Efficient broadband infrared absorbers based on core-shell nanostructures

Abstract

We suggest to exploit dielectric-metal core-shell nanostructures for efficient resonant and yet broadband absorption of infrared radiation with deeply subwavelength configurations. Realizing that nanostructures would efficiently absorb radiation only when their dielectric properties match those of the environment and making use of the effective medium approach, we develop the design strategy using core-shell nanostructures with very thin shells made of poor metals, i.e., metals having real and imaginary parts of their dielectric permittivities of the same order of magnitude. Analyzing in detail spherical and cylindrical core-shell nanostructures, we demonstrate that the resonant infrared absorption can be not only very efficient, i.e., with the absorption cross sections exceeding geometrical ones, but also broadband with the spectral width being of the order of the resonant wavelength. We obtain simple analytical expressions for the absorption resonances in spherical and cylindrical configurations that allow one to quickly identify the configuration parameters ensuring strong infrared absorption in a given spectral range. Relations to effective medium parameters obtained by the internal homogenization are established and discussed. We believe that our results can be used as practical guidelines for realization of efficient broadband infrared absorbers of subwavelength sizes desirable in diverse applications.

Maximizing absorption and scattering by spherical nanoparticles

“…The fact that their optical losses are higher than those of noble metals, along with very high melting temperatures, makes these materials beneficial for the applications involving the broadband sunlight absorption and efficient conversion [12]. Spherical and cylindrical core-shell structures made of poor metals have also been investigated as broadband absorbers for the same purposes [14,15].…”

“…1. CAF contours along with the dispersion curves for two DMs with different damping rates γ (green lines) normalized by the plasma frequency ω p , real metals (dark yellow lines), and coreshell nanospheres (blue lines) described by the effective permittivity [14,23]. Optical parameters for the Ti are taken from Refs.…”

“…It turns out that there is also more design room with respect to the deviation of the maximum absorption from the FC. Using the electrostatic approximation for spherical core-shell nanoparticles, one obtains the polarizability in the following form [14,15]:…”

“…ε c , ε s , and ε d are the dielectric constants of the core, (metallic) shell, and surrounding media, respectively; R 1 and R 2 are the corresponding radii of the core and total core-shell nanosphere. Using the Maxwell-Garnett approach, one can consider sufficiently small core-shell spheres as homogeneous with the following effective permittivity [14,23]:…”

“…It has already been emphasized [14] that the real and imaginary parts of the effective permittivity are not independent, while being also dependent on the core-shell geometry, i.e., on the ratio between the core radius and the shell thickness.…”