Electric field depolarization in high aperture focusing with emphasis on annular apertures

Electromagnetic focusing theory of light predicts that at high apertures fi eld components arise that are polarized perpendicular to the initial polari zation. Although vectorial depolarization has received considerable attenti on in focusing theory, no evidence has been presented as to its relevance i n experiments. We measure the intensity of the perpendicularly orientated f ield in the focal region by utilizing monomolecular, fluorescent polydiacet ylene layers whose transition dipoles are orientated in a single direction. For a 1.4 numerical aperture oil objective lens illuminated with linearly x-polarized light, we find that the integral of the modulus squared of the y-polarized focal field amounts to 1.5% of its x-polarized counterpart. In particular, we show here that the depolarization increases when using annul ar apertures. Annuli formed by a central obstruction with a diameter of 89% of that of the entrance pupil raise the integral to 5.5%. This compares we ll with the value of 5.8% predicted by electromagnetic focusing theory; how ever, the depolarization is also due to imperfections connected with focusi ng by refraction. Besides fluorescence microscopy and single molecule spect roscopy, the measured intensity of the depolarized component in the focal p lane is relevant to all forms of light spectroscopy combining strong focusi ng with polarization analysis.