Femtosecond optical nonlinearities in YBa2Cu3O7-x

摘要

Abstract: performed a series of femtosecond reflectivity experiments on various YBa$-2$/Cu$-3$/O$-7$MIN@x$/ (YBCO) thin films at temperatures ranging from 12 K to 300 K. In particular, we have measured the dependence of the transient reflectivity signal on probing laser frequency, oxygen content, sample's thickness, ambient temperature, and pumping laser intensity. Results obtained at room temperature provide quantitative information on the position of the Fermi level for films with different oxygen contents. Systematic analysis of the measurements performed in the superconducting state indicate that the optical response associated with nonequilibrium properties of YBCO depends strongly on excitation intensity and sample thickness. The observed ultrafast refractive nonlinearity under the low laser excitation cannot be comprehensively interpreted as the relaxation dynamics of quasi-particles, and is speculated to be correlated to the existence of the nonequilibrium intermediate state.!25