Abstract
We derive exact analytical formulas for the radiative heat transfer between parallel slabs separated by vacuum and subject to arbitrary temperature profiles. We show that, depending on the derivatives of the temperature at points close to the slab-vacuum interfaces, the flux can exhibit one of several different asymptotic low-distance (d) behaviors, obeying either 1/d^2,1/d, or logarithmic power laws, or approaching a constant. Tailoring the temperature profile within the slabs could enable unprecedented tunability over heat exchange, leading for instance to sign-flip transitions (where the flux reverses sign) at tunable distances. Our results are relevant to the theoretical description of on-going experiments measuring near-field heat transfer at nanometric distances, where the coupling between radiative and conductive transfer could result in temperature gradients.