In optical microscopy, the polarization state of the focal field strongly influences formed images due to its interactions
with the sample and the effective focal spot size. We demonstrate experimentally that control over the spatial profile of the
focal field polarization state improves spatial resolution in
laser-scanning third harmonic generation (THG) microscopy.
The focal field is manipulated by imaging a spatial light modulator to the focal plane of a moderate-numerical aperture
microscope. The resolution enhancement arises from exploiting the suppression, in isotropic media, of THG for circularlypolarized
field polarization. By synthesizing a focal field whose polarization state changes from linear at the beam center
to circular beyond radius rs, we quench THG beyond rs. A transverse spatial resolution of up to 2 times is demonstrated.
Targeted manipulation necessitates measurement techniques that allow us to determine of the focal field polarization state.
We develop two such techniques to characterize the field. We use a nano-particle with known third-order susceptibility
to localize THG scattering to a small focal volume. Scanning this nano-probe through the focal volume of the microscope
allows for complete reconstruction of the vector point spread function. Under moderate focusing conditions, where
the recorded THG signal is dominated by the incident paraxial polarization component, the spatial polarization state is
determined non-iteratively via three linear-polarization projection THG measurements. Under tight focusing conditions,
polarization scrambling occurs such that the input and focal fields are dissimilar, and we introduce an algorithm for focal
field retrieval through the collection of far-field THG images.
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