Haeberle, O. and Simon, B.
Saturated structured confocal microscopy with theoretically unlimited resolution. Optics Communications 282: 3657-3664 (2009).

The resolution of fluorescence microscopes is limited by diffraction, which determines the extension of their point spread functions. We propose and study numerically a simple method, based on a combination of subtraction microscopy with regular and annular excitation beams, which permits to double the resolution compared to wide field microscopy. When combined with the fluorescence saturation phenomenon, this approach would be able to deliver a resolution of a few tens of nanometers.