Abstract
We demonstrate that efficient broadband absorption of infrared radiation can be obtained with deeply subwavelength spherical dielectric particles covered by a thin metal layer. Considerations based on Mie theory and the quasi-static approximation reveala wide range of configuration parameters, within which the absorption cross section reaches the geometrical one and exceeds more than by order of magnitude the scattering cross section in the infrared spectrum. We show that the absorption is not only efficient but also broadband with the spectral width being close to the resonant wavelength corresponding to the maximum of the absorption cross section. We obtain a simple analytical expression for the absorption resonance that allows one to quickly identify the configuration parameters ensuring strong infrared absorption in a given spectral range. Relation between the absorption resonance and excitation of the short-range surface palsmon modes in the metal shell of particles is demonstrated and discussed. Our results can be used as practical guidelines for realization of efficient broadband infrared absorbers of subwavelength sizes desirable in diverse applications.
Highlights
Electromagnetic radiation absorbers are components in which the incident radiation at operating wavelengths can be efficiently absorbed and transformed into ohmic heat or other forms of energy
We demonstrate that efficient broadband absorption of infrared radiation can be obtained with deeply subwavelength spherical dielectric particles covered by a thin metal layer
Considerations based on Mie theory and the quasi-static approximation reveal a wide range of configuration parameters, within which the absorption cross section reaches the geometrical one and exceeds more than by order of magnitude the scattering cross section in the infrared spectrum
Summary
Electromagnetic radiation absorbers are components in which the incident radiation at operating wavelengths can be efficiently absorbed and transformed into ohmic heat or other forms of energy. Design challenges increase significantly when considering the infrared absorbers, because plasmonic characteristics of metal-based configurations deteriorate rapidly for long wavelengths [18] It is well-known that at low frequencies, due to sharp increases in metal dielectric constants and especially in their imaginary parts, unique properties of localized surface plasmons practically disappear, resulting (among other things) in a drastic decrease of the radiation. Small particles would efficiently absorb radiation only when their dielectric constants are of the same order as those of their environment, otherwise incident electromagnetic fields do not penetrate the material of particles [20] Realizing that this material matching can be facilitated by using composite materials, one is led to the idea of exploiting core-shell (dielectric-metal) particles for efficient and isotropic infrared absorption. This approach allowed us to cross check the accuracy of the quasi-static approximation, which is important for arriving at the simple design relationship that we established in our article
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