  A grand challenge in fundamental physics and practical applications is
overcoming wave diffusion to deposit energy into a target region deep inside a
diffusive system. While it is known that coherently controlling the incident
wavefront allows diffraction-limited focusing inside a diffusive system, in
many applications targets are significantly larger than such a focus and the
maximum deliverable energy remains unknown. Here, we introduce the "deposition
matrix", which maps an input wavefront to its internal field distribution, and
theoretically predict the ultimate limitations on energy deposition at any
depth. For example, the maximum obtainable energy enhancement occurs at 3/4 a
diffusive system's thickness: regardless of its scattering strength.
Experimentally we measure the deposition matrix and excite its eigenstates to
enhance/suppress the energy within an extended target region. Our theoretical
analysis reveals that such enhancement/suppression results from both selective
transmission eigenchannel excitation and constructive/destructive interference
among these channels.
