We show an intriguing connection between the coherence of spectral domain
interference of two electromagnetic modes in Fano resonance and the resulting
degree of polarization of light. A theoretical treatment is developed by
combining a general electromagnetic model of partially coherent interference of
a spectrally narrow and a broad continuum mode leading to Fano resonance and
the cross-spectral density matrix of the interfering polarized fields of light.
The model suggests a characteristic variation of the degree of polarization
across the region of spectral dip and the peak of Fano resonance as an
exclusive signature of the connection between the degree of polarization and
the coherence of the interfering modes. The predictions of the model is
experimentally verified in the partially polarized Fano resonance spectra from
metal Chalcogenides systems, which emerged due to the interference of a narrow
excitonic mode with the background continuum of scattered light in the
reflectance spectra from the system. The demonstrated connection between
polarization and coherence in the spectral domain Fano-type interference of
electromagnetic modes is fundamentally important in the context of a broad
variety of non-trivial wave phenomena that originate from fine interference
effects, which may also have useful practical implications