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Ultrafast carrier relaxation in the 2D topological insulator (TI) Bi2Se3[gapped Dirac surface states (SS)] and how it inherits ultrafast relaxation inthe 3D TI Bi2Se3 (gapless Dirac SS) remains a challenge for developing newoptoelectronic devices based on these materials. Here ultrashort (100 fs)pumping pulses of ~340 nm wavelength (~3.65 eV photon energy) were applied tostudy ultrafast electron relaxation in the 2D TI Bi2Se3 films with a thicknessof 2 and 5 quintuple layers (~2 and ~5 nm, respectively) using transientabsorption (TA) spectroscopy in the ultraviolet-visible spectral region (1.65 -3.85 eV). The negative and positive contributions of TA spectra were attributedto absorption bleaching that mostly occur in the bulk states and to the inversebremsstrahlung type free carrier absorption in the surface states,respectively. Owing to this direct and selective access to the bulk and surfacecarrier dynamics, we were able to monitor coherent longitudinal optical (LO)phonon oscillations, which were successively launched in the bulk and surfacestates by the front of the relaxing electron population within the LO-phononcascade emission. We have also recognized the coherent surface-to-bulkvibrational coupling that appears through the phase-dependent amplitudevariations of coherent LO-phonon oscillations. This unique behavior manifestsitself predominantly for the topologically trivial insulator phase of the 2D TIBi2Se3 (2 nm thick film) in the photon energy range (~2.0 - 2.25 eV) whereefficient energy exchange between the bulk and surface states occurs. We alsofound that the coherent surface-to-bulk vibrational coupling significantlyweakens with increasing both the Bi2Se3 film thickness and pumping power.

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