DNA sample spectra: The sample spectra were smoothed by averaging over the nearest neighbours. Linear interpolation was applied along the 𝜔 𝛽 -𝜔 𝛼 axis to match the glass spectrum. A background subtraction was applied to the spectra by subtracting the mean intensity in the region 𝜔 𝛼 = 1325-1375 cm -1 , 𝜔 𝛽 -𝜔 𝛼 = 1190-1210 cm -1 . The spectra were then background subtracted and normalised on the glass spectrum. As the FWM signal is proportional to the square of the analyte, the intensity of the spectra was square-rooted. The spectra were also normalised at the highest intensity peak at 𝜔 𝛼 = 1525 cm -1 , 𝜔 𝛽 -𝜔 𝛼 = 1450 cm -1 . Each sample spectrum is the average of three measurements.