Broadband negative refractive index in the visible spectrum

In this paper, a composite medium based metamaterial with random distribution of nanoparticles in vacuum host to achieve negative effective refractive index in the visible wavelength range is suggested for invisibility purposes. Our calculations show that structures including single metal (dielectric) spheres and core-shell structures with metallic core and dielectric shell, which consists two-layer particles with uniform sizes cannot support negative effective refractive index. For this purpose the structures consist of two-layer nanospheres with different sizes and fill fraction has been proposed. Since, band width of negative refractive index is narrow, the three layer nanospheres has been studied and investigated. We show that in this case with increasing the refractive index of middle and outer layers, negative value of effective refractive index can be increased. Also, we show that using different sizes of nanomaterials in host medium, band width is increased. Finally, superposition of three layer spherical nanoparticles with different outer radius and applied single doped semiconductor spheres, has been proposed. We show that Band width with negative permittivity and permeability can be optimized.