Selective Coherent Excitation of Charged Density Waves

Real time femtosecond pump-probe spectroscopy is used to study collective and single particle excitations in the charge density wave state of the quasi-1D metal, blue bronze. Along with the previously observed collective amplitudon excitation, the spectra show several additional coherent features. These additional resonances can be excited selectively by applying a sequence of pump pulses with intervals tuned to the period of the particular coherent excitation. A study of the pump power dependence shows a non-linear response of the amplitudon mode, in contrast to the linear power dependence of the single particle, phonon, and phason excitations, which is ascribed to the electron-amplitudon elastic scattering.