Fundamental limit to the rectification of near-field heat flow: The potential of intrinsic semiconductor films

We derive the fundamental limit to near-field radiative thermal rectification mediated by an intrinsic semiconductor film within the framework of fluctuational electrodynamics. By leveraging the electromagnetic local density of states, we identify {\epsilon}"_H/{\epsilon}"_L as an upper bound on the rectification magnitude, where {\epsilon}"_H and {\epsilon}"_L are respectively the imaginary parts of the film permittivity at high and low temperatures. This bound is tight and can be approached regardless of whether the film is suspended or supported. For intrinsic silicon the limit can in principle exceed 10^9. Our work highlights the possibility of controlling heat flow as effectively as electric current, and offers guidelines to potentially achieve this goal.