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Near-bandgap harmonic generation in solids from multiple scattering revealed by complex quantum trajectories
Phys. Rev. Research 8, 033303 – Published 11 September, 2026
DOI: https://doi.org/10.1103/l939-jglf
Abstract
High-harmonic generation in solids enables probing of strong-field electron dynamics in condensed matter and the development of compact ultrafast light sources. Whereas most studies have focused on above-bandgap high-order harmonics, which are usually interpreted through short electron-hole trajectories that recombine within one optical cycle, the microscopic origin of low-order emission near the bandgap remains poorly understood. Here, we investigate near-bandgap harmonic generation in ZnO by combining semiconductor Bloch equations with a complex-time quantum-trajectory analysis. Our results indicate that these harmonics are not a continuation of the short trajectory. Rather, they originate from multiple-scattering trajectories, in which the electron-hole pair undergoes repeated Bragg scattering at the Brillouin-zone boundary and electron-hole reencounter near the point before the actual recombination occurs. We identify that the imaginary part of the electron-hole displacement provides a practical diagnostic of the residual tunneling memory. Real-space electron-hole overlap alone does not imply coherent recombination unless the full complex displacement vanishes. These results clarify the microscopic picture of near-bandgap harmonic emission in ZnO and offer a framework for interpreting phase-resolved strong-field dynamics in solids.
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