AIRY PATTERN REORGANIZATION AND SUBWAVELENGTH STRUCTURE IN A FOCUS

An early result of optical focusing theory is the Lommel field, result ing from a uniformly illuminated lens; the dark rings in the focal pla ne, the Airy rings, hare been recognized as phase singularities. On th e other hand, it is well known that Gaussian illumination leads to a G aussian beam in the focal region without phase singularities. We repor t a theoretical and experimental study of the transition between the t wo cases. Theoretically, we studied this transition both within and ou tside the paraxial limit by means of diffraction theory. We show that in the gradual transition From uniform toward Gaussian illumination, t he Airy rings reorganize themselves by means of a creation/annihilatio n process of the singularities. The most pronounced effect is the occu rrence of extra dark rings (phase singularities) in front of and behin d the focal plane. We demonstrate theoretically that one can bring the se rings arbitrarily close together, thus leading to structures on a s cale arbitrarily smaller than 1 wavelength, although at low intensitie s. Experimentally, we have studied the consequences of the reorganizat ion process in the paraxial limit at optical wavelengths. To this end, we developed a technique to measure the three-dimensional intensity ( 3D) distribution of a focal field. We applied this technique in the st udy of truncated Gaussian beams; the experimentally obtained 3D intens ity distributions confirm the existence and the reorganization of extr a dark rings outside the focal plane. (C) 1998 Optical Society of Amer ica.