Polarization control and tomography for nonlinear microscopy

Abstract

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 r s , we quench THG beyond r s . 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.

Material characterisation with methods of nonlinear optics

“…These favorable properties linked to the THG process stimulated further improvements of the THGM technique. Namely, plasmon-resonance-enhanced THGM was demonstrated with a molecular specificity [73], a cylindrical focused beam enabled imaging of individual metal nanocones [74], the usage of Bessel beams facilitated rapid probing of large volumes [75], and the control of the spatial profile of the focal field polarisation state was shown to improve spatial resolution [76].…”