  We have investigated the transport and optical properties along the c-axis of
a-axis-oriented Sr2IrO4 epitaxial thin-films grown on LaSrGaO4 (100)
substrates. The c-axis resistivity is approximately one order of magnitude
larger than that of the ab-plane. Optical absorption spectra with E//b
polarization show both Ir 5d intersite transitions and charge-transfer
transitions (O 2p to Ir 5d), while E//c spectra show only the latter. The
structural anisotropy created by biaxial strain in a-axis-oriented thin-films
also changes the electronic structure and gap energy. These a-axis-oriented,
epitaxial thin-films provide a powerful tool to investigate the highly
anisotropic electronic properties of Sr2IrO4.
