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

Abstract

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.