Gigahertz longitudinal acoustic phonons originating from ultrafast ligand field transitions in hematite thin films

The creation and propagation of longitudinal acoustic phonons (LAPs) in high quality hematite thin films (α-Fe₂O₃) epitaxially grown on different substrates (BaTiO₃, SrTiO₃, and LaAlO₃) are investigated using the femtosecond pump-probe technique. Transient reflection measurements (ΔR/R) indicate the photo-excited electron dynamics, and the initial decay less than 1 ps and the slow decay of ～ 500 ps are attributed to the electron-LO phonon coupling and electron-hole nonradiative recombination, respectively. LAPs in α-Fe₂O₃ film can be created by ultrafast excitation of the ligand field state, such as the ligand field transitions under 800-nm excitation as well as the ligand to metal charge-transfer with 400-nm excitation. The strain modulations of the sound velocity and the out-of-plane elastic properties are demonstrated in α-Fe₂O₃ film on different substrates.