Comprehensive Study of the Topological Surface States through Ultrafast Pump-Probe Spectroscopy

In a broad probing spectral range covering visible to NIR, the transient reflectance study on bismuth selenide thin films, especially of 5-15 nm thickness, has not been carried out before. The independent carrier lifetimes in these films and the observation of resonant states in 10 nm thin films require an in-depth analysis of charge carriers and phonon modes to investigate the intrinsic states and to understand the probe-, pump-, and temperature-dependent effects. In this perspective, molecular beam epitaxy is used to grow 5-, 10-, and 15 nm-thin films of bismuth selenide, which are capped with a selenium layer of 1 nm on a c-cut sapphire substrate. After the structural and phonon-mode studies, reflectance spectroscopy is utilized to determine the refractive index of different thicknesses of films. Transient reflectance ultrafast spectroscopy is employed to probe the dynamics in the broad visible to NIR range, highlighting that the resonant feature is probe-and pump-dependent. Furthermore, temperature-dependent investigations in 10 nm films are performed to identify the intrinsic origin of the resonance effects down to 5 K. This article provides a comprehensive investigation of thin films of bismuth selenide to model the characteristic difference in the response of surface states with film thickness.