Giant asymmetric amplification of third-order nonlinearity in strained monolayer GeC

Despite the growing interest in two-dimensional germanium carbide (GeC) for optoelectronic applications, a comprehensive understanding of its nonlinear optical (NLO) properties and Optomechanical tunability remains largely unexplored. This study presents a theoretical investigation of the linear optical response and third-harmonic generation (THG) in monolayer GeC under biaxial strain, employing density-matrix formalism within the framework of perturbation theory coupled with an extended tight-binding model. The linear optical response of pristine GeC exhibits a primary absorption peak near 3 eV and an optical bandgap of approximately 2 eV, positioning it as a suitable candidate for visible-range optoelectronic devices. Strain engineering enables the systematic tuning of the linear absorption spectrum: compressive strain shifts the optical characteristics toward graphene-like behavior at higher energies, while tensile strain drives the response toward germanene-like features at lower energies. The complex NLO spectrum is systematically characterized, revealing distinct resonance features attributed to multi-photon sub-bandgap transitions and single-photon above-bandgap excitations at the high-symmetry K and M points of the Brillouin zone. Biaxial strain induces a pronounced and asymmetric modulation of the THG response. Compressive strain blue-shifts the NLO spectrum toward higher energies and induces peak splitting, whereas tensile strain red-shifts the spectrum and enhances the THG intensity by more than twofold. This giant enhancement is physically attributed to strain-induced band flattening near the critical points, which substantially amplifies both the joint density of states (JDOS) and transition dipole moments. These findings establish monolayer GeC as a highly tunable, high-efficiency platform for next-generation nonlinear nanophotonic devices, strain-tunable sensors and frequency converters operating across the visible to near-ultraviolet range.