Terahertz Acoustics in a Ferroelectric/Dielectric Superlattice*

The THz-frequency regime acoustic excitations of ferroelectric thin-film heterostructures underpin optical, electronic, and phase-transformation phenomena. Ferroelectric/dielectric superlattice (SL) heterostructures, in particular, can have atomic-scale periodicity and novel polarization configurations that pose a challenge to the prediction of the modes and frequencies of excitations and can be expected to produce previously unknown modes. We report an optical pump x-ray probe study using a free-electron laser x-ray diffraction to probe the wavevector-frequency dispersion in a ferroelectric/dielectric SL. The thin-film heterostructure consisted of a 100 nm-thick 8 unit-cell PbTiO₃/3 unit-cell SrTiO₃ SL deposited on a 20 nm SrRuO₃ layer on SrTiO₃ (001). The time-dependence of the intensity distribution of x-rays scattered from the SL was measured with high dynamic range and converted to a frequency-wavevector dispersion spectrum using a time-domain Fourier transform. The dispersion reveals folded longitudinal acoustic modes and frequency gaps at the centers and boundaries of the SL mini-Brillouin zones (BZs). The distribution of Fourier-transformed intensity indicates that novel modes at angular frequencies of 5 THz and 10 THz may arise at the BZ boundary.