  Infrared optical photonic crystals fabricated using direct laser writing,
which is based on the two-photon polymerization of suitable monomers, have
received substantial interest since the emergence of this process. Two-photon
polymerization could be a disruptive technology for the fabrication of
all-dielectric photonic crystals in the infrared spectral range, as it allows
the synthesis of large scale arrays of uniform structures with arbitrary
geometries and arrangements. However, all-dielectric photonic crystals that
provide birefringent optical responses in the infrared spectral range have not
yet been demonstrated using direct laser writing techniques. Here we explore
the form birefringence observed in photonic crystals composed of arrays of
subwavelength-sized slanted polymer microwires. The photonic crystals
investigated here were fabricated in a single fabrication step using direct
laser writing of an infrared transparent photoresist (IP-Dip). A strong
contrast of the cross-polarized reflectance of photonic crystals as a function
of the in-plane orientation is observed in the mid-infrared spectral range at
$\lambda$ $\approx$ 6.5~$\mu$m. This observation is indicative of an
anisotropic optical behavior. Finite element based techniques corroborate the
experimentally observed responses qualitatively.
