This article has a key explanation:
“ The laser diverts lightning bolts by creating an easier path for the electrical discharge to flow down. When laser pulses are fired into the sky, a change in the refractive index of the air makes them shrink and become so intense that they ionise air molecules around them. This leads to a long chain of what the researchers call filaments in the sky, where air molecules rapidly heat up and race away at supersonic speeds, leaving a channel of low density, ionised air. These channels, which last for milliseconds, are more electrically conductive than the surrounding air, and so form an easier path for the lightning to follow.”
https://amp.theguardian.com/science/2023/jan/16/scientists-s...
