Broadband Diffractive Optics*

In many regions of the EM spectrum refractive lenses are not practical, so that diffractive lenses are all that are available for high resolution focussing or imaging. For example, Fresnel structures (eg. zone plates) have demonstrated diffraction limited imaging at soft x-ray wavelengths, but only for narrowband illumination (Δλ <λO/N, where N = # of zones). There have been various attempts to design diffractive lens doublets or triplets to correct for and/or reduce the chromatic aberrations associated with diffractive optics. These approaches generally involve two or more diffractive elements separated by a finite distance. As such, they represent a "lens system" instead of compact, broadband lens, and they suffer from the compound inefficiency of the multiple diffractive elements.