The dependence of transverse and longitudinal resolutions on incident   Gaussian beam widths in the illumination part of optical scanning microscopy
  We studied both theoretically and experimentally the intensity distribution of a Gaussian laser beam when it was focussed by an objective lens with its numerical-aperture (NA) up to 0.95. Approximate formulae for full widths at half maximum (FWHM) of the intensity distribution at focus were derived for very large and very small initial beam waists with respect to the entrance pupil radius of the objective lens. In experiments the energy flux through a 0.5 micron pinhole was measured for various pinhole positions. We found that the FWHM's at focus in the transverse and the longitudinal directions do not increase much from the ultimate FWHM's until the input beam waist is reduced below the half of the entrance pupil radius. In addition, we observed significance of the spatial distribution of the input beam against a true Gaussian beam profile in the case of small initial beam waist. For high NA with resulting focal beam waists comparable to or smaller than the wavelength of the laser, the interaction between the electric field and the conducting surface of the pinhole caused the transverse FWHM to be measured slightly smaller than FWHM of the unperturbed intensity distribution convoluted with the pinhole opening. 
Hyung-Su Chon, Gisung Park, Sang-Bum Lee, Seokchan Yoon, Jaisoon Kim,   Jai-Hyung Lee and Kyungwon An (Seoul National Univer...