Giant Linear Dichroism and Valley Energetic Polarization in van der Waals Group-IV Monochalcogenide Semiconductors

van der Waals layered MX (M = Ge, Sn; X = S, Se) ferroelectric semiconductors present structural anisotropy that is responsible for the intriguing valley physics yet makes it challenging to determine direction-dependent optical-edge parameters in a rigorous and consistent manner. Here, we systematically explore the in-plane optical anisotropy in MX by polarized transmission spectroscopy combined with spherical aberration corrected scanning transmission electron microscopy. We demonstrate that the two orthorhombic armchair (AC) and zigzag (ZZ) optical edges are highly linearly polarized, with maximum linear dichroic ratios (LDRs) of ∼20 for GeX and ∼100 for SnX, corresponding to degrees of linear polarization (DoLPs) of ∼90% and ∼98%, respectively. The two polarization-selected optical channels yield a polarization-dependent optical band-edge splitting of up to ∼300 meV even in bulk-like flakes, with the high-energy edge occurring along the ZZ direction for GeX and along the AC direction for SnX. Based on the two polarization-locked optical edges, a semiempirical parametrization is introduced to evaluate the transmission spectra and the corresponding effective optical edge at arbitrary polarization conditions. This intrinsic characteristic in MX is robust in a wide temperature range and more interestingly in micrometer-scale thickness that is far beyond the well-accepted nanoscale. Our investigation not only provides a practical strategy for evaluating the anisotropic optical edges in low-symmetry MX semiconductors but also helps clarify the inconsistency of optical bandgaps frequently observed under different polarization conditions.