Spin Splitting Coupled Visible-Light Driven Injection Photocurrent in Bilayer Altermagnets

Altermagnets host symmetry-protected spin splitting without net magnetization, offering a new platform for spin-dependent transport and optical responses. Here we demonstrate that bilayer altermagnets enable efficient and tunable second-order injection photocurrents in the visible regime via interlayer coupling. Using a multiorbital tight-binding model, we show that interlayer coupling lifts the spin degeneracy and enables the normal injection current (NIC), while both the normal and magnetic injection currents (NIC and MIC) evolve in opposite directions as the interlayer hybridization increases. This contrasting behavior reflects distinct microscopic mechanisms: magnetic symmetry governs the emergence of the NIC, whereas the interlayer hybridization strengthens the Berry-curvature-weighted optical transitions and thereby enhances their magnitude. By contrast, the evolution of the MIC is primarily governed by band geometric effects. First-principles calculations for a realistic MnPTe₃ bilayer candidate confirm our predictions. Notably, moderate interlayer compression enhances the visible normal injection current by nearly a factor of five. Our results uncover a direct link between altermagnetic spin splitting and nonlinear photocurrent generation and establish interlayer engineering as a general route toward tunable nonlinear optoelectronics.