Multiple Linear Dichroism Inversions in SnO Monolayers for Polarization-Sensitive UV Photodetection: An Ab Initio Investigation

Tin monoxide (SnO) undergoes a phase transition from litharge-like tetragonal (space group P4/nmm) to orthorhombic geometry (layer group pmmn) in passing from multilayer to monolayer crystals. By means of ab initio ground and excited-state methods, we explore the impact of the reduced pmmn spatial symmetry on the electronic and optical properties of SnO monolayers. As a consequence of the in-plane anisotropy, the electronic states of the band edges show asymmetric projections onto the px and py atomic orbitals along orthogonal directions in the Brillouin zone. This results in optical absorption and exciton properties that are highly sensitive to the direction of in-plane polarized light. In contrast to typical linear dichroic materials, which generally favor the absorption of one polarization over the orthogonal one across a wide frequency range, we show that SnO monolayers display linear dichroism inversion. Here, the energy ordering of the exciton states causes the two orthogonal polarizations to be absorbed with different intensities depending on the light frequency. We observe multiple inversions of the linear dichroism across wavelengths from 200 to 400 nm. These properties make SnO monolayers promising candidates for further exploration of low-symmetry, two-dimensional materials for advanced applications in polarization-sensitive nanoscale devices. In addition, we propose utilizing optical dichroism measurements as a means to probe the recently predicted ferroelastic-to-paraelastic transition of SnO monolayers.