We study the relation between angular spectral absorptivity and emissivity for any thermal emitter, which consists of any linear media that can be dispersive, inhomogeneous, bianisotropic, or nonreciprocal. First, we establish an adjoint Kirchhoff’s law for mutually adjoint emitters. This law is based on generalized reciprocity and is a natural generalization of conventional Kirchhoff’s law for reciprocal emitters. Using this law, we derive all the relations between absorptivity and emissivity for an arbitrary thermal emitter. We reveal that such relations are determined by the symmetries of the system, which are characterized by a Shubnikov point group. We classify all thermal emitters based on their symmetries using the known list of all three-dimensional Shubnikov point groups. Each class possesses its own set of laws that relates the absorptivity and emissivity. We numerically verify our theory for all three types of Shubnikov point groups: Grey groups, colorless groups, and black/white groups. We also verify the theory for both planar and non-planar structures with single or multiple diffraction channels. Our theory provides a theoretical foundation for further exploration of thermal radiation in general media.
The low-grade heat wasted globally contains an enormous amount of exergy that can be recovered for renewable energy generation. Current solid-state techniques for recovering low-grade waste heat, such as thermoelectric generators and thermophotovoltaics, are still limited by low conversion efficiencies or power densities. In this work, we propose a high-performance solid-state near-field thermophotonic system. We utilize the thermal radiation from nonequilibrium bodies by replacing the passive emitters in traditional thermophotovoltaic systems with electrically biased light-emitting-diodes (LEDs). we show that the proposed system can achieve a power density of 24.4 W/cm2 and a conversion efficiency of 15.5%, significantly outperforming the current record-setting thermoelectric generators. We also propose an electronic circuit design for the system by feeding part of the power produced by the PV cell back to the LED, to make the whole system self-sustaining.
Within the last two decades, it has been theoretically shown and experimentally measured that the radiative heat transfer between bodies in the near-field significantly exceeds the blackbody limit. This enhancement in heat transfer arises from evanescent surface waves, for example surface plasmon and surface phonon polaritons, that can tunnel between bodies at different temperatures. This result holds promise for applications in nano-imaging and lithography, thermophotovoltaics, nanoscale refrigeration and thermal circuitry. Although significant progress has been made in near-field heat transfer using passive materials, such as plasmonic metals and polar dielectrics, realizing actively tunable near-field heat transfer modules is of fundamental importance for controlling the photon heat flux. In this talk, analogously to its electronic counterpart, the metal-oxide-semiconductor (MOS) capacitor, we propose a thermal switching mechanism based on accumulation and depletion of charge carriers in an ultra-thin plasmonic film, via application of external bias. In our proposed configuration, the plasmonic film is placed on top of a polaritonic dielectric material that provides a surface phonon polariton thermal channel, while also ensuring electrical insulation for application of large electric fields. The variation of carrier density in the plasmonic film enables the control of the surface plasmon polariton thermal channel. We show that the interaction of the surface plasmonic mode with the surface phonon polariton significantly enhances the net heat transfer. We study SiC as the oxide and explore three classes of gate-tunable plasmonic materials: transparent conductive oxides, doped semiconductors, and graphene, and theoretically predict contrast ratios as high as 225%.
Conference Committee Involvement (1)
New Concepts in Solar and Thermal Radiation Conversion VI
3 August 2025 | San Diego, California, United States
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