Comparison of coherent phonon generation by electronic and ionic Raman scattering in LaAlO3
Martin J. Neugebauer, Dominik M. Juraschek, Matteo Savoini, Pascal Engeler, Larissa Boie, Elsa Abreu, Nicola A. Spaldin, Steven L. Johnson
Abstract
In ionic Raman scattering, infrared-active phonons mediate a scattering process that results in the creation or destruction of a Raman-active phonon. This mechanism relies on nonlinear interactions between phonons and has in recent years been associated with a variety of emergent lattice-driven phenomena in complex transition-metal oxides, but the underlying mechanism is often obscured by the presence of multiple coupled order parameters in play. Here, we use time-resolved spectroscopy to compare coherent phonons generated by ionic Raman scattering with those created by more conventional electronic Raman scattering on the nonmagnetic and non-strongly-correlated wide-band-gap insulator LaAlO3. We find that the oscillatory amplitude of the low-frequency Raman-active Eg mode exhibits a sharp peak when we tune our pump frequency into resonance with the high-frequency infrared-active Eu mode, consistent with first-principles calculations. Our results suggest that ionic Raman scattering can strongly dominate electronic Raman scattering in wide-band-gap insulating materials. We also see evidence of competing scattering channels at fluences above 28mJ/cm2 that alter the measured amplitude of the coherent phonon response.