Coherent Electron-Phonon Coupling in Two-Dimensional Bi₂Se₃ Nanoplatelets Studied with Ultrafast Spectroscopy

Electron-phonon coupling in two-dimensional Bi₂Se₃ nanoplatelets was investigated using broadband ultrafast pump-probe spectroscopy (probe 1.6 to 2.8 eV) at 293 and 78 K. The high time resolution resolves coherent phonon oscillations on picosecond time scales, specifically the out-of-plane A_1g ⁽¹⁾ (2.12 THz) and A_1g ⁽²⁾ (4.95 THz) optical modes, and a low-frequency interlayer breathing mode (∼0.4 THz), arising from standing waves defined by the nanoplatelet thickness. By mapping the probe energy dependence of the oscillation amplitude, we find that the A_1g ⁽¹⁾ mode couples most strongly to an electronic transition near 1.97 eV, which we assign using the computed band structure to transitions along the high-symmetry Γ-K line. The absence of Raman-active E_g modes is explained by a symmetry analysis based on the displacive excitation of coherent phonons. These results characterize carrier-lattice interactions in 2D Bi₂Se₃, which are relevant for optoelectronic device applications.