Propagation and focusing of Gaussian laser beams beyond conventional diffraction limit

It has been commonly assumed that the maximum focused intensity of a Gaussian laser beam is obtained at the beam waist position by focusing with perfect spherical wavefront. that is, without wavefront deformation. In contrast to this conventional assumption, it is shown for the first time that in low Fresnel-number systems the maximum focused intensity can be made larger. by means of the addition of proper wavefront aberrations. than that of the aberration-free beam. This is illustrated with numerical examples by evaluating the axial intensity distributions of focused Gaussian beams with a negative spherical aberration which degrades the intensity at the geometrical focus by 20%. It is presented that the identical wavefront aberration increases the maximum focused intensity by 35% and 30%, respectively, for the beams with Fresnel numbers N = 0.5 and 1.