Super-Planckian near-field heat transfer between hyperbolic metamaterials

Heat transfer between closely-spaced objects may be greatly enhanced with efficiency far beyond than the classic blackbody radiation limit under the participation of evanescent waves. Metamaterials with high density of optical surface states have been particularly explored to improve the heat flux upper limit but mostly confined in theoretical discussions due to the experimental challenges. In the near-field context, the effective medium theory (EMT) as widely used in the discussions is also lack of practical inspections. In this work, we managed to fabricate two 2 x 2 cm(2) sized high-quality infrared hyperbolic metamaterials made of silicon nanorod array and studied their near-field thermophotonic interaction using home-made setup. At the 500 nm vacuum gap, a strong heat flux density of 830 W/m(2) is observed, which is 4.7 times larger than the blackbody value. A local uniaxial model with parameters retrieved from the full-wave simulation was built to reproduce the measured results, which unambiguously validated the widely-adopted semi-empirical condition on the relationship between vacuum gap and lattice constant for the application of the EMT in the near-field regime. These results are believed essential for the future development of metamaterials for novel thermophotonic devices.