Natural anisotropic nanoparticles with a broad absorption spectrum for solar energy harvesting

Optical properties of spherical nanoparticles, made of natural hyperbolic material-Bi2Te3, are studied numerically based on the Mie theory from the ultraviolet to the near-infrared region. The absorption efficiency factor of an anisotropic spherical nanoparticle is calculated by superimposing contributions from different polarizations. The numerical results show that Bi2Te3 nanoparticles exhibit broadband absorption in the wavelength range of 300-2400 nm with several absorption peaks, owing to the simultaneous excitation and superposition of electric and magnetic dipole resonances. In order to elucidate the mechanism of resonance absorption peaks of Bi2Te3 nanoparticles, the Mie theory is used to calculate the electric field inside these nanoparticles by assuming the nanoparticle to be isotropic, having the same dielectric function as either parallel or perpendicular dielectric function of Bi2Te3. It is shown that increasing the size of the Bi2Te3 nanoparticle causes a redshift of the resonance wavelength, an increase in the magnetic dipole absorption peak, and a decrease in the electric dipole absorption peak. The numerical results also show that Bi2Te3 nanoparticles have high absorption efficiency factor in the wavelength of 300-2400 nm, demonstrating their potential to be used for harvesting solar energy.