Methods and apparatus are provided for igniting, modulating, and sustaining a plasma for various doping processes. In one embodiment, a substrate (250) can be doped by forming a plasma (610) in a cavity (285) by subjecting a gas to an amount of electromagnetic radiation in the presence of a plasma catalyst (240) and adding at least one dopant material to the plasma. The material is then allowed to penetrate into the substrate. Various active and passive catalysts are provided.