Photo-induced strain in ferroelectric BiFeO₃ by resonant phonon excitation

Strong-field mid-infrared or terahertz laser pulses tuned to specific infrared-active phonons can directly modify structural parameters which are crucial to the physical properties of quantum materials. It provides a unique platform to engineer desired phases which may not exist at thermal equilibrium. Here we use time-resolved optical pump-probe spectroscopy to study the lattice dynamics in ferroelectric BiFeO₃. With resonant excitation of a mid-infrared optical phonon, an oscillatory response in the ellipticity change of the probe polarization is observed, indicating coherent excitation of an acoustic phonon. This response is absent when the pump frequency is tuned away from the optical phonon.