I never knew what the technology behind it was. In the show, the photon torpedo just looked like a big ball of light. Here's a description from the linked article.

From a technology standpoint, the Photon Torpedo is a (relatively) straightforward device. Take a missile, then drop in some antimatter—boom, photon torpedo.

I'm sorry I didn't get an upvote in before the post paid out. It's been a busy week and I fell behind on curation.