Solar energy harvesting by perfect absorbers made of natural hyperbolic material

A perfect absorber, with pyramidal nanostructures made of a natural hyperbolic material, for solar energy harvesting is proposed in this chapter. A numerical investigation is first carried out for regularly arranged bismuth telluride (Bi2Te3, an anisotropic and natural hyperbolic material) pyramidal nanostructures placed on top of a Ag substrate, and the metamaterial is submerged in water. The calculated results show that the absorptance of the absorber exceeds 99.9% in the wavelength range of 300-2400 nm. The underlying mechanisms are revealed by the electric field and power dissipation density distribution in the absorber. It is found that the slow light effect in the type-II hyperbolic region (300-1000 nm) and the gradient index effect in the long wavelength range (1000-2400 nm) contribute to the perfect absorption of the solar energy for the proposed absorber. Effects of geometry parameters of nano-pyramids and the substrate on optical properties of the proposed absorber are illustrated. In addition, a rough surface with sharp nanostructures made of Bi₂Te₃ is also numerically studied. Based on simulation results of rough Bi₂Te₃ surface, samples with nanostructures made of Bi₂Te₃ are experimentally manufactured and optical properties of the samples are measured by using an integrating sphere with a grating monochromator. The absorptance of the samples can be as high as 97.5%, and the lowest absorptance of the sample is still higher than 94% in the wavelength range of 380-1800 nm. Moreover, other samples are also fabricated and studied to validate underlying mechanisms of the perfect absorption of solar energy. The results of the present study open a new revenue for effectively harvesting solar energy by using metamaterials with nanostructures made of natural hyperbolic materials submerged in water.