Photons were particles of light. They could be produced by the recombination of an electron and a positron. (TV: Time and the Rani)
Weapons based on these particles included photon beams (TV: The Invasion of Time et. al.), photonic bombs (COMIC: Pureblood) and photonic missiles (TV: Horror of Fang Rock et. al.).
Artemis the Chronovore could pause time enough that the individual photons of sunlight could be observed. (PROSE: No Future)
The Time-Space Visualiser allowed one to seen any event in "past" history by examining photons, since light "recorded" an image and thus could be displayed. (TV: The Chase)
The photon accelerator coils were a TARDIS component. (PROSE: The Eye of the Giant)