These lenses work in the near infrared, from 1200nm to 1700nm, a range of 500nm. That is a bandwidth somewhat GREATER than the visible spectrum.
That's true, but the difficulty lies in getting a given bandwidth as a percentage of center wavelength.
500 nm bandwidth with a center of 1450 nm is a lot easier than 360 nm bandwidth with a center of 570 nm.
Fair point. It also looks like the structures act as waveguides. Making a 570nm waveguide will be more difficult than a 1400nm waveguide.
On a slightly different note, these things somewhat remind me of a Fresnel.