Optical properties of rutile near its fundamental band gap

We report low-temperature absorption, time-integrated photoluminescence, and resonant-Raman-scattering spectra near the fundamental band gap of TiO₂ single crystals. The photoluminescence spectrum comprises a first peak at {}{ω}=3.031 eV, followed by several peaks at lower energies. A polarization study of the emission spectrum indicates that the highest energy peak corresponds to 2pxy dipole-allowed second-class excitonic transitions while the lower-energy peaks are phonon replicas of the 1s quadrupolar exciton. This result is corroborated by time-resolved photoluminescence measurements. The near-band-gap optical response of TiO₂ is thus controlled by two distinct exciton states. The Raman-scattering intensity is found to increase slowly for excitation energies in the range 2.7--3 eV. This indicates that the Raman cross-section enhancement is dominated by virtual transitions involving the first dipole-allowed direct gap at 4.2 eV.