Enhanced far-field coherent thermal emission using mid-infrared bilayer metasurfaces

Abstract

A classical thermal source, such as an incandenscent fillament, radiates according to Planck’s law. The feasibility of super-Planckian radiation has been investigated with sub-wavelength-sized sources in the last decade. In...

Long Propagating Polaritonic Heat Transfer: Shaping Radiation to Conduction

“…On the contrary, the short-range SPhPs supported by highly confined modes would enhance thermal emissivity, and it is observed as heat loss. Our previous studies ,, have been proved that the slow SPhPs contributed to far-field thermal emission, and the thin and anisotropic design of NR emitters showed the high emissivity. The emission could be observed in the form of a reduced apparent thermal conductivity.…”

“…In a similar manner, we also analyzed the length dependency in the slow SPhP mode using the NR-integrated devices. We employed our previously developed frequency-dependent measurement to quantify the ε of varying NR lengths (Figure a). ,, The constant ε of 0.51 was observed, indicating the saturated γ at maximum with the L ≫ L p (Figure b). Indeed, our experimental samples are much longer than the L p by the slow SPhP mode (<100s nm).…”

“…Recent experimental investigations have demonstrated the surpassed blackbody limit of radiative heat transfer between two surfaces with nanogaps − and nanostructures , located far from each other, or thermal emission from a single nano-object − (see Figure S17). The exploit of polaritonic heat conduction and radiation shares the same origin of coupled energy of mid-IR photons and optical phonons within the Reststrahlen band in polar dielectric surfaces (e.g., SiO 2 in Figure S1).…”

Review on the Scientific and Technological Breakthroughs in Thermal Emission Engineering

“…Indeed, due to the resonant behavior of subwavelength emitters, ,− the absorption cross-section may largely exceed the geometrical cross-section of emitters, ,, resulting in an enhancement of the radiative heat transfer that appears to be super-Planckian. , Nonetheless, if we consider the absorption/emission cross-section as a suitable metric on which the thermal emission power is normalized (as customarily done in antenna theory), such an enhancement vanishes, and even subwavelength emitters are bounded by the usual upper limits imposed by Planck’s radiation law. Moreover, it is worth stressing that, due to the very low emitting power (on the order of nW), the experimental demonstration of super-Planckian emission in subwavelength emitters has turned out to be very challenging, and even though various orders of magnitude of enhancement in far-field radiation with respect to the blackbody spectrum have been claimed, − the highest experimental measurement of emissivity reported so far is still clearly below 1. − Yet, it is possible to overcome Planck’s radiation law just by disregarding each of the underlying constraints, namely, the near-field regime, or the conditions of thermal equilibrium. , In particular, a typical approach to deal with nonequilibrium systems relies on the use of nonlinear media. ,,,, Such is the case, for example, of a semiconductor externally biased either electrically or optically, which produces a redistribution of the energy of electrons and holes in different quasi-Fermi levels described by qV e and qV h , where q and Δ V = V e – V h stand, respectively, for the electron charge and the potential difference. This can be modeled by introducing a nonzero chemical potential, μ F = q Δ V , so that the spectral energy density of nonequilibrium thermal radiation is given by − I NE ( ω , T , μ F ) = ω 2 π 2 c 3 ℏ ...…”

“…However, very often, in both the near- and far-field regimes, the use of such a term can hardly be adequately justified. On the one hand, it should be noted that Planck’s law does not apply in the near-field regime, so that the term super-Planckian turns out to be directly meaningless. ,, Furthermore, even in the far-field regime, the occurrence of super-Planckian emission, often relying on the use of subwavelength emitters, ,,,− yet circumvents the applicability domain of Planck’s law, as far as the size of the emitters is concerned . Indeed, due to the resonant behavior of subwavelength emitters, ,− the absorption cross-section may largely exceed the geometrical cross-section of emitters, ,, resulting in an enhancement of the radiative heat transfer that appears to be super-Planckian. , Nonetheless, if we consider the absorption/emission cross-section as a suitable metric on which the thermal emission power is normalized (as customarily done in antenna theory), such an enhancement vanishes, and even subwavelength emitters are bounded by the usual upper limits imposed by Planck’s radiation law.…”