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Progress made on super-planckian near-field radiative heat between graphene sheets

2018-10-12

The research team headed by Prof. MA Yungui at the College of Optical Science and Engineering, Zhejiang University, made significant progress in experiments on near-field thermal radiation. Their findings are featured in a research article entitled “Observing of the super-Planckian near-field thermal radiation between graphene sheets” in the October 2 issue of Nature Communications.

Objects with temperature will radiate the infrared light arising from the mechanical oscillation of inner charges and the emitted light spectrum can be statistically characterized by the classic Plank’s radiation law. For the far-field scenario, a blackbody that absorbs all the impinging light has the capacity to yield the maximum energy transfer efficiency compared with nature materials. However, when a receiver is placed at a distance far smaller than the thermal de Broglie wavelength, the thermally excited evanescent waves that carry high density of states of photons can tunnel through the subwavelength gaps and thus substantially enhance the near-field heat transfer efficiency. Various applications based on the framework of near-field heat transfer have been proposed such as thermophotovoltaic (TPV) cells, thermal scanning imaging, thermal condensers or rectifiers. 

Nonetheless, the experiment on the near-field thermal radiation has posed immense challenges due to the technical difficulties primarily in controlling the gap distances. So far, the abnormal thermal radiation effect between usual metals or dielectrics has been well examined and the underlying description formulae have been verified in the experiment.

The research team led by MA Yungui made a direct measurement about plasmon-mediated thermal radiation between two macroscopic graphene sheets using a custom-made setup. Super-Planckian radiation with efficiency 4.5 times larger than the blackbody limit was observed at a 430-nm vacuum gap on insulating silicon hosting substrates. The positive role of graphene plasmons was further confirmed on conductive silicon substrates which have strong infrared loss and thermal emittance. 

This work validates the classic thermodynamical theory in treating graphene and also paves the way for the application of near-field thermal management.