Broadband negative optical constants in composite materials

• Flexible structures to achieve negative optical constants are introduced.
• Structures include random distribution of nanoparticles in polymeric host medium.
• Poly-tetrafluoroethylene due to a low refractive index is used as a host medium.
• The structures illustrate negative ɛeff and μeff in wavelengths 3–5 μm.

Capability of flexible composite substrates, consisting of randomly distributed nanoparticles in polymeric host medium, to illustrate negative effective permittivity and permeability in the mid infrared wavelengths (3–5 μm) is investigated. To produce negative permittivity in the desired wavelength range, we proposed a structure in which plasmonic nanoparticles (doped semiconductors or metallic nanoparticles) are inserted inside polytetrafluoroethylene as the low refractive index polymeric medium. Also, the optical properties of the structures including core/shell nanoparticles in polytetrafluoroethylene host (with polytetrafluoroethylene as core material and dielectric shells possessing higher refractive index compared to refractive index of the host medium) are investigated. It is shown that, high refractive index dielectric shells result in negative μeff in these structures. As a basic idea, to obtain negative optical constants in broad wavelength range, superposition of the mentioned nanoparticles in the polymeric host is examined. The advantages and limitations of the proposed structure are carefully investigated. Moreover, based on the simulation results, we will introduce flexible media that simultaneously display negative permittivity and permeability in the wavelength range of interest. Capability of two types of composites (the first one contains mixture of plasmonic nanoparticles with polymer-dielectric core–shell nanoparticles and the second one includes metal–dielectric core–shell nanoparticles in the polymeric host) to produce both negative effective parameters in the desired wavelength range are investigated and compared together. Finally a polymeric medium with random distribution of core–shell (metal–dielectric) nanoparticles and plasmonic nanoparticles is introduced as an optimal medium to illustrate negative optical constants in mid infrared wavelengths. Clausius–Mossotti formula is used to calculate the effective parameters.

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