If yes is it a remedial way to do laser irradiation in an oxygen atmosphere in order to prevent the colour change?
It is because you are reducing the Ti4+ to Ti3+ (the latter is typical showing the blue colour). Can you do some post-treatment such a oxygen reannealing ?
Yes, annealing afterwards in oxygen might help. What sort of laser wavelength, energy, and time are you using?
controlled .. the partial pressure of Oxygen while laser irradiation .. may be helpful ...
Have you checked that the thickness of your film did not varied? Remember possible interferometric effects...
At these experimental conditions, there is probably achieve a very intense surface modification. You should try reducing the energy per pulse and use shorter irradiation time, since at a given wavelength, absorption coefficient for TiO2 is quite high, so this is one way to avoid more significant modification. If your aim is to achieve moderate changes in morphology and composition, the best results are achieved with a fluence near the damage threshold for a given material.
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