Helicity-engineered nonlinear optical responses in photo-excited topological semimetals
Topological materials provide a transformative arena for light-driven control of electronic motion; yet, direct manipulation of electron dynamics on sub-cycle timescale remains a significant challenge. We demonstrate that the strong-field-driven high-harmonic generation of a Weyl semimetal can be controlled and manipulated through bicircular pump-probe driving fields. While a lone probe pulse generates exclusively odd-order harmonics, the addition of a pump pulse triggers a series of sidebands arising from nonlinear frequency mixing of pump and probe photons. Our results reveal that the sideband intensities are highly sensitive to the relative helicity of the pulses and the orientation of the polarization plane. This sensitivity stems from the chiral nature of the Weyl nodes, which couples efficiently to the light's helicity only when the node-separation axis is perpendicular to the polarization plane. Furthermore, the significant suppression of sideband intensity with increasing pump-probe delay identifies these features as a potential clock for electron-hole decoherence. These findings establish frequency-mixed high-harmonic generation as a sensitive probe of chiral quantum dynamics and suggest a robust framework for manipulating topological currents via structured light, with implications for lightwave electronics and ultrafast quantum information processing.