Maximum near-field radiative heat transfer between thin films

In this letter, we investigate the maximum near-field radiative heat transfer achievable between two thin films. By using frequency-independent permittivities for the films, we obtain optimum values of the real (epsilon'(opt)) and imaginary (epsilon ''(opt)) parts of the dielectric function maximizing the heat transfer for different thicknesses. We show that when the ratio D of the film thicknesses t(f) to the vacuum gap d is equal or less than 0.1, the maximum heat flux becomes independent of D. Based on the analysis performed in this study, it is possible to suitably choose film thicknesses maximizing near-field heat transfer at different vacuum gaps. The results obtained in this work also allow the interpretation of the physical details underlying near-field thermal radiation between films. (C) 2011 American Institute of Physics. [doi:10.1063/1.3600649]