  Radiative heat transfer between parallel objects separated by deep
sub-wavelength distances and subject to large thermal gradients (>100 K) could
enable breakthrough technologies for electricity generation and thermal
transport control. However, thermal transport in this regime has never been
achieved experimentally due to the difficulty of maintaining large thermal
gradients over nm-scale distances while avoiding other heat transfer mechanism
such as conduction. Previous experimental measurement between parallel planes
were limited to distances greater than 500 nm (with a 20 K thermal gradient),
which is much larger than the theoretically predicted distance (<100 nm)
required for most applications. Here we show near-field radiative heat transfer
between parallel nanostructures in the deep sub-wavelength regime using high
precision micro electromechanical (MEMS) displacement control. We also exploit
the high mechanical stability of structures under high tensile stress to
minimize thermal buckling effects and maintain small separations at large
thermal gradients. We achieve an enhancement of heat transfer of almost two
orders of magnitude relative to the far-field limit, which corresponds to a 54
nm separation. We also achieve a high temperature gradient (260 K) between the
cold and hot surfaces while maintaining a ~100 nm distance.
