We demonstrate a new photo-induced oxidation technique for tuning GaAs photonic crystal cavities using a $390~mathrm{nm}$ pulsed laser with an average power of $10~mathrm{mu W}$. The laser oxidizes a small $left(sim 500~mathrm{nm}right)$ diameter spot, reducing the local index of refraction and blueshifting the cavity. The tuning progress can be actively monitored in real time. We also demonstrate tuning an individual cavity within a pair of proximity-coupled cavities, showing that this method can be used to correct undesired frequency shifts caused by fabrication imperfections in cavity arrays.
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