We report the ultrafast optical response of quasiparticles (QPs) in both the pseudogap (PG) and superconducting (SC) states of an underdoped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+y}$ (Bi2212) single crystal measured with the time-resolved pump-probe technique. At a probe energy $\hslash {\omega }_{\mathrm{pr}}=1.55\mathrm{eV}$, it is found that the reflectivity change $\Delta R/R$ changes its sign at exactly $T_c$, which allows the direct separation of the charge dynamics of PG and SC QPs. Further systematic investigations indicate that the transient signals associated with PG and SC QPs depend on the probe beam energy and polarization. By tuning them below $T_c$, two distinct components can be detected simultaneously, providing evidence for the coexistence of PG and SC QPs.

• Received 8 February 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.137003