Coherent surface-to-bulk vibrational coupling in the 2D topologically trivial insulator Bi₂Se₃ monitored by ultrafast...

Ultrafast carrier relaxation in the 2D topological insulator (TI) Bi₂Se₃ [gapped Dirac surface states (SS)] and how it inherits ultrafast relaxation in the 3D TI Bi₂Se₃ (gapless Dirac SS) remains a challenge for developing new optoelectronic devices based on these materials. Here ultrashort (~ 100 fs) pumping pulses of ~ 340 nm wavelength (~ 3.65 eV photon energy) were applied to study ultrafast electron relaxation in the 2D TI Bi₂Se₃ films with a thickness of 2 and 5 quintuple layers (~ 2 and ~ 5 nm, respectively) using transient absorption (TA) spectroscopy in the ultraviolet-visible spectral region (1.65-3.9 eV). The negative and positive contributions of TA spectra were attributed to absorption bleaching that mostly occurs in the bulk states and to the inverse bremsstrahlung type free carrier absorption in the gapped Dirac SS, respectively. Owing to this direct and selective access to the bulk and surface carrier dynamics, we were able to monitor coherent longitudinal optical (LO) phonon oscillations, which were successively launched in the bulk and surface states by the front of the relaxing electron population within the LO-phonon cascade emission. We have also recognized the coherent surface-to-bulk vibrational coupling that appears through the phase-dependent amplitude variations of coherent LO-phonon oscillations. This unique behavior manifests itself predominantly for the topologically trivial insulator phase of the 2D TI Bi₂Se₃ (2 nm thick film) in the photon energy range (~ 2.0-2.25 eV) where efficient energy exchange between the bulk and surface states occurs. We also found that the coherent surface-to-bulk vibrational coupling significantly weakens with increasing both the Bi₂Se₃ film thickness and pumping power.