Electronic Origin of Ultrafast Photoinduced Strain in ${\mathrm{BiFeO}}_3$

Above-band-gap optical excitation produces interdependent structural and electronic responses in a multiferroic ${\mathrm{BiFeO}}_3$ thin film. Time-resolved synchrotron x-ray diffraction shows that photoexcitation can induce a large out-of-plane strain, with magnitudes on the order of half of one percent following pulsed-laser excitation. The strain relaxes with the same nanosecond time dependence as the interband relaxation of excited charge carriers. The magnitude of the strain and its temporal correlation with excited carriers indicate that an electronic mechanism, rather than thermal effects, is responsible for the lattice expansion. The observed strain is consistent with a piezoelectric distortion resulting from partial screening of the depolarization field by charge carriers, an effect linked to the electronic transport of excited carriers. The nonthermal generation of strain via optical pulses promises to extend the manipulation of ferroelectricity in oxide multiferroics to subnanosecond time scales.

Figure 1

(a) $\theta -2\theta$ scans of the BFO 002 Bragg reflection before and 100 ps after excitation by a 50 fs laser pulse with a central wavelength of 400 nm and the absorbed fluence of $3.2\mathrm{mJ}/{\mathrm{cm}}^2$. (b) The angular shift $\Delta \theta$ of Bragg peaks and induced strain as a function of delay at various absorbed fluences from 0.67 to $3.47\mathrm{mJ}/{\mathrm{cm}}^2$. The solid lines represent fits of a biexponential decay. (c) The angular shift and induced strain as a function of the absorbed fluence at 100 ps and 15 ns after the excitation. The solid lines represent linear fits.

Figure 2

(a) Optical absorption spectra in units of the optical density (OD) before (solid line) and after (dashed line) optical excitation of BFO film at absorbed fluence of $3.2\mathrm{mJ}/{\mathrm{cm}}^2$. The red curve shows the change in optical density ($\Delta \mathrm{OD}$) at 200 ps. (b) Transient absorption at 540 nm as a function of delay at various absorbed fluences from 0.37 to $3.2\mathrm{mJ}/{\mathrm{cm}}^2$. The solid lines represent the fits using a biexponential decay. The inset shows the initial ps dynamics at the absorbed fluence of $3.2\mathrm{mJ}/{\mathrm{cm}}^2$.

Figure 3

(a) The dynamics of the induced strain (triangles and diamonds) and optical absorption (solid lines) interpolated from Fig. 2b at various absorbed fluences from 0.67 to $3.47\mathrm{mJ}/{\mathrm{cm}}^2$. (b) Strain as a function of the induced absorption at various fluences. The solid lines are linear fits.

Figure 4

Decomposition of the experimentally observed strain at an absorbed fluence of $3.47\mathrm{mJ}/{\mathrm{cm}}^2$ (triangles) into thermal and nonthermal contributions. The red and blue curves are the carrier-induced [$(1-a)S_c(t)$] and thermally induced [$a{S}_q(t)$] strain, respectively. The blue curve is the sum of red and green curves that represents the overall induced strain $S(t)$ upon photoexcitation.