Time-Reversed Diffraction
  We show that a time-reversed formulation of Huygens-Kirchhoff diffraction can be used to deduce the transverse and longitudinal fields of a Gaussian laser beam, starting from a simple assumption of a Gaussian beam profile in the far field. An attempt to apply this technique to the far fields of a Hertzian dipole shows how the laws of diffraction do not permit a wave to be focused to a volume smaller than a cubic wavelength in a charge-free region. 
Max S. Zolotorev, Kirk T. McDonald