexagonal structure, but with broken inversion symmetry enabling the generation of even-order harmonics
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