Radiation greatly
exceeding blackbody between two objects separated
by microscale distances has attracted great interest. However, challenges
in reaching such a small separation between two plates have so far
prevented studies below a separation distance of about 25 nm. Here,
we report a study of radiation enhancement in the near-field regime
of less than 10 nm between two parallel plates. We make use of bulk,
rigid plates to approach small separation distances without the adverse
snap-in effect, develop embedded temperature sensors to allow near-zero
separation, and employ advanced sensing method to level the plates
and approach and maintain small separations. Our findings agree with
theoretical predictions between parallel surfaces with separations
down to 7 nm where an 18000 times enhancement in radiation between
two quartz plates is observed. Our method can also be used to explore
heat transfer between other materials and can possibly be extended
to smaller separation gaps.
Radiation as a heat transfer mode inside a bulk material is usually negligible in comparison to conduction. Here, the contribution of radiation to energy transport inside a hyperbolic material, hexagonal boron nitride (hBN), is investigated. With hyperbolic dispersion, i.e., opposite signs of dielectric components along principal directions, phonon polaritons contribute significantly to energy transport due to a much greater number of propagating modes compared to that in a normal material. A many-body model is developed to account for radiative heat transfer in a material with a nonuniform temperature distribution. The total radiative heat transfer through hBN is found to be largely contributed by the high-κ modes within the Reststrahlen bands, and is comparable to phonon conduction. Experimental measurements of temperature-dependent thermal transport also show that radiative contribution to thermal transport is of the same order as that from phonons. Therefore, this work shows, for the first time, radiative heat transfer inside a material can be comparable to phonon conductive heat transfer.
Thermal radiation has diffusive and broad emission characteristics. Controlling emission spectrum and direction is essential for various applications. Nanoparticle arrays, supporting collective lattice resonances, can be employed for controlling optical properties. However, thermal emission characteristics remain unexplored due to the lack of a theoretical model. Here, we develop an analytical model to predict thermal radiation from a nanoparticle array using fluctuation–dissipation theorem and lattice Green's functions. Our findings reveal that the periodicity and particle size of the particle array are main parameters to control both emission spectrum and direction. The derived simple expression for thermal emission enables insightful interpretation of physics. This model will lay a foundation for analytical derivation of thermal radiation from metasurfaces. Our study can be useful in engineering infrared thermal sources and radiative cooling applications.
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