Optical structures having an array of protuberances between two layers having different refractive indices are provided. The array of protuberances has vertical and lateral dimensions less than the wavelength range of lights detectable by a photodiode of a CMOS image sensor. The array of protuberances provides high transmission of light with little reflection. The array of protuberances may be provided over a photodiode, in a back-end-of-line interconnect structure, over a lens for a photodiode, on a backside of a photodiode, or on a window of a chip package.