Maximizing the electric field strength in the foci of high numerical   aperture optics
  Several applications require spatial distributions of the incident electric field that maximize the electric field at a focal point for a given input power. The field distributions are derived for various optical systems in a direct way based on fundamental physical properties. The results may prove useful for a wide range of applications, e.g., microscopy, scattering experiments or excitation of single atoms. For commonly used distributions - fundamental Gaussian modes and doughnut modes - we give the upper bounds of the achievable field amplitudes. 
Markus Sondermann, Norbert Lindlein, Gerd Leuchs