Second harmonic generation microscopy with tightly focused linearly and radially polarized beams

Abstract

It was found that the axial and transverse components of the electric field present in a tightly focused beam interact through the nonlinear susceptibility tensor resulting in SHG signals that have different polarizations. In particular SHG from rat-tail tendons sectioned transverse to the long axis of the tendon was imaged with tightly focused linearly and radially polarized beams. It was found that an x or y polarized beam generated SHG signals that had a cos^2 or sin^2 dependency on the orientation of the analyzer. It was also found that a tightly focused radially polarized fundamental beam generated radially polarized SHG that was not dependent on the orientation of the analyzer. A scalar approximation that assumed a homogeneous electric field at the focus did not explain the observed SHG polarization qualitatively. Rather the vectorial nature of the electric field at the focus had to be assumed so as to explain the experimental observations qualitatively.