  The photoluminescence (PL) spectra of Ar-ion irradiated single crystals of
SrTiO3 (STO) excited by the 325 nm line of a He-Cd laser are compared with
those of pristine crystals, epitaxial films, and amorphous layers of STO at
several temperatures down to 20 K. The 550 eV Ar-beam irradiation activates
three distinctly visible PL peaks: blue (~430 nm), green (~550 nm), and
infrared (~820 nm) at room temperature, making the photoluminescence
multicolored. The abrupt changes in PL properties below ~100 K are discussed in
relation with the antiferrodistortive structural phase transition in SrTiO3
from cubic to tetragonal symmetry, which makes it a direct bandgap
semiconductor. The photoluminescence spectra are also tuned by an electrostatic
gate field in a field-effect transistor geometry. At 20 K, we observed a
maximum increase of ~20% in PL intensity under back gating of SrTiO3.
