Time-resolved hard X-ray nano-diffraction study on the long-lived strain in BiFeO$_{3}$ induced by optical transient grating*

Optical transient grating excitation was applied to induce spatially modulated strain in a BiFeO$_{3}$ thin film. The spatial profile and the dynamics of the photo-induced strain were measured by time-resolved hard X-ray nano-diffraction at beamline 7-ID of the Advanced Photon Source. By interfering two ultrafast laser beams at 400nm wavelength and $\sim$ 100fs pulse duration, an optical transient grating with $\sim$ 5micron period sinusoidal profile was created and used to excite the BiFeO$_{3}$ sample. By using the nano-focused ultrafast X-ray pulses, both the strain profile and the dynamics induced by the photo-generated carriers were mapped out. We found the out-of-plane strain profile remained sinusoidal while the strain amplitude modulation lasted tens of microseconds, indicating the non-thermal nature of this photo-induced strain in BiFeO$_{3}$.