revealed by time-resolved optical reflectivity*

Bi₂Se₃ is an ideal three-dimensional topological insulator in which the chemical potential can be brought into the bulk band gap with antimony doping. Here, we utilize ultrafast time-resolved transient reflectivity to characterize the photoexcited carrier decay in Bi_2-xSb_xSe₃ nanoplatelets. We report a substantial slowing of the bulk carrier decay rate in bulk-insulating Bi2-xSbxSe3 as compared to n-type bulk-metallic Bi₂Se₃ at low temperatures, which approaches 0.30 ns^-1 in the zero pump fluence limit. This long-lived decay is correlated across different fluences and antimony concentrations, revealing unique decay dynamics not present in n-type Bi₂Se₃, namely the slow bimolecular recombination of bulk carriers.