Fig. 3: Residual map showing the oscillatory modulation of the A_2D signal averaged along the exciton region.

a, b For T < 2 ps (a), the residuals show fast, 107 fs (310 cm⁻¹) oscillations and (b) a dispersive lineshape centered around the maximum of the 2DES cross-section (cf. Figure 1d) at E₀ + δ (dashed line in b). c, d For T > 2 ps (c), oscillations at ~30 cm⁻¹ and 50 cm⁻¹ (FT map in the inset. See also Figs. S5, 6) with (d) an absorptive lineshape centered around E₀ (dashed line in d) are observed. This lineshape matches the predictions of a displaced harmonic oscillator model, showing that the oscillations reflect low-frequency coherent phonon wavepacket motion. The fast oscillations in (a) go beyond the displaced harmonic oscillator model and are assigned to intra-exciton Rabi oscillations.