Sub-wavelength focusing in the visible wavelength range realized by a one-dimensional ternary photonic crystal plano-concave lens

A plano-concave lens based on a one-dimensional (1D) ternary photonic crystal (PhC) was proposed, which was formed by periodic repetition of dielectric material layers MgF2, Ge, and SiO2. The dispersion relation and transmission spectrum of the 1D ternary PhC were obtained by the transfer matrix method (TMM). The plano-concave lens can achieve sub-wavelength focusing due to negative refraction. The negative refraction band can be moved to the visible light band with specific parameter adjustment. A broadband focusing in the visible wavelength range from 580 nm to 755 nm was demonstrated. Changes of light intensity and full width at half maximum (FWHM) with focusing wavelength (FW) were investigated and a focal point with almost a theoretical minimum FWHM of 0.275 λ was obtained at a wavelength of 675 nm. The functional relationship between the FW and the focal distance was derived theoretically. The theoretical calculation results are basically consistent with the numerical results. The results provide a promising methodology and theoretical guidance for the application of sub-wavelength focusing of 1D ternary PhC plano-concave lenses.