Selective coupling of coherent optical phonons in YBa2Cu3O7- with electronic transitions

We investigate coherent lattice dynamics in optimally doped YBa2Cu3O7-delta driven by ultrashort (similar to 12 fs) nearinfrared and near-ultraviolet pulses. Transient reflectivity experiments, performed at room temperature and under moderate (<0.1 mJ/cm2) excitation fluence, reveal Ag-symmetry phonon modes related to the O(2,3) bending in the CuO2 planes and to the apical O(4) stretching at frequencies between 10 and 15 THz, in addition to the previously reported Ba and Cu(2) vibrations at 3.5 and 4.5 THz. The relative coherent phonon amplitudes are in stark contrast to the relative phonon intensities in the spontaneous Raman scattering spectrum excited at the same wavelength. This contrast indicates mode-dependent contributions of the Raman and non-Raman mechanisms to the coherent phonon generation. We show that the particularly intense coherent Cu(2) phonon, together with its initial phase, supports its generation predominately via a displacive mechanism, possibly involving the charge transfer within the CuO2 planes. The small amplitude of the coherent out-of-phase O(2,3) bending mode at 10 THz also suggests the involvement of non-Raman generation mechanism. The generation of the other coherent phonons can in principle be explained within the framework of Raman mechanism. When the pump light has the polarization component perpendicular to the CuO2 plane, the coherent O(4) mode at 15 THz is strongly enhanced compared to the in-plane excitation, corresponding to the large polarizability component associated with the hopping between the apical and the chain oxygens, O(4) and O(1).