Ultrafast dynamics of carriers, coherent acoustic phonons, and strain pulses in BiSbTe1.5Se1.5 topological insulator thin films

Here, we investigate the ultrafast carriers, coherent acoustic phonons, and acoustic strain pulse dynamics in topological insulator BiSbTe1.5Se1.5 (BSTS) thin films of various thicknesses using degenerate pump-probe reflection spectroscopy. In this study, we have used sapphire as the main substrate since the BSTS-Sapphire interface has the maximum acoustic reflection in comparison to BSTS-GaAs, BSTS-Si, and BSTS-MgO interfaces. The transient reflectivity data for the films with thickness more than twice the penetration depth mainly contains the traveling acoustic strain pulses (TASP) and a single-exponential (~2 ps) electronic background whereas the data for the films with thickness less than penetration depth is dominated by coherent acoustic phonons (CAPs) and a biexponential electronic background of τ₁~2 ps and τ₂~260--380 ps. The experimentally observed TASP are reproduced by a well-known theoretical acoustic strain model. Further, to examine the underlying physics that is responsible for the dynamics of photoexcited carriers, CAPs and strain pulses, we have carried out carrier density and temperature dependent (7--294 K) studies on 22 nm and 192 nm thick BSTS films. For the 22 nm BSTS film, the two decay constants τ₁ and τ₂ increase with increase in carrier density at room temperature and decrease with increase in temperature at a carrier density of 1.7 ×10¹⁹4pt0excm^-3, respectively. A closer inspection of the behavior of τ₁ and τ₂ with carrier density and temperature indicates that τ₁ is due to the interplay of electron-phonon scattering and carrier diffusion whereas τ₂ plausibly stems from the combination of defect-assisted and phonon-assisted recombination. Furthermore, with increasing temperature, there is an anharmonic decay induced softening of ~14% in the phonon frequency and ~48% anomalous decrease in the phonon damping parameter due to decreased scattering between the Dirac surface electrons and the acoustic phonons. Additionally, temperature-dependent studies on the 192 nm film reveal ~7% reduction in sound velocity, compared to $14%$ reduction in 22 nm film.