Comparison of coherent phonon generation by electronic and ionic Raman scattering in LaAlO3

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 E_g mode exhibits a sharp peak when we tune our pump frequency into resonance with the high-frequency infrared-active E_u 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 28 mJ /cm² that alter the measured amplitude of the coherent phonon response.