  Transient reflectivity (TR) measured at laser photon energy 1.51 eV from the
indirectly intersurface coupled topological insulator Bi2-xMnxSe3 films (12 nm
thick) revealed a strong dependence of the rise-time and initial decay-time
constants on photoexcited carrier density and Mn content. In undoped samples (x
= 0), these time constants are exclusively governed by electron-electron and
electron-phonon scattering, respectively, whereas in films with x = 0.013 -
0.27 ultrafast carrier dynamics are completely controlled by photoexcited
electron trapping by ionized Mn2+ acceptors and their dimers. The shortest
decay-time (~0.75 ps) measured for the film with x = 0.27 suggests a great
potential of Mn-doped Bi2Se3 films for applications in high-speed
optoelectronic devices. Using Raman spectroscopy exploiting similar laser
photon energy (1.58 eV), we demonstrate that due to indirect intersurface
coupling in the films, the photoexcited electron trapping in the bulk enhances
the electron-phonon interaction strength in Dirac surface states.
