Metamaterials and Plasmonics for Efficient Energy Harvesting, Thermal Emission and Focusing of Radiation
Recently, they are increased theoretical and experimental efforts to show how plasmonic nanoparticle arrays or metamaterial gratings are helpful to boost the overall absorption in solar panels and semiconductors. However, such techniques are usually inherently sensitive to the frequency of operation, which severely limit their practical use in energy-harvesting devices. This severe limitation of narrowband operation can be surpassed by the recent concept of Brewster energy tunneling through plasmonic gratings. The interface between free-space or dielectric materials and plasmonic gratings may be completely matched to the surrounding material over an ultrabroad range of frequencies and at a specific angle. The plasmonic Brewster angle may be easily tuned as a function of the array geometry, the surrounding material or the loaded materials in the grating’s channels. This anomalous tunneling phenomenon is able to concentrate all the impinging energy inside very small apertures, overcoming the bandwidth limitations of resonant arrays or apertures commonly used for transmission enhancement and energy concentration, based mostly on Purcell effects.
These concepts can be translated to three dimensional (3D) crossed slit gratings, tapered inside a gold screen to adiabatically enhance the absorption. This is an optimal design for broadband optical absorption, for which the entrance grating ensures perfect matching to free-space, and the tapering allows absorption over a broad range of wavelengths. It can achieve maximal absorption for a broad set of angles. The absorption spectrum shows very large absorptivity spanning almost the entire THz spectrum, both optical and IR, for all incident angles. As a result, these structures can achieve high energy conversion and absorption in the entire solar spectrum and they can efficiently capture the solar power and re-use it in different applications, for example to produce electricity, of immediate interest for energy applications.
Due to reciprocity, the proposed plasmonic designs may also operate as an ultra-broadband black-body radiators. In this scenario, in particular for thermo-photovoltaic applications, we envision a system capable of emitting like a black body over a very broad range of frequencies, but with more angular selectivity, which may ensure directive radiation towards the desired location. In general, engineering the black body thermal emission using metamaterials promises to impact a variety of applications involving thermal management, energy harvesting, and novel coherent thermal sources. Moreover, efficient ultrabroadband absorbing and energy concentrating devices can also be designed based on the recent concept of Transformation Optics, such as the perfect metamaterial absorber, where the impinging radiation beam is concentrated and fully absorbed at the device’s core. Finally, we are particularly interested in proposing novel rectifying antenna designs (rectennas), which may efficiently transform the surrounding microwave radiation to electricity. We strongly believe that our findings may lead to groundbreaking efficiencies for the future alternative “environmentally-friendly” energy sources.