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Keywords:

  • Diffraction;
  • nondiffracting;
  • diffraction limit;
  • superresolution;
  • evanescent wave;
  • time reversal;
  • STED microscopy;
  • super lens;
  • optical antenna;
  • STORM microscopy;
  • high focal depth;
  • apodization;
  • longitudinal polarization;
  • nano-optics;
  • nano-photonics;
  • nanoscopy;
  • microscopy;
  • nano-focusing;
  • nano-lithography;
  • beam shaping.

Abstract

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.