Ultrafast Optical Excitation in YBa$_2$Cu$_3$O$_{7-\delta}$: Tracing the Optical Phonons

The time-resolved spectroscopy of nonequilibrium states proved to be a powerful tool for observation of the electron-phonon scattering dynamics and the recombination of photoexcited quasiparticles (QP), particularly in high-temperature cuprate superconductors. However, most of the reported experiments monitor only the electronic subsystem [1-5]. Thus, a detailed dynamics of the various phonon modes during an initial non-thermal regime has been beyond reach. Here we utilize the field-resolved ultrabroadband THz spectroscopy to resonantly trace ultrafast phonon and QP dynamics of optimally doped single crystals of YBa$_2$Cu$_3$O$_{7-\delta}$ [6]. The superconducting state is perturbed by 12-fs optical pump pulses, and the induced changes in the mid-infrared optical conductivity are probed by THz transients. Thus, we simultaneously observe the dynamics of nonequilibrium QPs and two specific phonon modes with a time resolution of 40 fs. A quantitative line shape analysis of the apex oxygen vibration allows us to separately follow its transient occupation and coupling to the Josephson plasma resonance. A strong phonon population and the maximum QP density are reached within the same time scale of 150 fs demonstrating that the lattice absorbs a major portion of the pump energy before the QPs are thermalized. Our results indicate substantial electron-phonon scattering in YBa$_2$Cu$_3$O$_{7-\delta}$ and introduce a powerful approach for characterizing transient phonon dynamics in a broad variety of solids.\\ [4pt] [1] S. G. Han et al., Phys. Rev. Lett. \textbf{65}, 2708 (1990);\\ [0pt] [2] R. A. Kaindl et al., Science \textbf{287}, 470 (2000);\\ [0pt] [3] R. D. Averitt et al., Phys. Rev. B \textbf{63}, 140502 (2001);\\ [0pt] [4] L. Perfetti et al., Phys. Rev. Lett. \textbf{99}, 197001 (2007);\\ [0pt] [5] R. P. Saichu et al., Phys. Rev. Lett. \textbf{102}, 177004 (2009);\\ [0pt] [6] A. Pashkin et al., Phys. Rev. Lett. \textbf{105}, 067001 (2010).