Ultrafast Optical Spectroscopy of Ba(Co$_{x}$Fe$_{1-x}$)$_{2}$As$_{2}$ Superconductors

High-temperature superconductivity (HTS) represents one of the most visited macroscopic quantum phenomena. A particularly interesting example is FeAs-based systems showing superconductivity from 20-50K. The development of ultrafast spectroscopy techniques has provide unique access to transiently photoexcited quasiparticles and their dynamics. Although the understandings of static thermodynamics, electronic transport, and magnetic properties are progressing, ultrafast optical studies on these systems are scarce. Here, we investigated the femtosecond optical responses of optimally-doped superconducting Ba(Co$_{x}$Fe$_{1-x})_{2}$As$_{2 }$(x=0.7) and antiferromagnetic parent compound BaFe$_{2}$As$_{2}$. Time-resolved pump-probe differential reflectivity revealed several distinct dynamic decay processes -- initial drop and slow recovery. The timescales and amplitudes of these components exhibited strong temperature dependence. Our results showed that the ultrafast optical response represents a new tool to study FeAs-based HTS.