A focused ultrashort pulse can reach high enough intensity that non-linear ionization dominates its interaction with transparent media while still having relatively low fluence. In this case, the energy extracted from the beam can counter self-focusing by energy depletion and plasma formation, providing controlled energy deposition that can modify the material in a highly local manner. We demonstrate that non-linear absorption limits the intensity that can be reached and that the energy is deposited prior to the focus. We model the energy distribution, and predict and measure the energy transmitted through the focus. We establish the threshold intensity for non-linear ionization in dielectrics at ∼ 1013 W cm-2. We use the refractive index modification that the non-linear ionization causes in glass to image the spatial distribution of the energy deposition.