Maximum energy transfer in near-field thermal radiation at nanometer distances

Abstract

Radiative energy transfer at nanoscale distances can exceed that of blackbody radiation by several orders of magnitude due to photon tunneling and the excitation of surface polaritons. While significant progress has been made recently in understanding near-field thermal radiation, an outstanding question remains as whether there exists an upper limit of near-field radiation for arbitrarily selected material properties at finite separation distances. We investigate the maximum achievable radiative heat flux between two parallel plates separated by a vacuum gap from 0.1 to 100 nm. By assuming a frequency-independent dielectric function and introducing a cutoff parallel wavevector component, we find that the ideal dielectric function for the two media that will maximize the near-field radiative transfer is −1+iδ, where δ is the imaginary part. For vacuum gaps greater than 1 nm, the near-field heat transfer peaks when δ⪡1, while at subnanometer gaps, the peak in the energy transfer shifts toward larger values of δ. The determination of the maximum radiative flux at nanometer distances will benefit emerging applications of near-field radiation for energy harvesting and nanothermal manufacturing.