Harnessing the point-spread function for high-resolution far-field optical microscopy.

The resolution limit of far-field optical microscopy is reexamined with a full vectorial theoretical analysis. A highly symmetric excitation optical field and optimized detection scheme are proposed to harness the total point-spread function for a microscopic system. Spatial resolution of better than 1/6λ is shown to be obtainable, giving rise to a resolution better than 100 nm with visible light excitation. The experimental measurement is applied to examine nonfluorescent samples. A lateral resolution of 1/5λ is obtained in truly far-field optical microscopy with a working distance greater than ∼500λ. Comparison is made for the far-field microscopic measurement with that of a nearfield scanning optical microscopy, showing that the proposed scheme provides a better image quality.