Non-thermal fluence threshold for femtosecond pulsed x-ray radiation damage in perovskite complex oxide epitaxial heterostructures

Abstract

Intense hard x-ray pulses from a free-electron laser induce irreversible structural damage in a perovskite oxide epitaxial heterostructure when pulse fluences exceed a threshold value. The intensity of x-ray diffraction from a 25-nm thick epitaxial BiFeO3 layer on a SrTiO3 (STO) substrate measured using a series of pulses decreases abruptly with a per-pulse fluence of 2.7 × 106 photons μm-2 at a photon energy of 9.7 keV but remains constant for 1.3 × 106 photons μm-2 or less. The damage resulted in the destruction of the BiFeO3 thin film within the focal spot area and the formation of a deep cavity penetrating into the STO substrate via the removal of tens of nanometers of material per pulse. The damage threshold occurs at a fluence that is insufficient to heat the absorption volume to the melting point. The morphology of the ablated sample is consistent with fracture rather than melting. Together, these results indicate that the damage occurs via a nonthermal process consistent with ultrafast ionization of the absorption volume.