Recently, Prof. MA Yungui’s group from College of Optical Science and Engineering (COSE) of Zhejiang University (ZJU) has achieved great progress in the research of near field thermal radiation. Their results were published on Nature Communications, titled “Observing of the super-Planckian near-field thermal radiation between graphene sheets”. (DOI: 10.1038/s41467-018-06163-8)
It is well known that objects with temperature radiate electromagnetic waves, while black body has the largest heat radiation efficiency in the far field. However, when the distance between objects is much smaller than the thermal wavelength, the near-field tunneling effect of the evanescent wave photon occurs. Since the evanescent wave has a much higher energy density, theoretically, it can break the Planck blackbody radiation limit and obtain an extremely high near-field energy transfer efficiency, which has important application potentials in thermophotovoltaic and thermal energy management. However, the near-field thermal radiation experiment is very challenging. It requires precise control of the physical spacing of the thermal radiation surface and the extraction of weak radiation signals in a high vacuum environment, which is also the current research focus. Up to now, the law of near-field thermal radiation between the surface of classical media such as silicon and silicon dioxide has been well verified. The group's innovative work is to design and manufacture a portable near-field thermal radiation test system based on magnetic attraction (Figure 1a), and for the first time, they use it to characterize the near-field heat transfer between graphenes (sample area: 2 cm x 2 cm).Based on the intrinsic silicon substrate, they used the near-field coupling effect of the graphene plasmons and successfully obtained 4.5 times the transfer efficiency of Planck blackbody radiation at 450 nm spacing(Figure 1b).The results not only confirm the validity of the classical thermal fluctuation dissipation theory in dealing with two-dimensional single-atom materials such as graphene, but also indirectly confirm the infrared response characteristics of graphene surface plasmons.
Figure 1. Near-field heat transfer between graphene-silicon heterostructures. (a) Schematic diagram of the test device; (b) heat transfer power between graphene and intrinsic silicon; (c) heat transfer power between graphene-doped silicon; (d) graphene-silicon thermal photovoltaic cell structure
Based on the above experiment, they further used a highly doped silicon substrate which gives rise to a Schottky junction effect between silicon and graphene and studied the change of near-field thermal radiation due to the modulation of the Fermi level of graphene. The experimental results are in good agreement with those predicted by the thermal fluctuation dissipation theory (Figure 1c). At the end of the paper, they discussed the application potential of the graphene-silicon heterojunction structures in near-field thermal photovoltaic cells (Figure 1d).
The first author of this paper is the postgraduate student YANG Jiang and Prof. Ma is the corresponding author. The work is financially support by the National Major Research Program Key Project, the Natural Science Foundations of China and Zhejiang Province.