Ultrafast photoinduced strain in super-tetragonal PbTiO3 ferroelectric films

The ultrafast photoinduced strain (UPS) resulting from the coupling of piezoelectric and photovoltaic effects in ferroelectric has been focused in the last decade, endowing them with extensive applications including ultrafast optical memories, sensors and actuators with strain engineering. The mechanism of screening of the depolarization field by photoinduced carriers is generally accepted for UPS in ferroelectrics, while the thermal component of the strain is usually diluted as the offset and has not been systematically confronted, leading to unnecessary confusion. Herein, both the positive and negative thermal expansion effects in composite ferroelectric epitaxial films are investigated by use of high-repetition-rate ultrafast X-ray diffraction, along with the piezoelectric and photovoltaic effects. The coupling of the positive/negative thermal effects and the piezoelectric/photovoltaic effects in ultrafast strain is evidenced and can be regulated. The opposite lattice responses due to different thermal effects of the samples with different axial ratios are observed. The maximum UPS is up to 0.24%, comparable to that of conventional ferroelectric. The interaction between the thermal and ferroelectric effects in the induced strain could promote the diversified applications with the coupling of light, heat and electricity.