Ultrafast Optical Properties of La0.7Sr0.3MnO3 Thin Films

Thin film solids often exhibit different physical properties in the ultra-thin regime. Enhancement of surface to bulk ratio results in the domination of surface/interface related phenomena such as surface recombination. Moreover, in the ultra-thin regime, quantum size and quantum confinement effects can alter the band gap of the system and constrain the strain wave propagation in the thin film. Ultrafast properties of solids can also be drastically altered in the ultra-thin regime due to the aforementioned phenomena. Experimentally, observation of these phenomena is challenging due to the insufficient material to absorb and interact with the electromagnetic wave. This dissertation addresses the altercation of the ultrafast phenomena in the ultra-thin regime in complex oxide La0.7Sr0.3MnO3 (LSMO) films. To accomplish this goal, a degenerate optical ultrafast pumpprobe setup was developed to perform transient reflectivity (TR) experiments in cross linear and circular polarization configurations. Analyzing methods were ranged from multi-decay component modeling to understand the ultrafast dynamics, to the newly introduced wavelet analysis to study the propagation of transient strain waves in ultra-thin films. Moreover, density functional theory has been used as a complementary method to further prove the claims made based on the observations. Spectroscopic ellipsometry of LSMO ultra-thin films illustrate optical transitions between the substrate (STO3) and thin film’s orbitals, at the interface. It has been indicated that, in the ultra-thin regime, the dominated surface recombination results in a complete excited carriers’ energy loss in less than a few hundreds of picoseconds. Hence, the surface recombination, that is known to be detrimental for device purposes, is introduced as a phenomenon that can be useful for ultrafast switches in ultra-thin films. Moreover, the ultrafast photoinduced spin polarization of LSMO thin films exhibit a sharp decrease in ultrafast regime (~ 1 ps) which suggest LSMO thin films to be used in ultrafast magnetic switches. Wavelet analysis was introduced as an efficient method, compared to the Fourier transform, to analyze oscillatory modes superimposed on the TR signal which are caused by the propagation of strain wave longitudinally in the thin film. As a result, it has been illustrated that the sound velocity in LSMO ultra-thin films increases by decreasing the film thickness. Strong energy transfer between the thin film and the substrate has also been observed using wavelet analysis.