Exploring band structure in MoS₂ with high harmonic generation spectroscopy

Solid-state High-harmonic generation (sHHG) from intense laser pulses is being developed as a probe of ultrafast electronic processes and crystalline symmetries in condensed matter [1]. Atomically thin semiconductors such as monolayer MoS₂ provide a model system in which to study band structure without the interference of light propagation effects through a bulk material [2]. In addition, few-layer transition metal dichalcogenides (TMDCs) offer a tunable material platform with unique properties to interrogate with solid-state HHG and diverse applications in optoelectronics and spintronics [3]. Here, we investigate the polarization properties of sHHG from monolayer MoS₂ as a function of crystal orientation relative to the mid-IR laser field polarization [4]. We experimentally observe that crystal symmetries impose constraints on harmonic emissions, leading to pronounced nodes in the HHG spectra at certain angles. Building on previous studies,[5] we find an angular shift such that for several mid-IR wavelengths, the parallel-polarized odd-order harmonics below (above) 3.5 eV are enhanced for driver polarization along the armchair (zigzag) direction. By solving the time-dependent density matrix equations involving a valence band and two conduction bands, we trace these angular shifts to electron-hole recombinations from different conduction bands, mapping lower energy harmonics to recombinations involving the first conduction band and higher energy harmonics to recombinations involving the second conduction band. This measurement effectively probes the vectorial nature of recombination dipoles from different bands in MoS₂, building up from the single conduction band picture of solid-state HHG that has been considered previously. This material specific analysis paves the way for solid-state HHG as a detailed probe of ultrafast dynamics in condensed matter systems.

References: [1] S Ghimire and D A Reis, Nat. Phys. 90, 10-16 (2019). [2] S Manzeli, et. al., Nat. Rev. Mater. 2, 17033 (2017). [3] I Kilen, et. al., Phys. Rev. Lett. 125, 083901 (2021). [4] L Yue, et. al., Phys. Rev. Lett. 129, 147401 (2022). [5] H Liu, et. al., Nat. Phys. 13, 262-265 (2017).