Revealing Optical Transitions and Carrier Dynamics within the Bulk Band Structure of Bi₂Se₃

Bismuth selenide (Bi₂Se₃) is a prototypical topological insulator which exhibits gapped states within the bulk and topologically protected conducting states on the surface. Here we use mid-infrared pump-probe spectroscopy on Bi₂Se₃ nanosheets exfoliated from Bridgeman grown single crystals to map the band-edge electronic structure and interrogate carrier relaxation processes over a wide energy range (0.3 to 1.2 eV). We observe direct optical transitions from spin-orbit split valence bands to the Fermi level above the lowest conduction band minimum. The photoexcited carriers thermalize rapidly to the lattice temperature within a couple of picoseconds due to optical phonon emission and scattering with the cold electron gas followed by slow electron-hole recombination within 150 ps at 10 K and 50 ps at 300 K. Knowledge of electronic structure and interaction of electrons and holes within the bulk band structure provides a foundation for understanding coupling of these states with the protected surface states.