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.

Subpicosecond Optical Stress Generation in Multiferroic BiFeO3

“…Time-resolved diffraction experiments were conducted at the Pohang Accelerator Laboratory X-ray free-electron laser (PAL-XFEL) using the arrangement in Figure a. , X-ray pulses with 25 fs duration, 9.7 keV photon energy, and 30 Hz repetition rate were focused to a 10 μm full width-at-half-maximum diameter spot. The X-ray fluence was below the damage threshold fluence, which was previously measured to be 1.3 × 10 6 X-ray photons μm –2 in these BiFeO 3 layers . The measurements spanned a range of wavevectors of 0.1 Å –1 along the Q z axis of reciprocal space near the BiFeO 3 002 Bragg reflection, which appears at wavevector Q z (002) in the pseudocubic notation.…”

“…The X-ray fluence was below the damage threshold fluence, which was previously measured to be 1.3 × 10 6 X-ray photons μm −2 in these BiFeO 3 layers. 20 The measurements spanned a range of wavevectors of 0.1 Å −1 along the Q z axis of reciprocal space near the BiFeO 3 002 Bragg reflection, which appears at wavevector Q z (002) in the pseudocubic notation. The intensity was recorded using the average of 10 FEL X-ray pulses for each measurement setting, normalized by the intensity of each incident pulse.…”

Ultrafast Optically Induced Perturbation of Oxygen Octahedral Rotations in Multiferroic BiFeO3 Thin Films

“…The incident X-ray beam was attenuated to avoid X-ray damage. 28 The full width at half maxima (fwhm) of the X-ray probe and optical beams were 150 and 684 μm, respectively. A MultiPort Charged Coupled Device (MPCCD) detector was used to record the diffracted intensity.…”

“…The X-rays had photon energy 8.97 keV, a duration of 25 fs, and a repetition rate of 30 Hz. The incident X-ray beam was attenuated to avoid X-ray damage . The full width at half maxima (fwhm) of the X-ray probe and optical beams were 150 and 684 μm, respectively.…”