Coherent Optical Phonons in the Iron Oxypnictide SmFeAsO1-xFx (x=0.075)

Coherent optical phonons with the Fe-related mode B-1g(Fe) in the iron oxypnictide SmFeAsO1-xFx (x = 0.075) are observed using femtosecond time-resolved reflection measurements with electrooptic sampling, in addition to the A(1g)(Sm) and A(1g)(As) modes. The time evolution of the frequency and amplitude of optical phonons is studied and the lifetimes of phonons are estimated. The frequency softening and positive frequency chirp related to the strong carrier-phonon coupling are found for the A(1g)(Sm) mode.