Abstract
We have observed coherent acoustic phonons in the hexagonal manganite LuMnO3 using two-color femtosecond optical pump-probe spectroscopy. The dependence of the oscillatory component of the photoinduced reflectivity on the probe wavelength and incident angle is consistent with a propagating strain pulse. Moreover, the frequency, dephasing, and phase of the oscillation are found to be temperature dependent. In particular, a large phase shift occurs in the vicinity of the Neel temperature (T-N), which we relate to the temperature-dependent on-site Mn d-d transition that is coupled to antiferromagnetic ordering, as recently observed in optical conductivity measurements. (C) 2003 American Institute of Physics.