Low-temperature ultrafast optical probing of topological bismuth selenide

Abstract

We report the optical response and temperature-dependent excited-state carrier dynamics in the flake of Bi2Se3, which is cleaved from its single crystal. The optical properties are explored using the visible-near infrared (VIS-NIR), infrared (IR), and photoluminescence (PL) spectroscopies. The VIS-NIR spectra are then used to find the Ɛ1 and Ɛ2 using Kramer’s Koning relations, confirming OBT of 2 eV and 1.4 eV. The DFT (density functional theory) calculations are also carried out to confirm the experimental optical transitions (OBT). To probe these OBT in the bismuth selenide, we have studied the temperature, fluence, and excitation dependent ultrafast transient reflectance over a wide spectral range from 2.58–0.77 eV (VIS-NIR) with different excitation energies 3.02 eV, 2.61 eV, 1.9 eV, and 1.4 eV to explore several previously unseen transitions that do not appear in PL spectroscopy and room temperature carrier analysis. The temperature-dependent excited-state dynamics are investigated at 5–300 K. This study clearly indicates the existence of Moss-Burstein shift in the visible region and Pauli blocking effect in the NIR region in the Bi2Se3 topological insulator. Moreover, the low-temperature TRUS confirms the transition to the second surface state present in bismuth selenide.