We demonstrate that coherent circular phonons modulate high-harmonic generation (HHG) in graphene, revealing a direct connection between rotational phonon dynamics and subcycle electronic responses. While monolayer graphene yields only odd-order harmonics due to inversion symmetry, the excitation of circular phonons induces elliptically polarized sidebands, spectrally offset by the phonon frequency. These sidebands encode both the phonon energy and the relative phase between degenerate in-plane phonon modes, whose coherent superposition gives rise to rotational lattice motion. Time-domain analysis of the HHG signal enables precise extraction of this phase. Leveraging chiral probe pulses further reveals the handedness of the phonons, with distinct sideband polarization features emerging from the symmetry interplay between the phonon mode and the probe field. These findings establish HHG as a powerful spectroscopic probe for exploring phase, handedness, and dynamical symmetries of coherently excited circular phonon modes, offering an approach to understanding temporally evolving quantum materials with subcycle temporal resolution.

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• High-order harmonic generation
• Lattice dynamics
• Phonons
• Chiral symmetry