Polarization-Resolved and Helicity-Resolved Raman Intensity of Monolayer and Bilayer β-InSe

The structural and vibrational properties along with the polarization-resolved and helicity-resolved Raman spectra of monolayer and bilayer β-InSe excited by two commonly used laser lines (532 and 633 nm) in experiments are investigated by first-principles calculations so that we can establish the correlation between layer stacks and optical features in two-dimensional InSe. The stability of both monolayer and bilayer InSe are examined and confirmed by phonon dispersion. The monolayer and bilayer InSe are found to exhibit different point group symmetries and thereby the Raman properties due to the symmetry breaking of bilayer structure caused by interlayer van der Waals interaction. All Raman-active modes for monolayer (bilayer) InSe are listed, and the polarization-angle-dependent Raman intensities are accordingly discussed separately in terms of the configuration of the incident light (parallel or perpendicular to certain planes). By calculating the Raman tensor of each Raman-active mode, we obtained polarization-angle-dependent Raman intensities and discussed the location of each maximum under different polarization configurations, as well as the nonpolarized Raman intensity. The helicity-resolved Raman intensity is also obtained. It is found that more Raman-active modes can be observed under helical excitation when the incident light wave vector is in the y-direction. Among all of the Raman-active modes, the Raman intensity of A′1(A1g) is the strongest. Our work might help material scientists to characterize monolayer and bilayer InSe in experiments.