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Individual relaxation dynamics of electrons and holes in optically pumpedsemiconductors is rarely observed due to their overlap. Here we report theindividual dynamics of long-lived (~200 mks) holes observed at room temperaturein a 10 nm thick film of the 3D topological insulator (TI) Bi2Se3 coated with a10 nm thick MgF2 layer using transient absorption spectroscopy in the UV-Visregion. The ultraslow hole dynamics was observed by applying multiphotonresonant pumping of massless Dirac fermions and bound valence electrons inBi2Se3 at a certain wavelength sufficient for their photoemission andsubsequent trapping at the Bi2Se3/MgF2 interface. The emerging deficit ofelectrons in the film makes it impossible for the remaining holes to recombine,thus causing their ultraslow dynamics measured at a specific probingwavelength. We also found an extremely long rise time (~600 ps) for thisultraslow optical response, which is due to the large spin-orbit coupling (SOC)splitting at the valence band maximum and the resulting intervalley scatteringbetween the splitting components. The ultraslow hole dynamics in Bi2Se3 due tothe presence of the Bi2Se3/MgF2 interface is nevertheless much faster than theknown ultraslow electron dynamics at the Si/SiO2 interface, also induced bymultiphoton excitation in Si. The observed dynamics of long-lived holes isgradually suppressed with decreasing Bi2Se3 film thickness for the 2D TI Bi2Se3(film thickness 5, 4, and 2 nm) due to the loss of resonance conditions formultiphoton photoemission caused by the gap opening at the Dirac surface statenodes. This behavior indicates that the dynamics of massive Dirac fermionspredominantly determines the relaxation of photoexcited carriers for both the2D topologically nontrivial and 2D topologically trivial insulator phases.

Observation of ultraslow hole dynamics in the 3D topological insulator  Bi2Se3 coated with a thin MgF2 layer using multiphoton pumped UV-Vis  transient absorption spectroscopy