Glass: A spectrum of the glass coverslip measured at time delays of 𝑇 𝛼𝛽 = 𝑇 𝛽𝛾 = 0 was used to normalise the sample spectra, to account for varying OPA output energies across the wavelength ranges. The glass spectrum was smoothed by averaging over the nearest neighbours. Interference fringes were observed in the 𝜔 𝛽 -𝜔 𝛼 axis of the spectrum. To remove the fringes, linear interpolation was applied along the 𝜔 𝛽 -𝜔 𝛼 axis, maintaining the same number of data points. A low-pass brick wall filter was applied to the Fourier transform of the subsequent glass spectrum, using a cut-off frequency of 0.05. Line cuts of the glass spectrum after each step are presented in Figure in the Supporting Information.