Carrier Dynamics and Optical Kerr Effect of Titanium Dioxide Single Crystals by Femtosecond Transient Grating Spectroscopy

The photogenerated carrier dynamics and optical Kerr effect in rutile titanium dioxide (TiO 2 ) single crystals with (100), (110), and (001) faces were examined by femtosecond transient grating spectroscopy. The diffraction signal of the intensity grating in bare TiO 2 single crystal at 670 nm was considered to be due to free electrons or weakly trapped electrons nearby Ti 4+ sites within the bulk. The decay curves of transient grating were strongly dependent on the excitation intensity but not on the types of crystal faces. The subsequent intensity-dependent relaxation was interpreted in terms of second-order electron-hole recombination kinetics with the rate constant of ∼ 2x 10 -11 cm 3 s -1 for both (110) and (100) faces. The optical Kerr dynamics of bare TiO 2 examined by resonant polarization grating and non-resonant intensity grating shows an ultrafast response similar to the system response function, indicating that the Kerr signal is mainly due to electronic polarization. By the excitation of conduction band electrons of reduced TiO 2 with the excess energy of 1.7 eV, the relaxation time constant of hot electrons within the conduction band was estimated to be 310 fs. In the long time region, the propagation of acoustic wave of 1.28 GHz was observed, which was due to the excess energy of hot electron converted into the thermal energy by electron-phonon scattering followed by the thermal relaxation.