We demonstrate extreme laser-induced modification confinement in a stack of dielectric layers, along the optical axis, without the use of tight focusing. Specifically, we show that an aperiodic distribution of layer thicknesses can effectively lead to the formation of subsurface laser-induced modifications, confined in a 100-nm single predefined layer only, without affecting the surrounding material. We argue that this confinement is correlated to a local electric field enhancement phenomenon occurring within a specific layer of the stack and we confirm this hypothesis with simulations. Concurrently, we demonstrate the presence of nanocrystallites in the laser-affected areas.