Hyperbolic secant lens with a slit for subwavelength light focusing

A modeling by FDTD method shown, that binary photonic crystal secant hyperbolic micro lens made of Si ( n 0 = 3,47), with size of 2×5 mm with a slit width of 50 nm and length of 250 nm focuses TM-polarized flat wave in a focal spot near exit surface of lens. Focal spot FWHM size is λ/23, diffractive efficiency of the lens is 44%, and the focal intensity is 10 times more then the intensity of the incident light.