A photonic or photon warhead was the a type of warhead technology. They were the payload component of photon and photonic torpedoes.
The Vissian torpedoes armed with photonic warheads were charged with sarium microcells aboard their starships. Prior to the encounter with the Vissians, Malcolm Reed was not familiar with "photonic weapons." According to Veylo, a Vissian tactical officer, the spatial torpedoes utilized by Starfleet in 2153 were "antiquated." (ENT: "Cogenitor")
A strategem using a photon warhead, deployed by the USS Shenzhou, played a decisive role in the Battle of the Binary Stars, in 2256. (DIS: "Battle at the Binary Stars")
In 2370, a number of photon warhead were stolen from a weapons locker on Deep Space 9. (DS9: "Tribunal")
In 2376, Tuvok was said to be able to dismantle a photonic warhead in less than thirty seconds. (VOY: "Riddles")