TABLE OF CONTENTS Heat transfer between surfaces placed at extremely short distances has important applications in near-field scanning thermal microscopy. [55] [58] The concept of microscale thermophotovoltaic devices has been proposed to improve the energy conversion efficiency, by bringing the hot source very close to the receiving surface so that photon tunneling can enhance the net radiant power flux. [18] Negative index materials can be used to enhance photon tunneling through longer distances. [23] , [24] The calculation of near-field radiation heat transfer between dielectric materials is rather straightforward and has already been described in Sec. 10.1.4. Nanoscale radiation heat transfer can be enhanced by several orders of magnitude when absorption is considered. While many metals support surface waves through surface plasmon polaritons, the plasma frequencies are usually much higher than the characteristic frequencies of thermal sources. Consequently, the near-field enhancement of thermal radiation is not very large for good conductors. On the other hand, semiconductors and semimetals, with smaller electric conductivities, may greatly enhance radiation heat flux at nanometer scales; see Polder and van Hove ( Phys. Rev. B, 4, 3303, 1971) and Loomis and Maris ( Phys. Rev. B, 50, 18517, 1994). Most of the theoretical works were centered on the prediction of the net heat flux between two parallel metallic plates, using a simple Drude model for the dielectric function. Several studies also considered the nanoscale energy transfer between a sphere and a surface or between two...
