Photoexcited carrier relaxation in a-axis oriented YBa₂Cu₃O_7-δ thin films measured by femtosecond time-resolved spectroscopy

The photoexcited carrier relaxation dynamics in a-axis oriented YBa₂Cu₃O_7-δ (YBCO) thin films has been investigated by femtosecond time-resolved spectroscopy. Distinct responses along b-axis (CuO₃ chain) and c-axis of YBCO have been separated by the polarization-dependent femtosecond pump-probe measurements. It is found that the transient reflectivity (ΔR/R) curves for the electric field E of the polarized light parallel to the b- or c-axes of the a-axis oriented YBCO film (E∥b or E∥c) are significantly different. The opening of superconducting gap in CuO₃ chain was obviously observed from the measurements of the ΔR/R curves for E∥b at temperatures below T_c. However, the ΔR/R curves for E∥c did not exhibit the similar characteristics.