Interesting. Sparked by your comment, I found this.
What’s the maximun frequency of light?
Approximately 2 x 10^43 Hz is the “Planck frequency” (the inverse of the Planck time). At that frequency, individual photons (or any other particle with this much energy) would be black holes (their Compton wavelength would be smaller than their Schwartzchild radius), and so until someone comes up with a theory of light which includes virtual black-holes, photons with a frequency above this cannot be considered sensible.