  Metadevices based on dielectric nanostructured surfaces with both electric
and magnetic Mie-type resonances have resulted in the best efficiency to date
for functional flat optics with only one disadvantage: a narrow operational
bandwidth. Here we experimentally demonstrate broadband transparent
all-dielectric metasurfaces for highly efficient polarization manipulation. We
utilize the generalized Huygens principle, with a superposition of the
scattering contributions from several electric and magnetic multipolar modes of
the constituent meta-atoms, to achieve destructive interference in reflection
over a large spectral bandwidth. By employing this novel concept, we
demonstrate reflectionless (~90% transmission) half-wave plates, quarter-wave
plates, and vector beam q-plates that can operate across multiple telecom bands
with ~99% polarization conversion efficiency.
