Ultrafast mid-infrared response of nonequilibrium excitations in the high T_c superconductor YBa₂Cu₃O_7-δ

The ultrafast mid-infrared reflectivity changes in YBa₂Cu₃O_7-δ after femtosecond near-infrared excitation provides new insights into the underlying quasi-particle dynamics are studied. The samples are optimally doped c-axis oriented films of 100 nm thickness sputtered onto a MgO substrate with T_C = 86 K. The mid-infrared reflection sharply increases on a sub-picosecond time scale by about ΔR/R₀≈10^-3 and then exhibits first a fast decay within 5 ps followed by a much slower decay on the ns time scale. As the temperature increases, the signal amplitude decreases and changes its sign at the transition temperature similar to behaviour at 1.5 eV. The signal strongly depends on probe frequency and was studied over a wide spectral range (10-18 μm).