Nanosecond dynamics in intrinsic topological insulator Bi 2-x Sb x Se 3 revealed by time-resolved optical reflectivity

${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ 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 ${\mathrm{Bi}}_{2\ensuremath{-}x}{\mathrm{Sb}}_{x}{\mathrm{Se}}_{3}$ nanoplatelets. We report a substantial slowing of the bulk carrier relaxation time in bulk-insulating ${\mathrm{Bi}}_{2\ensuremath{-}x}{\mathrm{Sb}}_{x}{\mathrm{Se}}_{3}$ as compared to $n$-type bulk-metallic ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ at low temperatures, which approaches $3.3\phantom{\rule{0.16em}{0ex}}\mathrm{ns}$ 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 ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, namely the slow bimolecular recombination of bulk carriers.