Ultrafast Anisotropic Optical-Gap Shift in Low-Symmetry Layered GeS

Abstract

Low-symmetry layered materials are emerging as promising platforms for polarization-driven nanophotonics. Understanding their nonequilibrium photoresponses, especially polarization dependence, is not only essential for designing high-performance devices but also provides new anisotropic light-matter interactions. Here, unique anisotropic optical-gap shifts are presented in layered germanium sulfide (GeS) using ultrafast differential transmission (DT) microscopy. Upon pump excitation, bandgap renormalization and lattice heating lead to overall redshifts in optical gaps. However, the redshift is substantially compensated by carrier-induced state filling at the band edge when probe light is polarized along the armchair (AC) direction of the crystal. In contrast, in the perpendicular zigzag (ZZ) polarization, the redshift is minimally compensated because the transition predominantly occurs at a band-nesting region, rather than at the band edge. As a result, the optical-gap redshift in ZZ-polarization is approximately three times larger than that in AC-polarization. Furthermore, this anisotropic redshift dynamically competes with isotropic spectral broadening, forming a strong dip in the DT spectrum, which shifts significantly to higher energies over time. It exhibits an approximate to 80 meV shift in ZZ-polarization-nearly three times that in AC-polarization. These findings offer new insights into the anisotropic nonequilibrium dynamics of low-symmetry materials, highlighting their potential for ultrafast polarization-driven nanophotonics.