Double-Stokes characterization of depolarized light using two-photon excited fluorescence

Describing depolarized light is a significant challenge for traditional polarimetry as linear Stokes parameters tend to vanish. Nonlinear optical processes that rely on the coherent interaction of two incident photons may enable depolarization analysis through fourth-order electric field correlations, providing a nonlinear extension of Stokes parameters - the double-Stokes parameters. In this work, we use two-photon absorption to experimentally demonstrate the application of double-Stokes parameters for analysis of polarized and depolarized sources, including Lyot and liquid-crystal depolarizers, revealing polarized features in depolarized light that have no linear counterpart. These results highlight the potential of nonlinear polarimetry as a tool for optical diagnostics.