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Local polarization of tightly focused unpolarized light


The polarization of light is important in a great variety of optical phenomena, ranging from transmission, reflection and scattering to polarimetric imaging of scenes and quantum-mechanical selection rules of atomic and molecular transitions. Among some less-well-known phenomena that illustrate the vectorial nature of light are the Pancharatnam1 (or geometric2) phase, singularities in the polarization pattern of clear sky3 and polarization of microwave background radiation4. Here, we examine the partial polarization of focused light. We experimentally demonstrate a rather surprising phenomenon, where the focusing of unpolarized light results in rings of full polarization in the focal plane of the focusing optics. The polarization rings are imaged with a resolution of <100 nm by probing the focal region using a gold nanoparticle.

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Figure 1: Distribution of P3D in the focal plane of a high-NA lens.
Figure 2: Experimental set-up for mapping the degree of polarization.
Figure 3: Polarization by focusing.
Figure 4: Measured P2D for different values of the degree of polarization of the incident beam Pinc2D.
Figure 5: Influence of the axial electric-field component on P2D.


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The authors from TKK acknowledge financial support from the Academy of Finland, project numbers 201293 and 118074, and A.T.F. acknowledges the support of the Swedish Foundation for Strategic Research. J. Pekola and O. Hahtela are thanked for loans of equipment.

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Correspondence to Klas Lindfors.

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Lindfors, K., Priimagi, A., Setälä, T. et al. Local polarization of tightly focused unpolarized light. Nature Photon 1, 228–231 (2007).

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