Machining Inside Bulk Material
As we all know, some materials are transparent at some wavelengths of light. Glass, for example, is transparent for all wavelengths of light in the visible. That is, it does not absorb visible light - provided the intensity stays below the threshold for multiphoton absorption. We can exceed that threshold by focusing ultrafast laser pulses to a spot inside the materials.
When the intensity exceeds the threshold for plasma formation, very localized absorption does occur at the focal point spot. Once again, this plasma expands. But this time it is confined by the surrounding material. The effect of the expansion is to create a void within a very dense shell of material - a pit within the glass itself. This process is not limited to glass. Pits can be created in any material by focusing an ultrafast laser pulse inside the material, whether it is amorphous or crystalline.