Percentage change of the real (a–e) and imaginary (f–j) part of the refractive index as a function of the delay Δτ between the ultraviolet pump and the infrared probe pulses. Multiple vertical plots are shown, for different delay Δt between the ultraviolet and the NIR pump pulses. Shaded areas indicate whether the ultraviolet pump, which is used as time reference, precedes (light blue) or follows (light red) the NIR pump. Overlapping the results obtained by simultaneous two-colour pumping (red curves), we plot the computed change in refractive index calculated by the algebraic summation of the results obtained from experiments with separate ultraviolet and NIR pump pulses (black dashed curves). The probe, the NIR and the ultraviolet pump wavelengths were set to 1,300 nm, 787 nm and 262 nm, respectively. The probe intensity was low: Ip≃200 MW cm−2, while the pump fluences were Fultraviolet=5 mJ cm−2 and FNIR=14 mJ cm−2.