Fighting against diffraction: apodization and near field diffraction structures

Diffraction is a natural phenomenon, which occurs when waves propagate or encounter an obstacle. Diffraction is also a fundamental aspect of modern optics: all imaging systems are diffraction systems. However, like a coin has two sides, diffraction also leads to some unfavorable effects, such as an increase in the size of a beam during propagation, and a limited minimal beam size after focusing. To overcome these disadvantages set by diffraction, many techniques have been developed by various groups, including apodization techniques to reduce the divergence of a laser beam and increase the resolution, and time reversal, STED microscopy, super lenses and optical antennas to obtain resolution down to nano-scale. This review concentrates on the diffraction of electromagnetic waves, and the ways to overcome beam divergence and the diffraction limit.