Depolarization by high aperture focusing

Wr introduce a method employing ferroelectric monomolecular layers, by whic h it is possible to measure the light field polarization in the focus of a lens. This method allowed us to noninvasively establish the perpendicularly oriented focal field that is anticipated at high apertures. For a numerica l aperture 1.4 oil immersion lens illuminated with linearly polarized plane waves, the integral of the modulus square of the perpendicular component a mounts to (1.51 +/- 0.2) % of that of the initial polarization. It is prove n that depolarization decreases with decreasing aperture angle. Whereas for regular imaging conditions depolarization is largely negligible, it plays a significant role in microscopy of highest resolution, microspectroscopy, and single molecule studies. (C) 2000 American Institute of Physics. [S0003 -6951(00)01631-4].