Optically induced lattice dynamics probed with ultrafast x-ray diffraction

We have studied the picosecond lattice dynamics of optically pumped hexagonal LuMnO₃ by using ultrafast x-ray diffraction. The results show a shift and broadening of the diffraction curve due to the stimulated lattice expansion. To understand the transient response of the lattice, the measured time- and angle-resolved diffraction curves are compared to a theoretical calculation based on the dynamical diffraction theory of coherent phonon propagation modified for the hexagonal crystal structure of LuMnO₃. Our simulations reveal that a large coupling coefficient (c₁₃) between the a-b plane and the c axis is required to fit the data. Though we interpret the transient response within the framework of thermal coherent phonons, we do not exclude the possibility of strong nonthermal coupling of the electronic excitation to the atomic framework. We compare this result to our previous coherent phonon studies of LuMnO₃ in which we used optical pump-probe spectroscopy.