A multi-state spectroscopic approach to carrier dynamics in thin-film topological insulators

Abstract

Understanding how to differentiate the dynamics of surface and bulk states is essential for advancing carrier physics in topological insulators. In this work, we systematically investigated ultrafast carrier dynamics in thin-film Bi₂Se₃ and Bi₂Te₃. The study combined reflective near-infrared excitation–mid-infrared detection spectroscopy with Fourier-transform infrared spectroscopy to achieve high temporal and spectral resolution. Under conditions that minimize surface-state interference, we directly observed pure bulk-state carrier intervalley scattering within the conduction band. The carrier lifetime increased significantly with higher excitation fluence, indicating that high-energy carriers relax into low-energy valleys through multichannel scattering pathways. By tuning the mid-infrared probe photon energy, we successfully distinguished between surface and bulk carrier responses. High-energy photons predominantly probed surface states, yielding shortened relaxation times, while mid-infrared probing captured bulk-carrier scattering dynamics. At elevated excitation densities, bulk carrier accumulation introduced competitive interactions with surface carriers. These interactions manifested as nonlinear evolution of relaxation times and measurable shifts in reflectivity peak positions. This study demonstrates a spectroscopic strategy for disentangling multi-state carrier interactions in topological insulators. The results provide experimental insights into carrier–phonon coupling and establish a foundation for valley-engineered optoelectronic device design.