Multilayer fishnet metamaterial. (a) Sketch of the structure. Thicknesses of MgF2 and Au layers are 45 and 30 nm, respectively. Thickness of Si3N4 membrane is 50 nm. Lattice period is 750 × 750 nm. Size of holes is 260 × 530 nm. (b) Experimentally measured transmission spectrum of the fishnet metamaterial. Inset shows a scanning electron microscopy image of the fabricated structure. (c) Effective refractive index of the fishnet metamaterial extracted for the normal incidence. The marked lines in b and c represent the wavelengths in the regions of elliptic dispersion (red), crossover optical topological transition (green) and hyperbolic dispersion (blue).
Physicists have discovered radical new properties in a nanomaterial which opens new possibilities for highly efficient thermophotovoltaic cells that harvest heat in the dark and turn it into electricity. The research team from the Australian National University (ARC Centre of Excellence CUDOS) and University of CA Berkeley demonstrated a new metamaterial, that glows in an unusual way when heated.
Thermophotovoltaic cells have been predicted to be > 2X more efficient than conventional solar cells. They do not need direct sunlight to generate electricity, and instead can harvest heat from their surroundings in the form of infrared radiation. They can also be combined with a burner to produce on-demand power or can recycle heat radiated by hot engines.
The team’s metamaterial, made of tiny nanoscopic structures of gold and magnesium fluoride, radiates heat in specific directions. The geometry of the metamaterial can also be tweaked to give off radiation in specific spectral range, in contrast to standard materials that emit their heat in all directions as a broad range of infrared wavelengths. This makes it ideal for use as an emitter paired with a thermophotovoltaic cell. “The size of individual building block of the metamaterial is so small that we could fit more than twelve thousand of them on the cross-section of a human hair.”
The key to the metamaterial’s remarkable behaviour is its novel physical property, magnetic hyperbolic dispersion. Dispersion describes the interactions of light with materials and can be visualized as a 3D surface representing how electromagnetic radiation propagates in different directions. For natural materials, such as glass or crystals the dispersion surfaces have simple forms, spherical or ellipsoidal vs hyperbolic form of the new metamaterial which arises from strong interactions with the magnetic component of light.
The efficiency of thermovoltaic cells based on the metamaterial can be further improved if the emitter and the receiver have just a nanoscopic gap between them. In this configuration, radiative heat transfer between them can be >10X more efficient than between conventional materials. http://www.anu.edu.au/news/all-news/nanomaterial-to-drive-new-generation-of-solar-cells
http://www.nature.com/ncomms/2016/160413/ncomms11329/full/ncomms11329.html
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