Photon

Photons were particles of light. 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)

Photons could be produced by the recombination of an electron and a positron. (TV: Time and the Rani)

Artemis the Chronovore could pause time enough that the individual photons of sunlight could be observed. (PROSE: No Future)