Abstract
This article reviews the recent advances in near-field radiative energy transfer, particularly in its fundamentals and applications. When the geometrical features of radiating objects or their separating distances fall into the sub-wavelength range, near-field phenomena such as photon tunneling and surface polaritons begin to play a key role in energy transfer. The resulting heat transfer rate can greatly exceed the blackbody radiation limit by several orders magnitude. This astonishing feature cannot be conveyed by the conventional theory of thermal radiation, generating strong demands in fundamental research that can address thermal radiation in the near field. Important breakthroughs of near-field thermal radiation are presented here, covering from the essential physics that will help better understand the basics of near-field thermal radiation to the most recent theoretical as well as experimental findings that will further promote the fundamental understanding. Applications of near-field thermal radiation in various fields are also discussed, including the radiative property manipulation, near-field thermophotovoltaics, nanoinstrumentation and nanomanufacturing, and thermal rectification.
Highlights
The theory of thermal radiation is based on the concept of blackbody, cast by Gustav Kirchhoff in 1860
Contrary to far-field thermal radiation carried by propagating EM waves, radiative heat transfer in the near field is dominated by evanescent EM waves and photon tunneling
Nearfield thermal radiation is influenced by the vacuum gap and radiative properties of materials
Summary
The theory of thermal radiation is based on the concept of blackbody, cast by Gustav Kirchhoff in 1860. Planck (1914) noted that the spectral distribution of blackbody radiation is derived based on the assumption that the geometric dimensions of the enclosure ( called a blackbody cavity) are much greater than the characteristic wavelength of thermal radiation This condition makes Planck’s law only applicable in the far field, i.e., away from the surface of any objects. Evanescent EM fields near the interface, which do not carry energy alone and exponentially decay from the interface, are coupled to carry a significant portion of energy across the gap when two objects are placed closer than the characteristic wavelength of thermal radiation This phenomenon is known as photon tunneling and is responsible for the enhanced energy transfer in the near field, along with other near-field effects such as interference and surface polaritons (Zhang, 2007; Fu and Zhang, 2006; Basu et al, 2009). A brief summary on the recent progresses in fundamentally understanding and engineering near-field thermal radiation is provided along with remarks on future research opportunities and challenges of the field
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