Levy, U., Nezhad, M., Kim, H.-C., Tsai, C.-H., Pang, L., & Fainman, Y. (2005). Implementation of a graded-index medium by use of subwavelength structures with graded fill factor. Journal of the Optical Society of America A, 22(4), 724-733. https://doi.org/10.1364/JOSAA.22.000724