  We report a systematic polarization-dependent angle-resolved photoemission
spectroscopy study of the three-dimensional electronic structure of the
recently discovered 112-type iron-based superconductor Ca1-xLaxFeAs2 (x = 0.1).
Besides the commonly reported three hole-like and two electron-like bands in
iron-based superconductors, we resolve one additional hole-like band around the
zone center and one more fast-dispersing band near the X point in the vicinity
of Fermi level. By tuning the polarization and the energy of incident
photons,we are able to identify the specific orbital characters and the kz
dependence of all bands. Combining with band calculations, we find As 4pz and
4px (4py) orbitals contribute significantly to the additional three-dimensional
hole-like band and the narrow band, respectively. Also, there are considerable
hybridization between the As 4p zand Fe 3d orbitals in the additional hole-like
band, which suggests the strong coupling between the unique arsenic zigzag bond
layers and the FeAs layers therein. Our findings provide a comprehensive
picture of the orbital characters of the low-lying band structure of 112-type
iron-based superconductors, which can be a starting point for the further
understanding of their unconventional superconductivity.
