Slawa Lang

Controlling thermal emission with refractory epsilon-near-zero metamaterials via topological transitions

Control of thermal radiation at high temperatures is vital for waste heat recovery and for high-efficiency thermophotovoltaic (TPV) conversion. Previously, structural resonances utilizing gratings, thin film resonances, metasurfaces and photonic crystals were used to spectrally control thermal emission, often requiring lithographic structuring of the surface and causing significant angle dependence. In contrast, here, we demonstrate a refractory W-HfO2 metamaterial, which controls thermal emission through an engineered dielectric response function. The epsilon-near-zero frequency of a metamaterial and the connected optical topological transition (OTT) are adjusted to selectively enhance and suppress the thermal emission in the near-infrared spectrum, crucial for improved TPV efficiency. The near-omnidirectional and spectrally selective emitter is obtained as the emission changes due to material properties and not due to resonances or interference effects, marking a paradigm shift in thermal engineering approaches. We experimentally demonstrate the OTT in a thermally stable metamaterial at high temperatures of 1,000 °C.

Large penetration depth of near-field heat flux in hyperbolic media

We compare super-Planckian thermal radiation between phonon-polaritonic media and hyperbolic metamaterials. In particular, we determine the penetration depth of thermal photons inside the absorbing medium for three different structures: two semi-infinite phonon-polaritonic media supporting surface modes, two multilayer hyperbolic metamaterials and two nanowire hyperbolic metamaterials. We show that for hyperbolic modes the penetration depth can be orders of magnitude larger than for surface modes suggesting that hyperbolic materials are much more preferable for near-field thermophotovoltaic applications than pure phonon-polaritonic materials.

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.

Effective medium model for the spectral properties of nanoporous gold in the visible

The spectral properties of nanoporous gold are distinguished by two peaks in the transmission spectrum. Unlike earlier works, we do not attribute the peaks in the transmission to two separate localized plasmon resonances. Instead we show that the spectral shape can be understood as that of diluted gold with a spectrally narrow dip in transmission that arises from the averaged electric field approaching zero. Thus, the transmission characteristics are rather featured by a dip in one broad transmission curve than by two distinct peaks. Nanoporous gold is approximated by the effective medium model of a cubic grid of gold wires.

Pulse compression and broadening by reflection from a moving front of a photonic crystal

Previously, the effect of pulse bandwidth compression or broadening was observed in reflection from a moving front together with the Doppler shift. In this letter, an approach is presented, which alters pulse bandwidth without change in the central frequency. It occurs when light is reflected from a moving front of an otherwise stationary photonic crystal. This means that the photonic crystal lattice as such is stationary and only its boundary to the environment is moving, thus extruding (or shortening) the photonic crystal medium. The compression (broadening) factor depends on the front velocity and is the same as for the conventional Doppler shift. Complete reflection and transformation of the pulse can be achieved even with weak refractive index contrast, what makes the approach experimentally viable.

Single mode thermal emission.

We report on the properties of a thermal emitter which radiates into a single mode waveguide. We show that the maximal power of thermal radiation into a propagating single mode is limited only by the temperature of the thermal emitter and does not depend on other parameters of the waveguide. Furthermore, we show that the power of the thermal emitter cannot be increased by resonant coupling. For a given temperature, the enhancement of the total emitted power is only possible if the number of excited modes is increased. Either a narrowband or a broadband thermal excitation of the mode is possible, depending on the properties of the emitter. We finally discuss an example system, namely a thermal source for silicon photonics.

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 Plancks blackbody theory and that the maximum of the spectral energy density determined by Wiens law is redshifted. Finally, we derive the Stefan-Boltzmann law for hyperbolic media which becomes a quadratic function of the heat bath temperature.Svend-Age Biehs1,∗ Slawa Lang, Alexander Yu. Petrov, Manfred Eich, and Philippe Ben-Abdallah4† Institut für Physik, Carl von Ossietzky Universität, D-26111 Oldenburg, Germany. 2 Institute of Optical and Electronic Materials, Hamburg University of Technology, 21073 Hamburg, Germany 3 ITMO University, 49 Kronverskii Ave., 197101, St. Petersburg, Russia. and 4 Laboratoire Charles Fabry,UMR 8501, Institut d’Optique, CNRS, Université Paris-Sud 11, 2, Avenue Augustin Fresnel, 91127 Palaiseau Cedex, France. (Dated: February 24, 2015)

Gold-silicon metamaterial with hyperbolic transition in near infrared

In this paper, we focus on the creation and characterization of a hyperbolic metamaterial for near infrared. To shift the hyperbolic transition there, a stack of alternating 7 nm gold and 42 nm silicon layers is chosen. Samples are manufactured using magnetron sputtering and different measurements confirm their structure. We fit the Drude model of gold to reproduce measured reflectivity and transmittance by simulations. The collision frequency of the thin film gold is increased by 9 times, which shifts the transition of our metamaterial to the hyperbolic regime to even larger wavelengths. The performance is comparable to other proposed metamaterials.

Radiative engineering with refractory epsilon-near-zero metamaterials(Conference Presentation)

Improvement in high-temperature stable spectrally selective absorbers and emitters is integral for the further development of thermophotovoltaic (TPV), lighting and solar thermal applications. However, the high operational temperatures (T>1000oC) required for efficient energy conversion, along with application specific criteria such as the operational range of low bandgap semiconductors, greatly restrict what can be accomplished with natural materials. Motivated by this challenge, we demonstrate the first example of high temperature thermal radiation engineering with metamaterials. By employing the naturally selective thermal excitation of radiative modes that occurs near topological transitions, we show that thermally stable highly selective emissivity features are achieved for temperatures up to 1000°C with low angular dependence in a sub-micron thick refractory tungsten/hafnium dioxide epsilon-near-zero (ENZ) metamaterial. We also investigate the main mechanisms of thermal degradation of the fabricated refractory metamaterial both in terms of optical performance and structural stability using spectral analysis and energy-dispersive X-ray spectroscopy (EDS) techniques. Importantly, we observe chemical stability of the constituent materials for temperatures up to 1000°C and structural stability beyond 1100°C. The scalable fabrication, requiring magnetron sputtering, and thermally robust optical properties of this metamaterial approach are ideally suited to high temperature emitter applications such as lighting or TPV. Our findings provide a first concrete proof of radiative engineering with high temperature topological transition in ENZ metamaterials, and establish a clear path for implementation in TPV energy harvesting applications.