Researchers at the Australian National University (ARC Centre of Excellence CUDOS) and the University of California Berkeley have discovered a new nano-metamaterial that could lead to highly efficient thermophotovoltaic cells. The new artificial material glows in an unusual way when heated.
As shown in the image, the metamaterial comprises 20 stacked alternating layers of 30-nm-thick gold and 45-nm-thick magnesium fluoride dielectric, perforated with 260 x 530 nm holes that are arranged into a 750 x 750 nm square lattice.
Thermophotovoltaics typically use a heated object as a source of radiation that is then converted to electricity by a photovoltaic cell. The caveat is that heated object emits light in all directions and over a broad spectral region, which reduces the efficiency of the light-to-electricity conversion. However, “The demonstrated metamaterial emits thermal radiation predominantly in particular directions and [within] a particular spectral region, which could make the conversion more efficient,” says Dr Sergey Kruk at the Nonlinear Physics Centre in the ANU Research School of Physics and Engineering.
The novel metamaterial could foster a new generation of highly efficient thermophotovoltaic cells. “The metamaterial has a potential to be used as an advanced thermal emitted that allows for a more efficient conversion of thermal energy into electricity,” the researcher agrees.
What is in fact truly revolutionary about this new metamaterial is its unusual optical property — magnetic hyperbolic dispersion. “This property has not been found in natural materials and is artificially engineered at the nanoscale,” Kruk notes. Dispersion describes the interactions of light with materials. The expert suggests to visualize it as a three-dimensional surface representing how electromagnetic radiation propagates in different directions. In natural materials, such as glass or crystals, these dispersion surfaces have simple forms; they could be spherical or ellipsoidal, for instance. “The dispersion of the new metamaterial is drastically different and takes hyperbolic form,” says Kruk. “As we know, hyperbola is a geometrical form that features branches extending towards infinity. These hyperbolic extensions of the dispersion lead to enhancement of radiation in specific directions at particular wavelength. The hyperbolic dispersion arises from the material's unusually strong interactions with the magnetic component of light. This is in contrast to usual natural materials, which interact strongly with electric component of light only.”
This new kind of nanoengineered metamaterial could find application in an advanced thermal system that, besides thermophotovoltaics, might include scanning thermal microscopy, coherent thermal sources and other thermal devices, according to the scientist.
The way Kruk and his team fabricated small quantities of the metamaterial in the laboratory, nevertheless, would not be scalable outside of a research setting as of yet, as becomes evident when the scientist describes their method: “The multi-layer structure was fabricated using electron-beam evaporation, the holes were fabricated using gallium focused ion beam milling. The focused ion beam milling technique is one of the favorite tools in nanofabrication research, as it allows for a fast and flexible prototyping,” he says. “However, it is expensive and has a low through-put. Therefore, for scalable manufacturing other nano-patterning techniques must be employed, as for example, deep-UV lithography, interference lithography, or nanoimprint.”
Kruk further elaborates that the metamaterial opens its full potential if a thermal emitter and a receiver (e.g., a photovoltaic cell) are placed extremely close together, at a distance comparable or smaller than the wavelength of radiation (sub-micrometer distance). He says the second law of thermodynamics applies the limitations to radiative heat transfer between the objects placed far apart from each other. The energy transfer does not exceed the transfer between two black bodies. This limit arises from a single mechanism of energy exchange via propagating light waves.
“If the objects are placed at a sub-micrometer distance, then there is an extra channel for the heat exchange: via evanescent waves,” Kruk explains. “These waves do not propagate in space and decay exponentially, therefore they are non-negligible only at small distances. However, if the objects are placed close enough such that evanescent waves of an emitter reach the receiver, the heat transfer can be boosted dramatically.”
“Magnetic hyperbolic dispersion of the metamaterial leads to extremely high energy confinement in the form of evanescent waves,” Kruk says. “It is estimated to be a least 10 times higher than in conventional materials. Therefore, the near-field heat transfer via evanescent waves should be much faster.”
The research is detailed in the article “Magnetic hyperbolic optical metamaterials,” published in Nature Communications.
Written by Sandra Henderson, Research Editor, Novus Light Technologies Today