Near-field radiative heat transfer with hyperbolic metamaterials

Max Plancks bedeutende Schwarzkörper Theorie kann den Anstieg der strahlenden Wärmeübertragung zwischen eng aneinanderliegenden Körpern, bekannt als Nahfeld Wärmeübertragung, nicht erklären. Wir demonstrieren diesen Effekt mit Hilfe einer neuen, dynamischen Messmethode und messen Wärmeströme bis zu 16-mal größer als die Schwarzkörper Grenze. Wir zeigen, dass thermische Strahlung in hyperbolischen Metamaterialien (HMMs) – äußerst anisotropen Nanostrukturen – sehr stark ist und ihre Eigenschaften sich von Schwarzkörper Strahlung deutlich unterscheiden. Schließlich erweisen sich Nahfeld Wärmeströme zwischen HMMs als sehr stark und deren Eindringtiefe als vergleichsweise groß.Despite its general significance, Max Planck’s blackbody theory cannot explain the increase in radiative heat flux between closely spaced bodies, known as near-field radiative heat transfer. We experimentally demonstrate this effect by utilizing a new, dynamic measuring technique and show heat fluxes up to 16 times above the blackbody limit. We also reveal that thermal radiation inside hyperbolic metamaterials (HMMs) – extremely anisotropic nanostructures – is very strong and has properties significantly different from blackbody radiation. Finally, near-field heat flux between HMMs turns out to be very strong and its penetration into the HMMs is comparatively large.Deutsche Forschungsgemeinschaft (DFG

Blackbody theory for hyperbolic materials

The blackbody theory is revisited in the case of thermal electromagnetic fields inside uniaxial anisotropic media in thermal equilibrium with a heat bath. When these media are hyperbolic, we show that the spectral energy density of these fields radically differs from that predicted by Planck's blackbody theory. We demonstrate that the maximum of their spectral energy density is shifted towards frequencies smaller than Wien's frequency making these media apparently colder. Finally, we derive Stefan-Boltzmann's law for hyperbolic media which becomes a quadratic function of the heat bath temperature.Deutsche Forschungsgemeinschaft (DFG

Electrochemical tuning of the optical properties of nanoporous gold

Using optical in-situ measurements in an electrochemical environment, we study the electrochemical tuning of the transmission spectrum of films from the nanoporous gold (NPG) based optical metamaterial, including the effect of the ligament size. The long wavelength part of the transmission spectrum around 800 nm can be reversibly tuned via the applied electrode potential. The NPG behaves as diluted metal with its transition from dielectric to metallic response shifted to longer wavelengths. We find that the applied potential alters the charge carrier density to a comparable extent as in experiments on gold nanoparticles. However, compared to nanoparticles, a NPG optical metamaterial, due to its connected structure, shows a much stronger and more broadband change in optical transmission for the same change in charge carrier density. We were able to tune the transmission through an only 200 nm thin sample by 30%. In combination with an electrolyte the tunable NPG based optical metamaterial, which employs a very large surface-to-volume ratio is expected to play an important role in sensor applications, for photoelectrochemical water splitting into hydrogen and oxygen and for solar water purification

Dynamic measurement of near-field radiative heat transfer

Super-Planckian near-field radiative heat transfer allows effective heat transfer between a hot and a cold body to increase beyond the limits long known for black bodies. Until present, experimental techniques to measure the radiative heat flow relied on steady-state systems. Here, we present a dynamic measurement approach based on the transient plane source technique, which extracts thermal properties from a temperature transient caused by a step input power function. Using this versatile method, that requires only single sided contact, we measure enhanced radiative conduction up to 16 times higher than the blackbody limit on centimeter sized glass samples without any specialized sample preparation or nanofabrication