  Guided waves are the perfect carriers of electromagnetic signals in planar
miniaturized devices due to their high localization and controlled propagation
direction. However, it is still a challenge to control the polarization of
propagating guided waves. In this work, we discover both theoretically and
experimentally the broadband polarization TE-TM degeneracy of highly localized
guided waves propagating along an all-dielectric metasurface and a
subwavelength chain of dielectric high-index cylinders. Using the discovered
near-field polarization degree of freedom, we demonstrate experimentally the
polarization transformation for guided waves propagating along a subwavelength
chain of dielectric cylinders at any frequency within the finite spectral
range. Namely, we implement the simplest planar near-field polarization device
-- the quarter-wave-retardation (linear-to-circular) polarization transformer
of guided waves. The results obtained discover the polarization degree of
freedom for guided waves paving the way towards numerous applications in flat
optics and planar photonics.
