  With their shielded 4f orbitals, rare-earth ions (REIs) offer optical and
electron spin transitions with good coherence properties even when embedded in
a host crystal matrix, highlighting their utility as promising quantum emitters
and memories for quantum information processing. Among REIs, trivalent erbium
(Er$^{3+}$) uniquely has an optical transition in the telecom C-band, ideal for
transmission over optical fibers, and making it well-suited for applications in
quantum communication. The deployment of Er$^{3+}$ emitters into a thin film
TiO$_2$ platform has been a promising step towards scalable integration;
however, like many solid-state systems, the deterministic spatial placement of
quantum emitters remains an open challenge. We investigate laser annealing as a
means to locally tune the optical resonance of Er$^{3+}$ emitters in TiO$_2$
thin films on Si. Using both nanoscale X-ray diffraction measurements and
cryogenic photoluminescence spectroscopy, we show that tightly focused
below-gap laser annealing can induce anatase to rutile phase transitions in a
nearly diffraction-limited area of the films and improve local crystallinity
through grain growth. As a percentage of the Er:TiO$_2$ is converted to rutile,
the Er$^{3+}$ optical transition blueshifts by 13 nm. We explore the effects of
changing laser annealing time and show that the amount of optically active
Er:rutile increases linearly with laser power. We additionally demonstrate
local phase conversion on microfabricated Si structures, which holds
significance for quantum photonics.
