Anomalous generation of high-order harmonics in triangular molybdenum disulfide quantum dots

High-order harmonic generation (HHG) in quantum dots has attracted significant attention due to its potential applications in ultrafast optics. In this study, we investigated the HHG from equilateral triangular molybdenum disulfide (MoS₂) quantum dots in linearly polarized laser fields using time-dependent density-functional theory (TDDFT). In the direction parallel to the polarization of the linear driving pulse, pure odd-order harmonics were generated in the low-energy section, and pure even-order harmonics were generated in the high-energy section. Interestingly, in the direction perpendicular to the polarization of the linear driving pulse, the generation pattern of the high-order harmonics was exactly the opposite. The mechanisms for generating the low-energy and high-energy sections of high-order harmonics were different. With the action of the laser field, when the ionized electron recombined to the highest occupied molecular orbital (HOMO), the low-energy section of the high-order harmonics was generated. The high-energy section of HHG was mainly caused by the following factors. Because of the ionization of electrons, the potential field of the quantum dot changed. This resulted in a phase difference of π between the dipole acceleration inside the quantum dot and the dipole acceleration in the entire space. In the high-energy region, this led to the generation of pure even harmonics along the laser polarization direction and pure odd harmonics perpendicular to the laser polarization direction. These findings provide new insights into the HHG mechanisms in transition metal dichalcogenide quantum dots.