  We demonstrate spatial modification of the optical properties of thin-film
metal oxides, zinc oxide and vanadium dioxide as representatives, using a
commercial focused ion beam (FIB) system. Using a Ga+ FIB and thermal
annealing, we demonstrated variable doping of a band semiconductor, zinc oxide
(ZnO), achieving carrier concentrations from 10^18 cm-3 to 10^20 cm-3. Using
the same FIB without subsequent thermal annealing, we defect-engineered a
correlated semiconductor, vanadium dioxide (VO2), locally modifying its
insulator-to-metal transition (IMT) temperature by range of ~25 degrees C. Such
area-selective modification of metal oxides by direct writing using a FIB
provides a simple, mask-less route to the fabrication of optical structures,
especially when multiple or continuous levels of doping or defect density are
required.
