An optical element includes a substrate and a pattern. The substrate has a top surface and a bottom surface. The pattern is provided on the top surface. The pattern includes multiple levels such that a thickness of the pattern is less than a design wavelength. The pattern is configured to focus an incident radiation, received at one of the top surface or the bottom surface of the substrate, at one or more prescribed focal locations on a detection plane. The one or more prescribed focal locations on the detection plane changes in proportion to a wavelength of the incident radiation. The detection plane is an achromatic focal plane when the incident radiation includes multiple wavelengths.