Super-resolving filters as diffractive optical elements

Super-resolving filters have potential applications in the areas of optical data storage,1-3 lithography4 and microscopy.5-6 In many applications the behaviour of the filter in the presence of defocus is important. For example, for lithography a large depth of focus is desirable, whereas in microscopy high axial resolution is often required. The effect of particular filters on the three-dimensional focal distribution has therefore been studied.7-9 It is found that filters which are super-resolving in the transverse direction can be either super-resolving or apodizing in the axial direction. Indeed, a minimum in intensity is often found along the axis at the focal point. For amplitude filters, simple design rules for the three-dimensional focusing properties have been proposed.7